Christensen H N
Biochim Biophys Acta. 1984 Sep 3;779(3):255-69. doi: 10.1016/0304-4157(84)90012-1.
The amino acids are ions of various charge combinations, and one can argue that historically they were the first ions for which the ongoing problem of membrane transport was presented; also that among transported ions these may undergo a highly detailed molecular recognition. Furthermore, the distribution of charge on the amino acid molecule determines by what route or routes it is conducted across the biological membrane, with what directional and structural specificity, and therefore what regulation is imposed, and where. Cases where a presumably charged chemical group behaves as if it were somehow absent from the amino acid have been observed to fall into several categories: Straightforward cases where the pH has been low enough or high enough to remove the charge by protonation or deprotonation, even in free solution. Cases where that protonation or deprotonation is facilitated at the binding site, and perhaps by the total transport process. The cystine molecule can apparently thus be rendered either a tripolar anion or a tripolar cation for transport. Cases where an otherwise co-transported Na+ is omitted to redress charge, or where a Na+ serves as a surrogate for a missing charged group on the amino acid molecule. A case where the protonation occurs reversibly at the receptor site rather than on the amino acid molecule.
氨基酸是具有各种电荷组合的离子，可以说从历史角度来看，它们是最早面临膜运输这一持续问题的离子；而且在被运输的离子中，它们可能会经历高度详细的分子识别。此外，氨基酸分子上电荷的分布决定了它通过何种途径穿过生物膜，具有何种方向和结构特异性，进而决定了施加何种调节以及调节的位置。据观察，推测带电的化学基团表现得好像在氨基酸中不存在的情况可分为几类：在pH值足够低或足够高以至于即使在游离溶液中也能通过质子化或去质子化去除电荷的简单情况。在结合位点以及可能通过整个运输过程促进质子化或去质子化的情况。胱氨酸分子显然因此可以成为三极阴离子或三极阳离子用于运输。在其他情况下，共运输的Na⁺被省略以补偿电荷，或者Na⁺替代氨基酸分子上缺失的带电基团。在受体位点而不是在氨基酸分子上发生可逆质子化的情况。
