9. The amino acid with a side-chain pKa near neutrality and which therefore plays an important role as proton donor and acceptor in many enzyme catalyzed reactions is:
(A)histidine.
(B)cysteine.
(C)proline.
(D)serine.
(E)methionine.
【hsiao1997】評論
側鏈pKa接近中性，在許多酶催化反應中作為質子供體和受體發揮重要作用的氨基酸是：(A)histidine