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在生物化学的分析与研究中,选择性切割多肽和蛋白质是一种非常有用的手段。除了蛋白酶以外,溴化氰、羟胺等试剂也常用来进行切割。近来发现Pd(Ⅱ)的配合物对含有半胱氨酸(Cys)或蛋氨酸残基(Met)的多肽的选择性水解非常有效,它可以促进与半胱氨酸或蛋氨酸残基相邻的羧基端肽键水解。但最近作者之一,在用Pd(Ⅱ)的配合物选择性切割马心细胞色素c(Cyt c)的过程中观察到,水解位置与以前有所不同。在细胞色素c中,有两个半胱氨酸残基(Cys14和Cys17)和两个蛋氨酸残基(Met65和Met80),而细胞色素c却专一地被切断在His18-Thr19处。不但Cys14-Ala15-Gln16和两个蛋氨酸残基羧端处的酰胺键未被水解,就连Cys17-His18的酰胺键也未水解。这说明在蛋白质大分子中,仅有Cys或Met残基不足以被Pd(Ⅱ)配合物水解,与Cys17相邻的咪唑环N3质子化的组氨酸残基(His18)也许起了重要的作用(在pH=2时,His18咪唑环的N3已从血红素上解离并接受一质子)。我们利用AM1半经验分子轨道方法来计算N-乙酰化二肽N-Ac-His-Gly正离子(以[AcHis-Gly]~+表示)的构象,期望能有助于了解组氨酸残基在切割过程中所起的作用。
In biochemical analysis and research, selective cleavage of peptides and proteins is a very useful tool. In addition to proteases, reagents such as cyanogen bromide and hydroxylamine are also commonly used for cutting. It has recently been found that complexes of Pd (II) are very effective for the selective hydrolysis of polypeptides containing cysteine (Cys) or methionine residues (Met) which promote the carboxyl groups adjacent to cysteine or methionine residues Peptide bond hydrolysis. However, one of the recent authors observed during the selective cleavage of horse heart cytochrome c (Cyt c) with a complex of Pd (II) that the hydrolysis site was different from before. In cytochrome c, there are two cysteine residues (Cys14 and Cys17) and two methionine residues (Met65 and Met80), whereas cytochrome c is exclusively cleaved at His18-Thr19. Not only did the amide bond at the Cysl4-Alal5-Glnl6 and the carboxy terminal end of both methionine residues were not hydrolyzed, nor was the amide bond of Cysl 7-Hisl8 hydrolyzed. This suggests that among protein macromolecules, only histidine residues (His18), protonated with the imidazole ring N3 adjacent to Cys17, may not be sufficient for hydrolysis of the Cys or Met residues by the Pd (II) complex may play an important role (At pH = 2 N3 of His18 imidazole ring has dissociated from heme and received a proton). We calculated the conformation of the N-acetylated dipeptide N-Ac-His-Gly cations (expressed as [AcHis-Gly] ~ +) using the AM1 semiempirical molecular orbital method in hopes of understanding the histidine residues The role of the cutting process.