Fact-checked by Grok 2 weeks ago

Cytochrome b

Cytochrome b is a heme-binding integral membrane protein encoded by the mitochondrial MT-CYB gene, serving as a core subunit of the cytochrome bc1 complex (complex III) in the electron transport chain of mitochondria.^[1] It consists of approximately 400 amino acid residues and spans the inner mitochondrial membrane with eight transmembrane α-helices, coordinating two heme b prosthetic groups (bL and bH) that enable redox reactions.^[2] Through its participation in the Q-cycle mechanism, cytochrome b facilitates the oxidation of ubiquinol (QH2) at the Qo site and reduction of ubiquinone (Q) at the Qi site, bifurcating electron flow—one to the Rieske iron-sulfur protein and cytochrome c1 for transfer to cytochrome c, and the other across the complex to recycle electrons—while translocating protons to establish the electrochemical gradient essential for ATP synthesis via oxidative phosphorylation.^[3] The cytochrome bc1 complex, a dimeric assembly of 10-11 subunits per monomer (with cytochrome b being the only mitochondrially encoded component in mammals), integrates into the respiratory chain between complexes I/II and IV, coupling electron transfer from ubiquinol to cytochrome c with proton pumping across the membrane.^[1] Mutations in MT-CYB can disrupt complex III function, leading to mitochondrial disorders such as exercise intolerance and myopathy due to impaired energy production.^[1] Structurally conserved across eukaryotes, bacteria, and archaea, cytochrome b's transmembrane helices form binding pockets for quinones and hemes, with helix packing enabling the spatial separation of low- and high-potential hemes critical for the Q-cycle's efficiency.^[4] Its evolutionary role underscores the ancient origins of bioenergetic systems, with homologs like cytochrome b6 in photosynthetic organisms performing analogous functions in the cytochrome b6f complex.^[5]

Molecular Structure

Primary Sequence and Topology

Cytochrome b, a core subunit of the ubiquinol-cytochrome c reductase (complex III) in the mitochondrial respiratory chain and the analogous cytochrome b6f complex in photosynthetic organisms, typically consists of a polypeptide chain of approximately 380-400 amino acids in both eukaryotic and prokaryotic species.^[6]^[7] For example, the human mitochondrial cytochrome b (MT-CYB) comprises 380 residues, while the yeast ortholog is 385 amino acids long, reflecting high conservation across diverse taxa despite minor length variations.^[6]^[8] The protein adopts an integral membrane architecture characterized by 8 transmembrane α-helices that form a compact bundle embedded within the inner mitochondrial membrane of eukaryotes or the cytoplasmic membrane of prokaryotes.^[6]^[8] These helices, often labeled A through H, span the lipid bilayer and are connected by loops of varying lengths, with the structure confirmed by crystallographic studies such as the bovine heart bc1 complex at 2.1 Å resolution, which resolved exactly 8 helices rather than the 9 sometimes predicted by sequence-based algorithms. In the photosynthetic cytochrome b6f complex of thylakoid membranes, the equivalent cytochrome b6 subunit features 4 transmembrane helices, complemented by 3 in the associated subunit IV, collectively mimicking the 8-helix bundle of the bc1-type cytochrome b.^[9] The membrane topology positions both the N- and C-termini on the matrix side of the inner mitochondrial membrane in eukaryotes, consistent with the even number of transmembrane helices in the canonical eight-helix model.^[10] Biochemical assays using protease protection and antibody accessibility in yeast mitochondria confirm the N-terminus protrudes into the matrix, with the C-terminus similarly oriented due to the symmetric helical crossings.90035-3) In prokaryotes, this corresponds to both termini facing the cytoplasm, while in thylakoid-embedded b6f complexes, the termini face the stroma. The helices are organized into two distinct bundles: an N-terminal domain encompassing helices A-D and a C-terminal domain with helices E-H, creating a central core that accommodates non-covalently bound heme groups.^[9]^[11] Several conserved motifs define the primary sequence, notably histidine-rich regions critical for structural integrity and heme coordination. These include paired axial histidine ligands (e.g., His97 and His198 in bovine cytochrome b) located in helices B and D, spaced approximately 14 residues apart to axially ligate the low- and high-potential hemes (b_L and b_H).^[8]^[11] The "cyt b signature," a broader conserved sequence pattern encompassing these histidine motifs and adjacent residues (such as the PEWY quinol-binding loop), is preserved across species, ensuring proper folding and membrane integration.^[9] These elements underscore the evolutionary stability of cytochrome b's architecture, as evidenced by sequence alignments from bacteria to mammals.

Heme Binding and Cofactors

Cytochrome b, the core subunit of the ubiquinol:cytochrome c oxidoreductase (complex III), non-covalently binds two distinct b-type heme groups, designated heme b_H and heme b_L, which are essential for its redox activity. These hemes differ in their midpoint redox potentials and spectroscopic signatures: heme b_H exhibits a higher potential of approximately +50 mV and an α-band absorption maximum at 562 nm in the reduced state, while heme b_L has a lower potential of approximately -60 mV and an absorption maximum at 566 nm.^[12] These properties arise from their asymmetric positioning within the protein and enable their selective monitoring via difference spectroscopy, where shifts in the α-band allow distinction between the two cofactors during redox titrations.^[12] The iron atoms of both hemes are axially coordinated by pairs of conserved histidine residues acting as ligands, forming a bis-histidine ligation geometry that stabilizes the high-spin ferric state. In the bovine mitochondrial cytochrome b (numbered according to UniProt P00157), heme b_L is ligated by His83 (on transmembrane helix B) and His182 (on helix D), whereas heme b_H is coordinated by His97 (helix B) and His196 (helix D).^[13] This ligation pattern is highly conserved across species, ensuring proper heme incorporation during biogenesis and maintaining the hemes' accessibility for electron transfer without covalent attachment to the polypeptide.^[12] Structurally, the heme groups are embedded within a four-helix bundle formed by transmembrane helices A–D of cytochrome b, with their planes oriented roughly perpendicular to the membrane plane to facilitate inter-heme electron equilibration. The porphyrin rings are anchored to the protein matrix primarily through electrostatic interactions involving their carboxylate propionate side chains, which extend into hydrophilic pockets and interact with positively charged residues or water molecules.^[12] This orientation positions the hemes optimally relative to the quinone-binding sites while minimizing exposure to the lipid bilayer, contributing to the overall stability and functional asymmetry of the cofactor pair.^[13]

Structural Variations Across Organisms

Cytochrome b in mitochondrial and prokaryotic bc₁ complexes typically consists of approximately 380–400 amino acid residues, forming a polypeptide with eight transmembrane helices that span the inner mitochondrial or plasma membrane.^[12] These helices are organized into two bundles: helices A–D on the cytoplasmic (or periplasmic in bacteria) side and E–H on the matrix (or cytoplasmic) side, accommodating two non-covalently bound b-type hemes in a near-symmetric placement within the central four-helix bundle.^[12] The hemes, denoted b_L (low potential) and b_H (high potential), are axially ligated by histidine residues, with b_L positioned closer to the positive side (p-side) and b_H toward the negative side (n-side) of the membrane.^[5] In many prokaryotic variants, such as those from Rhodobacter capsulatus, the structure is conserved.^[5] In contrast, cytochrome b₆ in chloroplast and cyanobacterial b₆f complexes is notably shorter, comprising about 215–250 amino acid residues per monomer, and features only four transmembrane helices (A–D).^[14] This subunit corresponds to the C-terminal domain of mitochondrial cytochrome b, with the N-terminal domain homolog replaced by the separate subunit IV, which adds three additional transmembrane helices (E–G), resulting in a total of seven helices for the b₆/subunit IV core.^[15] The two b-type hemes in b₆ (b_p and b_n) are bound similarly to those in bc₁, but the complex includes a unique non-covalently bound c-type heme (heme c_n or heme x) near the n-side, ligated by a CXGG motif in subunit IV, facilitating distinct electron routing.^[12] The Rieske iron-sulfur protein partner in b₆f contains a [2Fe-2S] cluster essential for electron transfer, though this is extrinsic to cytochrome b₆ itself.^[16] Both bc₁ and b₆f complexes function as homodimers, with cytochrome b (or b₆) monomers forming the structural core through inter-monomer contacts via transmembrane helices, enabling concerted electron bifurcation.^[12] However, in b₆f, cytochrome b₆ engages in heterodimeric interactions with small subunits like PetG (a single-transmembrane helix protein), which stabilizes the dimer and influences quinol access pathways, a feature less prominent in bc₁ where cytochrome b homodimers suffice for core stability.^[17] These structural divergences reflect evolutionary adaptations: the unified eight-helix cytochrome b in bc₁ supports respiratory ubiquinol oxidation, while the split architecture in b₆f accommodates photosynthetic plastoquinol cycling and additional cofactors like chlorophyll a and β-carotene in subunit IV.^[5]

Biological Function

Role in Respiratory Chain Complex III

Cytochrome b serves as an essential subunit of the cytochrome bc1 complex, also known as Complex III, which is a central component of the mitochondrial electron transport chain in aerobic respiration. This complex catalyzes the transfer of electrons from ubiquinol (QH2) to cytochrome c, coupling this redox reaction to proton translocation across the inner mitochondrial membrane to generate the proton motive force. In eukaryotic mitochondria and many bacteria, cytochrome b works in concert with two other core catalytic subunits: the Rieske iron-sulfur protein and cytochrome c1, which together form the minimal functional unit for electron bifurcation.^[18]^[19] The cytochrome bc1 complex operates as a symmetric dimer, with each monomer comprising 11 subunits in mammalian mitochondria, including the three respiratory subunits (cytochrome b, cytochrome c1, and Rieske protein), two core proteins (UQCRC1 and UQCRC2), and six low-molecular-weight subunits that stabilize the structure. Cytochrome b, encoded by the mitochondrial genome in eukaryotes, forms the transmembrane core of each monomer, anchoring the complex within the lipid bilayer through eight transmembrane helices and coordinating two heme b cofactors essential for electron relay. This dimeric architecture ensures coordinated electron transfer between the two Qo sites (on the positive side of the membrane) and Qi sites (on the negative side), with cytochrome b's hemes bridging these quinone-binding pockets.^[19]^[18]^[20] Within the electron transport chain, cytochrome b positions the bc1 complex downstream of Complexes I and II, where it accepts electrons from ubiquinol oxidation at the Qo site; one electron branch reduces the Rieske protein for transfer to cytochrome c1 and ultimately cytochrome c, while the other traverses cytochrome b's hemes bL (low-potential, near Qo) to bH (high-potential, near Qi) for delivery to the Qi site. This bifurcated pathway enables efficient coupling of electron flow to energy conservation. As part of the Q-cycle mechanism, cytochrome b's role in the complex results in the net translocation of four protons from the matrix (or bacterial cytoplasm) to the intermembrane space per two electrons transferred to cytochrome c, amplifying the proton gradient for ATP synthesis without direct proton pumping by the protein itself.^[3]^[21]

Role in Photosynthetic b6f Complex

Cytochrome b6 serves as a core transmembrane subunit of the cytochrome b6f complex, embedded in the thylakoid membranes of chloroplasts in plants, algae, and cyanobacteria. This complex functions as a key redox hub in photosynthetic electron transport, where cytochrome b6 partners with cytochrome f, the Rieske iron-sulfur protein (also known as the ISP), and smaller subunits such as PetD (subunit IV) to form a dimeric structure essential for quinone oxidation and reduction. The integration of these components enables the coordinated movement of electrons and protons across the membrane, contributing to the generation of a proton motive force.^[22]^[23]^[16] In its role within the b6f complex, cytochrome b6 facilitates the linkage between photosystem II (PSII) and photosystem I (PSI) by mediating the oxidation of plastoquinol (PQH2) at the Qo (or Qp) site on the lumenal side and the subsequent reduction of plastoquinone at the Qi (or Qn) site on the stromal side. This process bifurcates electrons from PQH2, with one electron transferred via the Rieske protein and cytochrome f to plastocyanin for PSI reduction, while the other traverses the low-potential chain through the hemes of cytochrome b6 to reduce plastoquinone at the Qi site. These site-specific reactions support the overall electron transport chain, enhancing the efficiency of photosynthetic energy conversion.^[22]^[16]^[23] Structurally adapted for its photosynthetic environment, cytochrome b6 exhibits a lower molecular weight of approximately 20-25 kDa per subunit, reflecting its compact transmembrane architecture with multiple alpha-helices and bound hemes (including bp, bn, and the unique cn). This reduced size compared to its mitochondrial counterpart facilitates rapid quinone diffusion and electron shuttling within the crowded thylakoid membrane, making it indispensable for both linear electron flow from PSII to PSI (producing NADPH and ATP) and cyclic electron flow around PSI (primarily boosting ATP synthesis). The presence of an additional heme cn near the Qi site further optimizes semiquinone stabilization and proton-coupled electron transfer in this light-driven context.^[24]^[16]^[22] Evolutionarily, the cytochrome b6f complex, including its b6 subunit, derives from an ancient prokaryotic ancestor resembling the bc1 complex found in respiring bacteria and mitochondria, with the b6f form predating the longer cytochrome b in bc1 types. This divergence likely occurred in early oxygenic phototrophs, where cytochrome b6 specialized to couple plastoquinone redox reactions with light-dependent proton translocation, thereby establishing the trans-thylakoid proton gradient crucial for ATP production in oxygenic photosynthesis. The conservation of a Q-cycle mechanism underscores this shared heritage, adapted for photosynthetic rather than respiratory demands.^[25]^[26]^[22]

Electron and Proton Transfer Mechanisms

Cytochrome b plays a central role in the Q-cycle mechanism within the bc₁ (respiratory) and b₆f (photosynthetic) complexes, where it facilitates bifurcated electron transfer from quinol (QH₂) oxidation. In this process, oxidation of QH₂ at the Qo site releases two protons to the positive side of the membrane and bifurcates the electrons: one electron travels via the high-potential chain through the Rieske iron-sulfur protein [2Fe-2S] cluster to cytochrome c (in bc₁) or cytochrome f (in b₆f), while the second electron is transferred to the low-potential chain consisting of the two b hemes within cytochrome b. This bifurcation enables semiquinone (SQ) formation at the Qo site, with the low-potential electron reducing heme b_L, followed by rapid transfer to heme b_H.^[3] The full Q-cycle requires two turnovers of QH₂ at the Qo site to reduce one ubiquinone (Q) or plastoquinone to QH₂ at the Qi site, effectively coupling scalar proton release and uptake to vectorial translocation. At the Qo site, the initial QH₂ oxidation yields the equation: QH₂ + 2 cyt c^{ox} (or f^{ox}) → Q + 2 cyt c^{red} (or f^{red}) + 2 H^+ (released to intermembrane or lumen space), but the bifurcated path ensures the second electron contributes to Qi site reduction in the cycle. Proton pumping nets four protons translocated per Q-cycle (four released at Qo from two QH₂ oxidized and two taken up at Qi), establishing a proton motive force essential for ATP synthesis.^[27] The redox potentials of the b hemes drive the directional electron flow and stabilize the SQ intermediate, with heme b_L exhibiting a lower potential (approximately -90 mV) compared to heme b_H (approximately -50 mV), creating a thermodynamic gradient that favors transfer from b_L to b_H and subsequent SQ_i stabilization at the Qi site. This potential difference, spanning about 40 mV, prevents backflow and supports the low-potential chain's role in bifurcated transfer across both bc₁ and b₆f complexes.^[18] Inhibitor studies highlight the mechanistic specificity: antimycin binds near heme b_H at the Qi site, blocking electron transfer to Q and causing b hemes to accumulate in the reduced state, while stigmatellin binds at the Qo site adjacent to the Rieske [2Fe-2S] cluster, preventing QH₂ oxidation and bifurcation by stabilizing the enzyme-substrate complex. These sensitivities confirm the distinct roles of the Qo and Qi sites in electron and proton management.^[28]

Genetics and Evolution

Encoding Genes in Humans and Model Organisms

In humans, the core subunit of cytochrome b in mitochondrial respiratory chain complex III is encoded by the mitochondrially located MT-CYB gene (also known as CYTB), which spans positions 14,747 to 15,887 on the heavy strand of the mitochondrial genome and produces a 380-amino acid polypeptide essential for electron transport.^[29] This gene is maternally inherited due to the uniparental transmission of mitochondrial DNA and is transcribed and translated within the mitochondria to support oxidative phosphorylation.^[30] Distinct nuclear-encoded variants belong to the cytochrome b5 family, which facilitate electron transfer in non-respiratory pathways; CYB5A, located on chromosome 18q22.3, encodes a microsomal form anchored to the endoplasmic reticulum membrane for roles in fatty acid desaturation and methemoglobin reduction, while CYB5B on chromosome 16q22.1 produces an outer mitochondrial membrane isoform involved in nitrite reduction.^[31]^[32]^[33] A related nuclear gene, CYBASC3 (alias for CYB561A3 on chromosome 2q31.1), encodes a lysosomal transmembrane ferrireductase that uses ascorbate as an electron donor; recent studies as of 2024 have characterized it as essential for lysosomal ferric iron reduction and iron homeostasis, particularly in cancer cells such as those in Burkitt lymphoma.^[34]^[35] Among model organisms, the bovine MT-CYB gene, encoding a 379-amino acid cytochrome b, serves as a standard reference for residue numbering in comparative structural analyses and human mutation studies due to its high-resolution crystal structures of the bc1 complex.^[36] In the yeast Saccharomyces cerevisiae, the CYTB (or COB) gene is mitochondrially encoded between tRNA-Glu and replication origin ORI6, producing the cytochrome b subunit of complex III; cytoplasmic petite (ρ⁻) mutants, which lack functional mitochondrial DNA, have been instrumental in sequencing the gene and revealing its split structure with introns.^[37]^[38] Bacterial systems feature the petB gene encoding cytochrome b of the bc1 complex, typically within the petABC operon alongside genes for the Rieske iron-sulfur protein (petA) and cytochrome c1 (petC), enabling quinol oxidation in the plasma membrane.^[39] In cyanobacteria, such as Synechococcus elongatus, the cytochrome b₆ subunit of the photosynthetic b₆f complex is encoded by petB, which forms a bicistronic operon with petD (encoding subunit IV) to facilitate electron transfer between photosystems I and II in the thylakoid membrane.^[40]

Evolutionary Conservation and Sequence Variability

Cytochrome b exhibits remarkable evolutionary conservation, particularly in its core structural elements essential for function. The histidine residues serving as axial ligands to the heme groups are invariant across diverse taxa, ensuring proper heme coordination and electron transfer capabilities. Similarly, the eight transmembrane helices that form the membrane-spanning core of the protein in mitochondrial bc1 complexes (and analogous structures in b6f complexes) display high sequence identity, with over 80% similarity in vertebrates, reflecting strong selective pressure to maintain the bc1/b6f scaffold inherited from ancient bacterial ancestors approximately 2 billion years ago. This core architecture originated from prokaryotic membrane-anchored dehydrogenases in response to early redox challenges posed by biogenic photosynthesis.^[5]^[41]^[42] Sequence variability in cytochrome b is pronounced in non-functional regions, allowing for neutral evolutionary drift without compromising core activity. The third codon positions of the gene are hypervariable, accumulating synonymous substitutions at rates up to 14 times faster than other positions, which supports neutral evolution models and facilitates phylogenetic resolution at shallow taxonomic levels. In certain fungi and plants, the cytochrome b genes contain introns, particularly group I introns inserted after specific codons like the 143rd, with frequent gain and loss events contributing to genomic diversity; these introns are often mitochondrial but can influence nuclear-mitochondrial interactions in splicing. Such variability contrasts with the rigid conservation of functional motifs, highlighting modular evolution.^[43]^[44]^[45] Gene duplication events have shaped the cytochrome b family, leading to functional diversification in electron transfer roles across eukaryotes. The divergence of bc1 (respiratory) and b6f (photosynthetic) complexes occurred post-endosymbiosis, as alphaproteobacterial progenitors adapted to oxygenated environments, resulting in distinct cytochrome b isoforms tailored to mitochondrial and chloroplast functions. Recent phylogenomic analyses of proteobacterial diversity, including 2022 studies resolving deep-branching lineages, reveal variant cytochrome b forms in Alphaproteobacteria that illuminate the mitochondrial ancestor's genetic toolkit, underscoring ongoing bacterial contributions to eukaryotic evolution.^[46]^[5]^[47]

Applications and Significance

Use in Molecular Phylogenetics

Cytochrome b sequences from mitochondrial DNA (mtDNA) are widely utilized as molecular markers in phylogenetics due to several advantageous properties. The gene's maternal inheritance pattern ensures uniparental transmission, minimizing complications from biparental contributions, while the absence of recombination maintains linkage among mutations, facilitating clear tracing of evolutionary lineages.^[48] Additionally, at approximately 1.1 kb in length, the cytochrome b gene is amenable to efficient PCR amplification and sequencing, making it practical for large-scale studies across diverse taxa.^[49] These sequences have proven effective in DNA barcoding for species identification, particularly in challenging groups like fish families such as Salmonidae, where cytochrome b distinguishes closely related salmon and trout species in commercial and ecological contexts. In mammals and birds, the gene aids in resolving intra-genus phylogenies, such as reconstructing relationships among great apes and rodents in mammals or among passerine species in birds, by capturing sufficient variation for recent divergences.^[50]^[51] Common analytical methods involve multiple sequence alignment followed by phylogenetic tree construction using maximum likelihood approaches, which account for rate heterogeneity and improve resolution when weighting transversions over transitions.^[49] In vertebrates, the substitution rate of approximately 2% per million years provides a reliable molecular clock for estimating divergence times in these analyses.^[52] Despite these strengths, cytochrome b has limitations, notably nucleotide saturation at third codon positions, which obscures signal in deep evolutionary divergences beyond 5-10 million years.^[49] To address this, post-2010 studies have increasingly supplemented cytochrome b with nuclear markers, such as introns or multilocus datasets, to enhance resolution in complex phylogenies.^[53] Recent advances, including 2021 research on salmonid hybrids, integrate cytochrome b with genomic approaches to detect hybridization events, as seen in Chinook and Coho salmon crosses, enabling finer-scale identification of admixed populations in trout and related species.^[54]

Clinical Mutations and Associated Disorders

Mutations in the MT-CYB gene, encoding the cytochrome b subunit of mitochondrial respiratory complex III, are a recognized cause of isolated complex III deficiency and related mitochondrial disorders in humans. These mutations disrupt electron transfer and proton translocation in the Q-cycle mechanism, leading to reduced ATP production and elevated reactive oxygen species (ROS) levels, which contribute to cellular damage across affected tissues. Over 50 pathogenic variants in MT-CYB have been documented, predominantly point mutations resulting in amino acid substitutions that impair complex III assembly or stability.^[29]^[55] A representative example is the m.15257G>A variant (also denoted G15257A), which causes an aspartic acid to asparagine substitution at position 171 (p.Asp171Asn) and is associated with Leber hereditary optic neuropathy (LHON)-like phenotypes, including acute or subacute vision loss mimicking primary LHON mutations. This variant occurs in approximately 9% of LHON cases but is also found in 0.3% of controls, often acting as a secondary mutation that increases phenotypic penetrance when combined with primary LHON variants. The m.14846G>A variant (p.Gly34Ser) similarly contributes to isolated complex III deficiency, manifesting as mitochondrial myopathy with prominent exercise intolerance, muscle weakness, and myoglobinuria.^[29]^[56]^[57] The prevalence of mitochondrial complex III deficiency is unknown, though it is considered rare; overall mitochondrial diseases from mtDNA variants occur at approximately 1 in 5,000 births, with complex III defects being among the rarest respiratory chain disorders.^[58] Diagnosis typically involves muscle biopsy to assess complex III activity via spectrophotometry, combined with targeted mtDNA sequencing to identify heteroplasmic variants, as clinical presentation varies widely from isolated myopathy to multisystem involvement like cardiomyopathy and encephalopathy. Therapeutic options remain supportive, with coenzyme Q10 supplementation used to potentially bypass the defect and mitigate ROS, though clinical trials have shown limited efficacy in improving motor function or overall outcomes. As of 2025, emerging mitochondrial gene editing approaches hold promise for correcting MT-CYB variants, but no specific phase trials for these mutations have advanced to clinical use.^[59]^[29]^[60]^[61]

Target for Fungicides and Drug Resistance

Cytochrome b serves as a primary target for quinone outside inhibitor (QoI) fungicides, also known as strobilurins and classified under FRAC group 11, which bind to the Qo site of the bc1 complex in fungal mitochondria, thereby disrupting electron transfer and ATP production essential for fungal respiration.^[62] These fungicides have been widely used in agriculture since the late 1990s to control pathogens such as Botrytis cinerea and Zymoseptoria tritici, but resistance has emerged rapidly due to point mutations in the cytochrome b gene. A prominent example is the G143A mutation, first identified in field isolates of B. cinerea in the early 2000s, which confers high-level resistance by altering the Qo binding pocket and preventing inhibitor attachment without severely impacting fungal fitness.^[63] Similarly, the G143A mutation has been detected in Z. tritici, the causal agent of septoria leaf blotch in wheat, leading to widespread QoI failure in European fields by the mid-2010s.^[64] In parasitic protozoa, cytochrome b is targeted by the antimalarial drug atovaquone, which inhibits the Plasmodium falciparum bc1 complex by binding to the Qo site, collapsing the mitochondrial membrane potential and halting parasite replication. Resistance arises primarily from mutations at codon 268, such as Y268S and Y268N, which reduce drug affinity while maintaining partial enzyme function; these were first linked to clinical failures in the late 1990s and have since spread globally. Y268 mutations have been detected in clinical isolates from sub-Saharan Africa, though at low frequencies (typically <1%), underscoring the drug's relative resilience in combination therapy despite occasional emergence.^[65] These mutations often emerge de novo during treatment, as seen in patient recrudescences where resistant parasites outcompete wild-type within days.^[66] The evolution of cytochrome b-mediated resistance is accelerated by mitochondrial DNA (mtDNA) heteroplasmy, where cells harbor a mixture of wild-type and mutant genomes, enabling rapid selection under fungicide or drug pressure as resistant mtDNA variants replicate preferentially and segregate into daughter cells. In fungal pathogens like Podosphaera xanthii, heteroplasmy at the G143A site allows intermediate resistance levels that escalate quickly upon QoI exposure, contributing to the fixation of mutations in populations within 2-3 seasons. Cross-resistance is universal among QoI fungicides due to their shared binding site, rendering all FRAC 11 compounds ineffective against G143A or Y268 mutants. To mitigate this, resistance management strategies emphasize rotating QoIs with fungicides from unrelated FRAC groups (e.g., group 3 DMIs or group 7 SDHIs) and limiting applications to no more than two per season, as recommended by international guidelines to delay selection.^[67]^[68] Recent studies as of 2025 have highlighted the emergence of dual mutations in cytochrome b, such as combinations of F129L and G143A, in wheat pathogens like Z. tritici, which confer resistance to both conventional strobilurins and newer QoIs like pyribencarb while imposing variable fitness costs. These dual mutants, detected in European surveillance programs post-2021, exhibit moderate to high resistance and reduced growth rates, complicating disease control in cereals and prompting updated FRAC monitoring protocols. Such findings underscore the ongoing need for integrated pest management to track and counter evolving resistance profiles beyond single-point changes.

References

[1]
MT-CYB gene: MedlinePlus Genetics
### Summary of MT-CYB Gene and Cytochrome b
[2]
CDD Conserved Protein Domain Family: Cytochrome_b_N - NCBI
Feb 2, 2016 · Cytochrome b spans the mitochondrial membrane with 8 transmembrane helices (A-H) in eukaryotes. In plants and cyanobacteria, cytochrome b6 ...<|control11|><|separator|>
[3]
The Q Cycle of Cytochrome bc Complexes: a Structure Perspective
III. The Q cycle. The coupling of the oxidation-reduction and deprotonation-protonation of lipophilic quinone/ol (Q/QH2) within the cytochrome bc complex is ...
[4]
Bioinformatics Analysis the Complete Sequences of Cytochrome b ...
Sep 22, 2009 · In this study, homology modeling method and computer programs were performed to predict the tertiary structure of cyt b proteins of T.sylvaticus.
[5]
Evolution of cytochrome bc complexes: from membrane-anchored ...
The cytochrome bc1 complexes are functional dimers, capable of electron exchange between the monomers via closely placed heme bp [32–34]. In the bc1-type ...
[6]
MT-CYB - Cytochrome b - Homo sapiens (Human) - UniProt
Oct 3, 2012 · The full-length protein contains only eight transmembrane helices, not nine as predicted by bioinformatics tools. By similarity. Cofactor.Missing: topology | Show results with:topology
[7]
A Novel Mutation in the Mitochondrial DNA Cytochrome b Gene ...
The cyt b protein is 380 amino acids long and the MTCYB gene encompasses nucleotide positions 14747 to 15887 of mitochondrial DNA. Isolated complex III ...
[8]
Biogenesis of the cytochrome bc1 complex and role of assembly ...
Nov 22, 2011 · Cytochrome b is an integral membrane protein that contains eight hydrophobic α-helices and two heme b molecules with distinct redox chemistry.
[9]
https://doi.org/10.1016/j.bbabio.2013.07.006
[10]
Biochemical evidence for the orientation of cytochrome b ... - PubMed
These results indicate that the N-terminus of yeast cytochrome b protrudes from the matrix side of the inner membrane consistent with the eight-helix model for ...Missing: topology | Show results with:topology
[11]
https://doi.org/10.1016/S0969-2126(96)00062-7
[12]
Catalytic Reactions and Energy Conservation in the Cytochrome bc ...
Jan 19, 2021 · This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) ...
[13]
Crystal Structure of the Cytochrome bc1 Complex from Bovine Heart ...
The eight transmembrane helices of cytochrome b form two groups consisting of helices A to E and F to H, with the myxothiazol binding site in between. The ...
[14]
Cytochrome b6 - Arabidopsis thaliana (Mouse-ear cress) | UniProtKB
Arabidopsis thaliana (Mouse-ear cress). Amino acids. 215 (go to ... Note: Binds 2 heme b groups non-covalently with two histidine residues as axial ligands.
[15]
Structural-Functional Characterization of Cytochrome b in bc1 and ...
The present review provides the structural comparison of mitochondrial cyt b in ten model organisms targeting archaea, prokaryotes, and eukaryotes, highlighting ...
[16]
High-resolution cryo-EM structures of plant cytochrome b6f at work
Jan 13, 2023 · Here, we describe two high-resolution cryo-EM structures of the spinach Cyt b 6 f homodimer with endogenous plastoquinones and in complex with plastocyanin.
[17]
PetG and PetN, but not PetL, are essential subunits of the ...
The cytochrome bc1 and the cytochrome b6f complexes are homologous in their structure and function and both complexes most likely evolved from an ancient ...
[18]
Structural Analysis of Cytochrome bc1 Complexes - PubMed Central
Structures of the bc1 complex can be divided into three regions: the membrane spanning, the inter-membrane space (periplasm in bacteria or positive side of the ...
[19]
Complete Structure of the 11-Subunit Bovine Mitochondrial ...
We report the complete atomic resolution structures of the entire 11-subunit bovine heart mitochondrial cytochrome bc 1 complex from crystals belonging to two ...Intermembrane Side · Transmembrane Region · Matrix Side
[20]
Complete structure of the 11-subunit bovine mitochondrial ... - PubMed
Refined crystal structures of the 11-subunit bc1 complex from bovine heart reveal full views of this bifunctional enzyme.
[21]
Protonmotive pathways and mechanisms in the cytochrome bc1 ...
Jun 12, 2003 · The cytochrome bc 1 complex catalyzes electron transfer from ubiquinol to cytochrome c by a protonmotive Q cycle mechanism.
[22]
Structure-Function Studies of the Cytochrome b 6 f Complex
The cytochrome b 6 f complex occupies a central position in the sequence of photosynthetic electron transport carriers, oxidizing plastoquinol (PQH 2 )
[23]
Molecular basis of plastoquinone reduction in plant cytochrome b6f
Oct 3, 2024 · A multi-subunit enzyme, cytochrome b6f (cytb6f), provides the crucial link between photosystems I and II in the photosynthetic membranes of ...
[24]
A cytochrome f/b6 complex of five polypeptides with plastoquinol ...
The complex consists of five polypeptides with a molecular mass of 34, 33, 23.5, 20 and 17.5 kDa, and contains one cytochrome f, two cytochromes b6 and the ...
[25]
Evolution of cytochrome bc complexes: From membrane-anchored ...
This review traces the evolution of the cytochrome bc complexes from their early spread among prokaryotic lineages and up to the mitochondrial cytochrome bc 1 ...
[26]
SOME NEW STRUCTURAL ASPECTS AND OLD ... - Annual Reviews
Jun 1, 1996 · The cytochrome b6f complex functions in oxygenic photosynthetic membranes as the redox link between the photosynthetic reaction center ...
[27]
https://pubs.acs.org/doi/10.1021/acs.jpcb.6b10524
[28]
A semiquinone intermediate generated at the Q o site of the ... - PNAS
The cytochrome bc1 and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains.Missing: seminal | Show results with:seminal
[29]
Entry - *516020 - CYTOCHROME b OF COMPLEX III; MTCYB - OMIM
MTCYB is encoded by the guanine-rich heavy (H) strand of the mtDNA and located between nucleotide pairs (nps) 14747 and 15887 ( · The MTCYB gene encompasses 1140 ...
[30]
Mitochondrial Deoxyribonucleic Acid (mtDNA), Maternal Inheritance ...
Jun 1, 2023 · The mtDNA and its mutations could be maternally inherited thereby predisposing the offspring to various cancers like breast and ovarian cancers among others.
[31]
1528 - Gene ResultCYB5A cytochrome b5 type A [ (human)] - NCBI
Aug 19, 2025 · CYB5A encodes a membrane-bound cytochrome that reduces ferric hemoglobin to ferrous hemoglobin, required for stearyl-CoA-desaturase activity. ...
[32]
Entry - *611964 - CYTOCHROME b5, TYPE B (OUTER ... - OMIM
Hartz (2010) mapped the CYB5B gene to chromosome 16q22.1 based on an alignment of the CYB5B sequence (GenBank AB009282) with the genomic sequence (GRCh37).
[33]
Defects in CYB5A and CYB5B impact sterol-C4 oxidation in ...
Nov 26, 2024 · CYB5A was located in the ER, whereas CYB5B was anchored at the outer membrane of mitochondria with the majority of the protein in cytosol. The ...
[34]
CYB561A3 cytochrome b561 family member A3 [ (human)] - NCBI
Sep 9, 2025 · Predicted to enable transmembrane ascorbate ferrireductase activity. Predicted to be involved in intracellular iron ion homeostasis. Located in nucleolus.Missing: CYBASC3 | Show results with:CYBASC3
[35]
MT-CYB - Cytochrome b - Bos taurus (Bovine) | UniProtKB | UniProt
Component of the ubiquinol-cytochrome c reductase complex (complex III or cytochrome b-c1 complex) that is part of the mitochondrial respiratory chain.
[36]
COB - Saccharomyces Genome Database | SGD
May 29, 2007 · Summary: COB is located on the mitochondrial chromosome between tRNA-Glu and replication origin ORI6; coding sequence is 7108 nucleotides ...
[37]
Assembly of the mitochondrial membrane system. DNA sequence ...
Oct 25, 1980 · The mitochondrial genomes of cytoplasmic "petite" (rho-) mutants of Saccharomyces cerevisiae have been used to sequence the cytochrome b gene.
[38]
exchangeable Rieske subunits in the cytochrome bc-complex of ...
Bacterial cytochrome bc1-complex encoded by the petABC operon consists of three subunits, the Rieske iron-sulphur protein, the b-type cytochrome, and the c1 ...
[39]
Cloning and Sequencing of the petBD Operon From the ... - PubMed
The genes encoding the photosynthetic cytochrome b6 (petB) and subunit 4 (petD) have been cloned and sequenced from the unicellular, photoheterotrophic, ...
[40]
The Molecular Evolution of the Q o Motif - Oxford Academic
Mitochondrial cyt b consists of eight transmembrane helices (helices A–H, fig. 2), which can be grouped in N- and C-terminal region (Xia et al. 1997). The N ...
[41]
Tracking molecular evolution of photosynthesis by characterization ...
This cytochrome contains two conserved histidine pairs (His-84 and His-185 pair and His-98 and His-200 pair), which noncovalently bind two heme groups, located ...Results And Discussion · Photosynthesis Gene Cluster · Evolutionary Analyses<|control11|><|separator|>
[42]
Comparative evolution of the mitochondrial cytochrome b gene and ...
Cyt b evolves 2.8 times as fast as beta-fibint7 (14. 0 times as fast at third codon positions). Despite its relatively slow substitution rate, the ...Missing: fungi plants<|separator|>
[43]
Frequent Gain and Loss of Introns in Fungal Cytochrome b Genes
Nov 7, 2012 · An intron is considered as phase “0” if it is inserted between two consecutive codons, “1” if inserted between the first and the second ...Missing: variability | Show results with:variability
[44]
Discovery of new group I-D introns leads to creation of subtypes and ...
May 23, 2020 · In some fungal species, one intron interrupts the cytochrome b gene (COB) directly after its 143th codon (i.e. the 429th base pair into the ...
[45]
Cytochrome b5 diversity in green lineages preceded the evolution of ...
These observations suggest that the CB5 gene family has experienced early gene duplication and potential functional diversification in the basal land plants.
[46]
[PDF] genes in Paramecium tetraurelia - Genetics and Molecular Research
These findings suggest that cyt b5 genes underwent a similar evolutionary history in ancient species, possibly as a result of adaptation to a particular ...
[47]
On the bacterial ancestry of mitochondria: new insights ... - bioRxiv
May 16, 2022 · Here we triangulate across multiple phylogenomic and molecular approaches to pinpoint the most likely living relatives of the ancestral bacteria from which ...
[48]
Comparison of Universal mtDNA Primers in Species Identification of ...
Nov 13, 2024 · Abstract. Analysis of mitochondrial DNA, specifically the cytochrome b gene (cyt b), has become an essential tool for species identification. In ...
[49]
[PDF] The cytochrome b gene as a phylogenetic marker
The cytochrome b gene is used to resolve divergences, but has limitations like base biases and limited variation, making its utility questioned.
[50]
Reconstructing Mammalian Phylogenies: A Detailed Comparison of ...
Nov 30, 2010 · Comparison of the DNA sequences of 217 mammalian species reveals that cyt b more accurately reconstructs their phylogeny and known relationships ...<|separator|>
[51]
Molecular phylogeny of some avian species using Cytochrome b ...
This study used the cytochrome b gene to identify avian species, finding 88.60% similarity between chicken and Japanese quail, and 80.46% between chicken and ...Missing: mammals examples
[52]
Mitochondrial Cytochrome b Sequence Variations and Population ...
In general, the standard divergence rate for the mammalian cytochrome b gene is 2% per million years (Myr) (Avise et al., 1998). However, some studies ...
[53]
Phylogenetic Utility of Nuclear Introns in Interfamilial Relationships ...
Jan 20, 2011 · The present study not only resolves some of the ambiguous relationships in Caniformia phylogeny but also shows that the noncoding nuclear ...
[54]
Chinook and Coho salmon hybrids linked to habitat and climatic ...
Nov 11, 2021 · The cytochrome b for Coho salmon amplified in 41 individuals indicated that these were the offspring of Coho females. Two of the remaining four ...
[55]
Comprehensive analysis of mitochondrial DNA variants ...
Sep 26, 2023 · Previous studies have revealed that deleterious variants in the MT-CYB gene caused isolated complex III deficiency, leading to a variety of ...
[56]
NC_012920.1(MT-CYB):m.15257G>A AND Leber optic atrophy - NCBI
Condition(s) · Name: Leber optic atrophy (LHON) · Synonyms: Optic Atrophy, Hereditary, Leber; Leber hereditary optic neuropathy; Leber's disease; See all synonyms ...
[57]
Effective treatment of choreaballism due to an MT‐CYB variant with ...
Jun 20, 2023 · He was diagnosed with a non-syndromic mitochondrial disorder after detection of the variant m.15043G > A in MT-CYB. Seizures have been ...
[58]
Exercise Intolerance Due to Mutations in the Cytochrome b Gene of ...
Sep 30, 1999 · Exercise intolerance is a common symptom of the encephalomyopathies that are associated with mutations in mitochondrial DNA (mtDNA). These ...
[59]
Mitochondrial complex III deficiency - Genetics - MedlinePlus
Apr 1, 2014 · Frequency. The prevalence of mitochondrial complex III deficiency is unknown, although the condition is thought to be rare.
[60]
Mitochondrial diseases: from molecular mechanisms to therapeutic ...
Jan 10, 2025 · Previous studies estimate the global prevalence of mitochondrial diseases at approximately 1 in 5,000 births, with pathogenic mtDNA mutations ...
[61]
Phase III Trial of Coenzyme Q10 in Mitochondrial Disease
The types of primary purpose are: treatment, prevention, diagnostic ... mutations, and that this beneficial action is reflected in improved motor ...
[62]
Mitochondrial Diseases: Molecular Pathogenesis and Therapeutic ...
Sep 12, 2025 · Most pathogenic point mutations can potentially be corrected or modeled using mitochondrial gene editing technology. Locus, Allele, Position ...
[63]
Qol Fungicides | FRAC
QoI FRAC code 11A fungicide is: metyltetraprole. Fungicides within each code group are all cross-resistance to each other and should be managed accordingly.Introduction And General... · General Use Recommendations... · Specific Recommendations By...
[64]
Characterization of QoI resistance in Botrytis cinerea and ...
Jan 11, 2009 · QoI-resistant field isolates with the G143A mutation have been found only in species without the bi-143/144 intron, and QoI-resistant mutants ...
[65]
Mutations in the complex III of Zymoseptoria tritici and implications ...
Aug 12, 2025 · Evolution of the G143A mutation in cytochrome b led to the development of resistance, resulting in a loss of effectiveness of FRAC Group 11 ...Missing: 2024 | Show results with:2024
[66]
Clinically relevant atovaquone-resistant human malaria parasites ...
Oct 12, 2023 · Here we generate atovaquone-resistant parasites that differ from wild type parent by only a Y268S mutation in cytochrome b, a modification ...
[67]
Late clinical failure associated with cytochrome b codon 268 ...
Jan 21, 2020 · Confirmation of emergence of mutations associated with atovaquone–proguanil resistance in unexposed Plasmodium falciparum isolates from Africa.Missing: N268 2022
[68]
[PDF] Quinone 'outside' inhibitor (QoI) Working Group - FRAC
Mar 27, 2025 · Strategies for the management of QoI fungicide resistance, in all crops, are based on the statements listed below. These statements serve as a ...
[69]
Heteroplasmy for the Cytochrome b Gene in Podosphaera xanthii ...
Jul 3, 2018 · The rapid development of fungicide resistance by P. xanthii is a major problem, and resistance to multiple classes of fungicides including ...