複選題

36. The enzymes of fatty acid oxidation are located in the mitochondria matrix, so free fatty acids must be transported through the shuttle into the mitochondria. Which of the following statements regarding the shuttle is correct?
(A) Free fatty acid must be activated by a fatty acyl-CoA synthetase, through the hydrolysis of one high-energy bond in ATP, to yield a fatty acyl-CoA before entering the shuttle.
(B) The enzyme responsible for esterifying carnitine with acyl-CoA in both inner and outer mitochondria membranes is the rate-limiting step for oxidation.
(C) Coenzyme A in the cytosol is largely used in the oxidative degradation of fatty acids.
(D) This shuttle plays a crucial role in transporting medium-chain fatty acids into the mitochondria for beta-oxidation.
(E) Fatty acyl-CoA destined for mitochondria oxidation must be attached to carnitine to be shuttled across the inner mitochondria membrane.

? 先給你一個正確的流程框架

長鏈脂肪酸要進粒線體 matrix 做 β-oxidation，流程是：

1. FA → fatty acyl-CoA（在細胞質/外膜）
   用 acyl-CoA synthetase
   ATP → AMP + PPi（= 用掉 兩個高能鍵）

2. fatty acyl-CoA + carnitine → acyl-carnitine
   用 CPT-I（outer mitochondrial membrane）

3. acyl-carnitine 被 translocase 送進 matrix

4. acyl-carnitine → fatty acyl-CoA + carnitine
   用 CPT-II（inner membrane side）

只有這樣，acyl group 才能進到 matrix。

選項逐一判斷

(A) ❌ 錯

through the hydrolysis of one high-energy bond in ATP

錯在這一句。
Acyl-CoA synthetase 用的是
ATP → AMP + PPi → 2 Pi
= 消耗兩個高能磷酸鍵，不是一個。

這是考試最愛埋的地雷。

(B) ❌ 錯

enzyme responsible for esterifying carnitine in both inner and outer membrane is rate-limiting

首先：

• 外膜是 CPT-I

• 內膜是 CPT-II
  不是同一個 enzyme。

其次：

• β-oxidation 的速率限制步驟是 CPT-I
  不是整個「兩側都在酯化 carnitine 的酵素」。

敘述錯得很精緻。

(C) ❌ 錯

CoA in cytosol is largely used in oxidative degradation

錯。
β-oxidation 發生在 mitochondrial matrix，
用的是 mitochondrial CoA pool，
不是 cytosolic CoA。

(D) ❌ 錯

transporting medium-chain fatty acids

Medium-chain (C8–C12) 可以 直接進粒線體，
不需要 carnitine shuttle。

Carnitine shuttle是為了
? long-chain fatty acids (≥C14)。

(E) ✅ 正確

Fatty acyl-CoA destined for mitochondria oxidation must be attached to carnitine to be shuttled across the inner mitochondrial membrane.

完全正確。

內膜 對 acyl-CoA 不通透，
只能讓 acyl-carnitine 通過 translocase。

所以：

要進去 matrix 做 β-oxidation
→ 一定要先變成 acyl-carnitine

? 結論

正確答案： (E)

這題本質上是在問：

「誰可以穿過 inner mitochondrial membrane？」

答案不是 acyl-CoA，而是 acyl-carnitine。