Q.27 In zinc finger proteins, the amino acid residues that coordinate zinc are
(A) Cys and His (B) Asp and Glu
(C) Arg and Lys (D) Asp and Arg
Zinc finger proteins feature a structural motif where zinc ions stabilize DNA-binding domains through specific amino acid coordination. The correct answer is option (A) Cys and His, forming the classic C2H2 configuration.
Zinc Finger Structure Overview
Zinc fingers are compact domains of about 30 amino acids that bind zinc via tetrahedral coordination, enabling DNA or RNA recognition. The classical zinc finger, identified in TFIIIA, uses two cysteines and two histidines (Cys2His2 or C2H2) to chelate Zn2+, forming a finger-like loop for target binding.
Correct Answer: Option A
Option (A) Cys and His accurately describes the residues coordinating zinc in classical zinc fingers. Cysteines provide thiolate sulfur (S-) ligands, while histidines offer imidazole nitrogen (N) ligands, creating a stable Zn(S-)2(N)2 geometry essential for folding.
Option Analysis
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Option A (Cys and His): Standard C2H2 motif; two Cys and two His residues anchor zinc, as confirmed by X-ray studies and sequencing of proteins like Zif268.
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Option B (Asp and Glu): Asp and Glu coordinate via carboxylate oxygens in other motifs like zinc ribbons or treble-clef fingers, but not classical zinc fingers.
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Option C (Arg and Lys): Positively charged Arg and Lys bind DNA phosphates electrostatically but do not chelate zinc due to lacking suitable donor atoms.
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Option D (Asp and Arg): Asp can coordinate metals in some zinc sites, but Arg does not; this pair lacks the specificity of Cys-His for zinc fingers.
Biological Importance
These motifs regulate transcription, with over 700 human C2H2 zinc fingers influencing gene expression. Mutations disrupting coordination impair DNA binding, linking to diseases like cancer.
