This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
In carboxylation of clotting factors by Vitamin K, which amino acid is carboxylated: 
Select and explain. 
A. Glutamate, B. Aspaate, C. Histamine, D. Histidine
A: Glutamate
Answer: A: Glutamate 
Explanation: Glutamate