Question

Vitamin K is involved in the post-translational modification of which amino acid?

• A. Glutamate
• B. Aspartate
• C. Tyrosine
• D. Tryptophan

Hint:

Think gamma-carboxylation and the Gla domains of clotting factors.

Answer 1

The Correct Option is A

Anchor on what vitamin K chemically does: it is the cofactor that lets gamma-glutamyl carboxylase add an extra carboxyl group to selected amino acid side chains in clotting-related proteins. The only residue that receives this gamma-carboxylation is glutamate, which is thereby turned into gamma-carboxyglutamate. The functional payoff is a calcium-binding site, because the twin carboxyl groups grip calcium ions and let factors II, VII, IX, and X plus proteins C and S dock onto phospholipid membranes during coagulation. That is why warfarin, by blocking vitamin K recycling, produces under-carboxylated, non-functional clotting factors. Aspartate, tyrosine, and tryptophan are not the substrates for this particular reaction, so they are excluded.

\[\boxed{\text{Glutamate}}\]