Protein structural classification primarily relies on motifs, which are recurring 3D arrangements defining folds and functions. The correct answer to Question 29 is (D) Motif.

Correct Answer

Option (D) Motif is correct. Major databases like SCOP and CATH classify proteins hierarchically by structural motifs—recurring supersecondary structures (e.g., helix-turn-helix, beta-hairpin) that define folds, topologies, and evolutionary families beyond mere sequence or composition.

Option Explanations

• (A) Length: Incorrect as primary criterion; long/short proteins can share folds (e.g., multi-domain vs single-domain). Length influences stability but not SCOP/CATH class/family assignment.​

• (B) Genus: Refers to taxonomic grouping (e.g., bacterial vs eukaryotic proteins), not structural basis. Structural classification is sequence-independent, focusing on 3D topology.​

• (C) Composition: Amino acid content (e.g., % hydrophobic) aids prediction but isn’t primary; analogous structures arise from different compositions via convergent evolution.​

• (D) Motif: Correct; motifs (e.g., EF-hand in calcium-binding) form hierarchical levels: class (secondary structure ratio), architecture, topology/fold, superfamily in CATH/SCOP.

Classification Comparison