Brennan T V, Clarke S
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024-1569.
Biochem Biophys Res Commun. 1993 Jun 30;193(3):1031-7. doi: 10.1006/bbrc.1993.1728.
The maturation of the lectin concanavalin A from the jack bean, Canavalia ensiformis, involves an unusual post-translational cleavage at three internal asparagine residues and the subsequent rearrangement and ligation of two of the resulting fragments. It is presently unclear whether these reactions are enzymatically catalyzed or occur autocatalytically. A specific model for non-enzymatic cleavage has been proposed where the attack of the side chain amide nitrogen atom of asparagine on its alpha-carbonyl carbon cleaves the peptide-bond and leaves a C-terminal succinimide residue. Spontaneous hydrolysis of the succinimide would result in the formation of both terminal asparagine and isoasparagine (aspartic acid alpha-amide) residues. We tested this model by chemically analyzing the C-terminal tryptic peptide of mature concanavalin A that contains the cleavage site residue Asn-148 as its C-terminal residue. We found that only free asparagine was released from this peptide with either elastase or leucine aminopeptidase digestion under conditions that released isoasparagine from a similar synthetic peptide. These results suggest that precursor processing in concanavalin A does not in fact follow a succinimide pathway, although other types of autocatalytic mechanisms remain possible.
刀豆球蛋白A（伴刀豆球蛋白A）来源于刀豆（Canavalia ensiformis），其成熟过程涉及在三个内部天冬酰胺残基处发生异常的翻译后切割，以及随后两个产生的片段的重排和连接。目前尚不清楚这些反应是由酶催化还是自动催化发生。有人提出了一种非酶促切割的特定模型，其中天冬酰胺的侧链酰胺氮原子对其α-羰基碳的攻击会切断肽键，并留下一个C端琥珀酰亚胺残基。琥珀酰亚胺的自发水解会导致末端天冬酰胺和异天冬酰胺（天冬氨酸α-酰胺）残基的形成。我们通过化学分析成熟伴刀豆球蛋白A的C端胰蛋白酶肽来测试该模型，该肽含有切割位点残基Asn-148作为其C端残基。我们发现，在从类似合成肽中释放异天冬酰胺的条件下，用弹性蛋白酶或亮氨酸氨肽酶消化该肽时，只有游离天冬酰胺被释放出来。这些结果表明，伴刀豆球蛋白A中的前体加工实际上并不遵循琥珀酰亚胺途径，尽管其他类型的自动催化机制仍然是可能的。
