生物化学教学课件：2 Amino acids

1、1,2.1 结构特征 2.2 种类 2.3 主要化学性质,2 氨基酸/Amino Acids,2,R=“Remainder of the molecule”,结构通式,为什么羧基不写成COOH，氨基不写成NH2？,或,3,Dextrorotary， Levorotary,与甘油醛比较定D/L构型,4,The horizontal bonds project out of the plane of the paper, the vertical behind.,Perspective formulas 透视式,projection formulas 费歇尔(Fischer)投影式,通常使用费歇尔

2、(Fischer)投影式,5,除少数列外，大多数氨基酸只有 -carbon 是手性碳原子。 绝大多数蛋白质中的氨基酸是L-AAs，D-AAs在细菌细胞壁的肽聚糖以及抗菌肽中出现。 氨基酸的D，L 构型与其具体的旋光性（左旋还是右旋？）没有直接对应关系。,小结,6,2.1 结构特征 2.2 种类 2.3 主要化学性质,2 氨基酸/Amino Acids,7,Standard AAs: L-enantomer directly incorporated into a protein,Nonstandard AAs: Derivative from Standard AAs,标准与菲标准氨基酸,8,

3、Trivial (common) name Asparagine from asparagus (1st, in 1806) Glutamate from wheat gluten (sticky) Tyrosine from cheese (“tyros” in Greek),Standard AAs,Three-letter abbreviation and one-letter symbol. They often use the first three letter and the first letter. When there is confusion, an alternativ

4、e is used.,9,10,W,11,Lambert-Beers law: A=Log Io/I = ecl e, extinction coefficient; c, concentration; l, optical length,12,13,Cystine is dimer of cysteine,14,15,16,Nature may yet surprise us with more directly encoded amino acids.,Selenocysteine: encoded by the RNA nucleotide triplet UGA found in Ar

5、chaea, eubacteria and animals including mammals. Pyrrolysine: encoded by the RNA nucleotide triplet UAG, found in Archaea, and eubacteria. Science (2002) vol. 296, 1409; 1459; 1462,Two rare standard AAs,17,Two rare standard AAs,18,in collagen (胶原),in collagen (胶原),in myosin(肌球蛋白),in prothrombin (凝血酶

6、原）,Some in certain proteins (由蛋白质中的某些氨基酸经修饰而成）,Nonstandard AAs (1),19,鸟氨酸,瓜氨酸,Nonstandard AAs (2),About 300 in cells, but not in proteins (e.g. ornithine/ 鸟氨酸, citrulline/瓜氨酸. 一般是蛋白质和氨基酸代谢的中间产物),20,2.1 结构特征 2.2 种类 2.3 主要化学性质,2 氨基酸/Amino Acids,酸碱滴定曲线 特征反应,21,氨基酸的酸碱性,pKa = 2.3,pKa = 4.8,pKa = 9.6,pKa = 10.6,22,A pH at which an AA has equal positive and negative charge-isoelectric point (PI).,滴定曲线与等电点,两个结论： （1）等电点时，氨基酸兼性离子比列达到最大 （2）等电点时，氨基酸离子正电荷之和等于负电荷之和,23,甘氨酸（类）的滴定曲线与等电点,25,谷氨酸（类）的滴定曲线与等电点,27,组氨酸（类）的滴定曲线与等电点,29,找到净电荷为零的兼性离子，将其两侧的pK求和