Fonzé E, Vermeire M, Nguyen-Distèche M, Brasseur R, Charlier P
Centre d'Ingénierie des Protéines, Université de Liège, Institut de Physique, B5, Sart Tilman B-4000, Belgium.
J Biol Chem. 1999 Jul 30;274(31):21853-60. doi: 10.1074/jbc.274.31.21853.
The serine DD-transpeptidase/penicillin-binding protein of Streptomyces K15 catalyzes peptide bond formation in a way that mimics the penicillin-sensitive peptide cross-linking reaction involved in bacterial cell wall peptidoglycan assembly. The Streptomyces K15 enzyme is peculiar in that it can be considered as an intermediate between classical penicillin-binding proteins, for which benzylpenicillin is a very efficient inactivator, and the resistant penicillin-binding proteins that have a low penicillin affinity. With its moderate penicillin sensitivity, the Streptomyces K15 DD-transpeptidase would be helpful in the understanding of the structure-activity relationship of this penicillin-recognizing protein superfamily. The structure of the Streptomyces K15 enzyme has been determined by x-ray crystallography at 2.0-A resolution and refined to an R-factor of 18.6%. The fold adopted by this 262-amino acid polypeptide generates a two-domain structure that is close to those of class A beta-lactamases. However, the Streptomyces K15 enzyme has two particular structural features. It lacks the amino-terminal alpha-helix found in the other penicilloyl-serine transferases, and it exhibits, at its surface, an additional four-stranded beta-sheet. These two characteristics might serve to anchor the enzyme in the plasma membrane. The overall topology of the catalytic pocket of the Streptomyces K15 enzyme is also comparable to that of the class A beta-lactamases, except that the Omega-loop, which bears the essential catalytic Glu(166) residue in the class A beta-lactamases, is entirely modified. This loop adopts a conformation similar to those found in the Streptomyces R61 DD-carboxypeptidase and class C beta-lactamases, with no equivalent acidic residue.
链霉菌K15的丝氨酸DD-转肽酶/青霉素结合蛋白催化肽键形成的方式，类似于细菌细胞壁肽聚糖组装过程中涉及的对青霉素敏感的肽交联反应。链霉菌K15酶的独特之处在于，它可被视为经典青霉素结合蛋白（苄青霉素是其非常有效的失活剂）与低青霉素亲和力的抗性青霉素结合蛋白之间的中间体。凭借其适度的青霉素敏感性，链霉菌K15 DD-转肽酶将有助于理解这个青霉素识别蛋白超家族的构效关系。链霉菌K15酶的结构已通过X射线晶体学以2.0埃分辨率确定，并精修至R因子为18.6%。这条262个氨基酸的多肽所采用的折叠方式产生了一个与A类β-内酰胺酶相似的双结构域结构。然而，链霉菌K15酶有两个特殊的结构特征。它缺少其他青霉素酰丝氨酸转移酶中存在的氨基末端α-螺旋，并且在其表面展示出额外的一个四链β-折叠。这两个特征可能有助于将该酶锚定在质膜中。链霉菌K15酶催化口袋的整体拓扑结构也与A类β-内酰胺酶相似，只是在A类β-内酰胺酶中携带关键催化性谷氨酸（Glu166）残基的Ω-环完全不同。这个环采用了一种与链霉菌R61 DD-羧肽酶和C类β-内酰胺酶中发现情况相似的构象，没有等效的酸性残基。
