氨基酸Amino acid

氨基呈碱性、羧基呈酸性，氨基酸既能与酸反应又能与碱反应，是两性化合物。在固态和中性水溶液中，羧基的 H⁺ 转移到氨基上，形成内盐 ⁺H₃N–CHR–COO⁻，称为两性离子，因此氨基酸熔点高、易溶于水。溶液中正负电荷恰好相等时的 pH 称为等电点，此时氨基酸在电场中不移动。

The amino group is basic and the carboxyl group acidic, so amino acids react with both acids and bases: they are amphoteric. In the solid and in neutral solution, H⁺ moves from the carboxyl to the amino group, giving the internal salt ⁺H₃N–CHR–COO⁻, a zwitterion; this is why amino acids have high melting points and dissolve in water. The pH at which positive and negative charges balance is the isoelectric point, where the amino acid does not move in an electric field.

除甘氨酸（R = H）外，α-碳连有四个不同基团，是手性中心，存在一对对映异构体。构成天然蛋白质的氨基酸几乎都是 L 型。两种对映体的化学性质相同，但旋光方向相反，在生物体内的作用可能完全不同。

Except for glycine (R = H), the α-carbon carries four different groups and is a chiral centre, so each amino acid exists as a pair of enantiomers. The amino acids in natural proteins are almost all of the L form. The enantiomers have the same chemical properties but rotate plane-polarised light in opposite directions, and can behave quite differently in living things.

一个氨基酸的羧基与另一个氨基酸的氨基脱水缩合，形成酰胺键 –CO–NH–，在生物化学中称为肽键。两个氨基酸缩合得到二肽，许多氨基酸缩合得到多肽，多肽链折叠成具有特定空间结构的蛋白质。蛋白质在酸、碱或酶作用下水解，又可得到氨基酸。

The carboxyl group of one amino acid condenses with the amino group of another, losing water to form an amide link –CO–NH–, called a peptide bond in biochemistry. Two amino acids give a dipeptide and many give a polypeptide, which folds into a protein with a specific three-dimensional structure. Hydrolysis by acid, alkali or enzymes breaks proteins back down into amino acids.

人体不能合成、必须从食物中获得的氨基酸称为必需氨基酸，成人有 9 种：组氨酸、异亮氨酸、亮氨酸、赖氨酸、甲硫氨酸、苯丙氨酸、苏氨酸、色氨酸和缬氨酸。

Amino acids the human body cannot make and must obtain from food are called essential amino acids; for adults there are nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.