In carboxylation of clotting factors by Vitamin K, which amino acid is carboxylated:
**Core Concept**
The carboxylation of clotting factors by Vitamin K is a critical post-translational modification that enables the binding of calcium ions (Ca2+) to these proteins, facilitating their interaction with phospholipid surfaces and subsequent blood coagulation. This process involves the conversion of glutamate residues to gamma-carboxyglutamate (Gla) residues.
**Why the Correct Answer is Right**
The correct answer is **B. Glutamate**. In this reaction, Vitamin K epoxide acts as a co-factor, allowing the enzyme gamma-glutamyl carboxylase to catalyze the carboxylation of glutamate residues. This process is essential for the activation of clotting factors II, VII, IX, and X, as well as proteins C and S. The carboxylation of glutamate residues to Gla residues creates a negatively charged site that is critical for the binding of calcium ions and subsequent blood coagulation.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because aspartate is not the amino acid carboxylated by Vitamin K. While aspartate is a negatively charged amino acid, it is not the target of this reaction.
* **Option C:** This option is incorrect because serine is not directly involved in the Vitamin K-dependent carboxylation reaction. Serine is involved in other post-translational modifications, such as phosphorylation.
**Clinical Pearl / High-Yield Fact**
Warfarin, an oral anticoagulant, inhibits Vitamin K epoxide reductase, leading to a decrease in the levels of active clotting factors and an increased risk of bleeding. This highlights the importance of Vitamin K-dependent carboxylation in blood coagulation and the mechanism of action of warfarin.
**Correct Answer: B. Glutamate**