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首页> 外文期刊>Biochemistry >The Catalytically Active Serine Protease Domain of Human Complement Factor I
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The Catalytically Active Serine Protease Domain of Human Complement Factor I

机译：人类补体因子I的催化活性丝氨酸蛋白酶结构域

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摘要

Factor I (fI) is a major regulator of complement.As a protease it has very restricted specificity,cleaving only C3b or C4b in the presence of a cofactor such as factor H (fH).Cleavage of C3b by ft yields iC3b,a major opsonin.The cleavage occurs through the formation of a ternary complex between the enzyme,the substrate,and the cofactor.The catalytic subunit of fI,the SP domain,accommodates substrate recognition and cleavage.The role of the fI heavy chain within the catalysis complex is unknown.Using partial proteolysis and affinity chromatography an intact form of the SP domain was generated and isolated from fl in high yield,fl and the SP domain were found to have similar amidolytic activities but strikingly different proteolytic activities on C3(NH_3).fI did not cleave C3(NH3) in the absence of fH,while in its presence it cleaved C3(NH3) rapidly at two sites.The SP domain,however,slowly cleaved C3(NH_3) in the absence of fH,at more than two sites.Cleavage by the SP domain was inhibited,not stimulated,by fH.Pefabloc SC and antipain inhibited the proteolytic activity of both fI and the SP domain,but suramin inhibited only fI and not the SP domain.The contrast in the proteolytic activities suggests that the heavy chain domains and the cofactor must have roles in orienting the natural substrates and restricting cleavage to the two sites which yield iC3b through a highly specific catalysis.
机译：因子I（fI）是补体的主要调节剂。作为蛋白酶，它具有非常有限的特异性，在辅因子H（fH）的存在下仅裂解C3b或C4b.ft裂解C3b会产生iC3b。裂解是通过在酶，底物和辅因子之间形成三元复合物而发生的.fI的催化亚基，SP结构域可适应底物的识别和裂解.fI重链在催化复合物中的作用使用部分蛋白水解和亲和色谱法可生成完整形式的SP结构域，并以高收率从fl中分离出来，发现fl和SP结构域对C3（NH_3）具有相似的酰胺分解活性，但蛋白水解活性却截然不同。在不存在fH的情况下，它不会切割C3（NH3），而在存在fH的情况下，它在两个位点上迅速切割C3（NH3）。但是，在不存在fH的情况下，SP域在两个以上的位置上缓慢地切割C3（NH_3）。 SP域的切割被抑制。 Pefabloc SC和抗痛药抑制fI和SP结构域的蛋白水解活性，但苏拉明仅抑制fI而不抑制SP结构域。蛋白水解活性的对比表明重链结构域和辅因子必须具有定向天然底物的作用，并限制通过高特异性催化产生iC3b的两个位点的裂解。

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