The article by Cyrus Chothia and Arthur M. Lesk explores the relationship between sequence divergence and structural divergence in homologous proteins. They found that as the amino acid sequence diverges, the structural differences in the regions with the same general fold increase. The root mean square deviation (RMSD) in the positions of the main-chain atoms (Δ) is related to the fraction of mutated residues (H) by the equation Δ(Å) = 0.40 e^1.87H. This relationship is significant because it suggests that the success of predicting a protein's structure from its sequence, using a known homologous protein structure, depends on the extent of sequence identity. Proteins with >50% sequence identity can be modeled closely, while those with <20% identity show large structural differences that are currently difficult to predict. However, active sites of distantly related proteins can have similar geometries due to the coupling of structural changes during evolution.The article by Cyrus Chothia and Arthur M. Lesk explores the relationship between sequence divergence and structural divergence in homologous proteins. They found that as the amino acid sequence diverges, the structural differences in the regions with the same general fold increase. The root mean square deviation (RMSD) in the positions of the main-chain atoms (Δ) is related to the fraction of mutated residues (H) by the equation Δ(Å) = 0.40 e^1.87H. This relationship is significant because it suggests that the success of predicting a protein's structure from its sequence, using a known homologous protein structure, depends on the extent of sequence identity. Proteins with >50% sequence identity can be modeled closely, while those with <20% identity show large structural differences that are currently difficult to predict. However, active sites of distantly related proteins can have similar geometries due to the coupling of structural changes during evolution.