Template modeling score
The Template Modeling score (TM-score) is a quantitative metric designed to evaluate the topological similarity between protein structures, particularly for assessing the quality of structural templates and predicted models in protein structure prediction pipelines.[1] Introduced in 2004 by Yang Zhang and Jeffrey Skolnick, it addresses limitations in traditional measures like root-mean-square deviation (RMSD) by incorporating alignment coverage and using a length-independent scale, with scores ranging from 0 (no similarity) to 1 (identical structures).[1] A TM-score greater than 0.5 typically indicates that two proteins share the same fold, while values below 0.17 suggest random structural similarity.[2] The TM-score is calculated after optimal superposition of structures as the maximum value of the average distance score over aligned residues, normalized by protein length:\text{TM-score} = \max \left[ \frac{1}{L_N} \sum_{i=1}^{L_A} \frac{1}{1 + \left( \frac{d_i}{d_0} \right)^2} \right]
where L_N is the length of the native structure, L_A is the number of aligned residues, d_i is the distance between the i-th pair of aligned residues, and d_0 is a size-dependent scaling factor (e.g., d_0 = 1.24 (L_N - 15)^{1/3} - 1.8 for intrachain comparisons).[3] This formulation weights smaller deviations more heavily than larger ones for the aligned residue pairs, providing a more robust correlation to the accuracy of full-length predicted models compared to metrics like the Global Distance Test (GDT) or MaxSub.[1] TM-score has become a standard in benchmarks such as the Critical Assessment of Structure Prediction (CASP) competitions and tools like AlphaFold, where it informs confidence in predicted structures.[4]