细菌细胞壁肽聚糖由通过短肽茎交联的聚糖链制成。交联由DD-转肽酶(4,3-交联)和LD-转肽酶(3,3-交联)催化。然而,最近对非模型物种的研究揭示了新的交联类型,表明存在未表征的酶。这里,我们鉴定出一种LD-转肽酶,LDTGo,在乙酸细菌氧化葡糖杆菌中产生1,3-交联。在缺乏LD3,3-转肽酶的α-和β-变形杆菌中发现了LDTGo样蛋白。与典型的LD-和DD-转肽酶的严格特异性相反,LDTGo可以使用非末端氨基酸部分进行交联。与LD3,3-转肽酶相比,LDTGo的高分辨率晶体结构揭示了独特的特征,包括似乎限制底物进入的富含脯氨酸的区域,和一个容纳聚糖链和多肽的腔,源于供体muropeptides。最后,我们表明,DD交联周转参与为LD1,3-转肽化提供必要的底物。这种现象强调了不同的交联机制在维持氧化银中细胞壁完整性方面的相互作用。
The bacterial cell-wall peptidoglycan is made of glycan strands crosslinked by short peptide stems. Crosslinks are catalyzed by DD-transpeptidases (4,3-crosslinks) and LD-transpeptidases (3,3-crosslinks). However, recent research on non-model species has revealed novel crosslink types, suggesting the existence of uncharacterized enzymes. Here, we identify an LD-transpeptidase, LDTGo, that generates 1,3-crosslinks in the acetic-acid bacterium Gluconobacter oxydans. LDTGo-like proteins are found in Alpha- and Betaproteobacteria lacking LD3,3-transpeptidases. In contrast with the strict specificity of typical LD- and DD-transpeptidases, LDTGo can use non-terminal amino acid moieties for crosslinking. A high-resolution crystal structure of LDTGo reveals unique features when compared to LD3,3-transpeptidases, including a proline-rich region that appears to limit substrate access, and a cavity accommodating both glycan chain and peptide stem from donor muropeptides. Finally, we show that DD-crosslink turnover is involved in supplying the necessary substrate for LD1,3-transpeptidation. This phenomenon underscores the interplay between distinct crosslinking mechanisms in maintaining cell wall integrity in G. oxydans.