辅酶Coenzyme

酶蛋白（脱辅基酶）本身无活性，与辅因子结合成全酶才有活性。辅酶在一个反应中被修饰，在另一个反应中再生，起“穿梭载体”作用：NAD⁺/NADH 和 NADP⁺/NADPH 传递氢负离子；FAD/FADH₂ 传递氢原子；辅酶 A 携带酰基；焦磷酸硫胺素（TPP）参与 α-酮酸脱羧；磷酸吡哆醛参与氨基转移；四氢叶酸转移一碳单位；生物素携带 CO₂。

The protein part alone (apoenzyme) is inactive and becomes active only as the holoenzyme with its cofactor. A coenzyme is modified in one reaction and regenerated in another, acting as a shuttle: NAD⁺/NADH and NADP⁺/NADPH carry hydride, FAD/FADH₂ carries hydrogen atoms, coenzyme A carries acyl groups, thiamine pyrophosphate (TPP) serves α-keto acid decarboxylation, pyridoxal phosphate serves transamination, tetrahydrofolate carries one-carbon units and biotin carries CO₂.

1906 年哈登和扬发现酵母汁经透析后失去发酵能力，加回透析液可恢复，提出存在耐热的“辅酶”。1930 年代瓦尔堡阐明了 NAD 和 FAD 的作用。1945 年李普曼发现辅酶 A，阐明其携带乙酰基的功能，获 1953 年诺贝尔生理学或医学奖。辅酶与维生素的关系由此逐步清晰。

In 1906 Harden and Young found that dialysed yeast juice lost its ability to ferment, which returned when the dialysate was added back, revealing a heat-stable 'coenzyme'. In the 1930s Warburg worked out the roles of NAD and FAD. In 1945 Lipmann discovered coenzyme A and its acetyl-carrying function, sharing the 1953 Nobel Prize in Physiology or Medicine. The link between coenzymes and vitamins gradually became clear.

维生素 B₁ → 焦磷酸硫胺素；B₂ → FMN、FAD；B₃（烟酸）→ NAD⁺、NADP⁺；B₅（泛酸）→ 辅酶 A；B₆ → 磷酸吡哆醛；B₇（生物素）→ 羧化酶辅基；B₉（叶酸）→ 四氢叶酸；B₁₂ → 钴胺素辅酶。因此维生素缺乏会导致相应酶活性下降和代谢障碍。辅酶 Q（泛醌）可在体内合成，不属于维生素。

Vitamin B₁ becomes thiamine pyrophosphate; B₂ becomes FMN and FAD; B₃ (niacin) NAD⁺ and NADP⁺; B₅ (pantothenic acid) coenzyme A; B₆ pyridoxal phosphate; B₇ (biotin) the prosthetic group of carboxylases; B₉ (folate) tetrahydrofolate; and B₁₂ the cobalamin coenzymes. Vitamin deficiencies therefore impair the corresponding enzymes and metabolism. Coenzyme Q (ubiquinone) is made in the body and is not a vitamin.