Citric Acid Cycle
개요
Citric Acid Cycle (TCA cycle, Krebs cycle)은 미토콘드리아 matrix에서 Acetyl CoA를 완전히 산화하여 NADH, FADH₂, GTP, CO₂를 생성하는 순환 대사 경로이다.
“The oxidation of acetyl CoA is carried out by the citric acid cycle, also called the Krebs cycle. This cycle produces CO₂ and more important saves a great deal of the bond energy released by this oxidation in the form of NADH.” — Chapter 14
위치: 미토콘드리아 matrix
Figure 14-10: 미토콘드리아에서의 에너지 변환 대사 요약.
8단계 반응 개요
| Step | 효소 | Reactant | Product |
|---|---|---|---|
| 1 | Citrate synthase | Acetyl CoA + OAA | Citrate |
| 2 | Aconitase | Citrate | Isocitrate |
| 3 | Isocitrate dehydrogenase | Isocitrate | α-Ketoglutarate + CO₂ + NADH |
| 4 | α-Ketoglutarate dehydrogenase | α-Ketoglutarate | Succinyl CoA + CO₂ + NADH |
| 5 | Succinyl CoA synthetase | Succinyl CoA | Succinate + GTP |
| 6 | Succinate dehydrogenase | Succinate | Fumarate + FADH₂ |
| 7 | Fumarase | Fumarate | Malate |
| 8 | Malate dehydrogenase | Malate | Oxaloacetate + NADH |
Net Products (Acetyl CoA 1분자, 1 turn 기준)
| 분자 | 수량 |
|---|---|
| NADH | 3 (Step 3, 4, 8) |
| FADH₂ | 1 (Step 6) |
| GTP | 1 (Step 5) |
| CO₂ | 2 |
교과서 Table 14-3:
“2 acetyl CoA → 6 NADH + 2 FADH₂ + 2 GTP” (2 turns for 1 glucose)
특이점: Succinate Dehydrogenase
Step 6의 succinate dehydrogenase는 미토콘드리아 inner membrane에 embedded 되어 있으며, ETC의 Complex II로도 기능한다.
“There is an unusual additional respiratory chain complex that is an important component of the citric acid cycle, where it is known as succinate dehydrogenase.” — Chapter 14
→ Complex II (Succinate dehydrogenase)
조절
| 효소 | 억제 | 활성화 |
|---|---|---|
| Citrate synthase | NADH, Succinyl CoA, Citrate, ATP | |
| Isocitrate dehydrogenase | NADH, ATP | ADP |
| α-KG dehydrogenase | NADH, Succinyl CoA |