The metabolism of "surplus" amino acids
The metabolism of "surplus" amino acids
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DOI：
10.1017/s0007114512002292
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发表时间：
2012-08-01
影响因子：
3.6
通讯作者：
Bender, David A.
中科院分区：
文献类型：
--
作者：
Bender, David A.
对于氮平衡的成年人来说，除了合成神经递质、激素等所需的少量氨基酸外，几乎等量于从饮食中吸收的氨基酸可以被认为是“过剩的”，因为它将被分解代谢。饮食诱导的蛋白质产热高于碳水化合物或脂肪，通常被认为是由于蛋白质合成增加，这是ATP昂贵的。除此之外，还必须增加通过泛素-蛋白酶体途径进行蛋白质分解代谢的ATP成本。氨基酸的分解代谢会增加产热作用。除使用丝氨酸和苏氨酸脱氨酶外，脱氨可导致净ATP的形成，但在肝外组织中合成谷氨酰胺是有能量成本的。当富马酸代谢产生的三磷酸腺苷的产量与尿素循环的三磷酸腺苷成本相抵时，合成尿素的净成本只有1.5倍三磷酸腺苷，但这种抵消是产热的。在禁食和低碳水化合物饮食中，尽可能多的氨基酸碳将用于糖异生--这是一种昂贵的ATP，因此也是一种产热过程。大多数氨基酸碳骨架的完全氧化还包括一些生热步骤，其中利用了ATP(或GTP)或还原辅酶。在丙酮酸、乙酰辅酶A或乙酰乙酸酯的代谢中没有这样的生热步骤，但对于通过柠檬酸循环中间体代谢的氨基酸，存在从每摩尔1到7倍ATP当量的生热。
For an adult in N balance, apart from small amounts of amino acids required for the synthesis of neurotransmitters, hormones, etc, an amount of amino acids almost equal to that absorbed from the diet can be considered to be "surplus" in that it will be catabolized. The higher diet-induced thermogenesis from protein than from carbohydrate or fat has generally been assumed to be due to increased protein synthesis, which is ATP expensive. To this must be added the ATP cost of protein catabolism through the ubiquitin-proteasome pathway. Amino acid catabolism will add to thermogenesis. Deamination results in net ATP formation except when serine and threonine deaminases are used, but there is the energy cost of synthesizing glutamine in extra-hepatic tissues. The synthesis of urea has a net cost of only 1.5 x ATP when the ATP yield from fumarate metabolism is offset against the ATP cost of the urea cycle, but this offset is thermogenic. In fasting and on a low carbohydrate diet as much of the amino acid carbon as possible will be used for gluconeogenesis - an ATP-expensive, and hence thermogenic, process. Complete oxidation of most amino acid carbon skeletons also involves a number of thermogenic steps in which ATP (or GTP) or reduced coenzymes are utilized. There are no such thermogenic steps in the metabolism of pyruvate, acetyl CoA or acetoacetate, but for amino acids that are metabolized by way of the citric acid cycle intermediates there is thermogenesis ranging from 1 up to 7 x ATP equivalent per mol.