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首页> 外文期刊>Biochemistry >On the mechanism of peptidoglycan binding and cleavage by the endo-specific lytic transglycosylase MltE from Escherichia coli
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On the mechanism of peptidoglycan binding and cleavage by the endo-specific lytic transglycosylase MltE from Escherichia coli

机译：大肠杆菌内切裂解糖基糖基化酶MltE结合肽聚糖的机理研究

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摘要

The lytic transglycosylase MltE from Escherichia coli is a periplasmic, outer membrane-attached enzyme that cleaves the β-1,4-glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine residues in the cell wall peptidoglycan, producing 1,6-anhydromuropeptides. Here we report three crystal structures of MltE: in a substrate-free state, in a binary complex with chitopentaose, and in a ternary complex with the glycopeptide inhibitor bulgecin A and the murodipeptide N-acetylglucosaminyl-N-acetylmuramyl-l-Ala-d-Glu. The substrate-bound structures allowed a detailed analysis of the saccharide-binding interactions in six subsites of the peptidoglycan-binding groove (subsites -4 to +2) and, combined with site-directed mutagenesis analysis, confirmed the role of Glu64 as catalytic acid/base. The structures permitted the precise modeling of a short glycan strand of eight saccharide residues, providing evidence for two additional subsites (+3 and +4) and revealing the productive conformational state of the substrate at subsites -1 and +1, where the glycosidic bond is cleaved. Full accessibility of the peptidoglycan-binding groove and preferential binding of an N-acetylmuramic acid residue in a ~4C _1 chair conformation at subsite +2 explain why MltE shows only endo- and no exo-specific activity toward glycan strands. The results further indicate that catalysis of glycosidic bond cleavage by MltE proceeds via distortion toward a sofa-like conformation of the N-acetylmuramic acid sugar ring at subsite -1 and by anchimeric assistance of the sugar's N-acetyl group, as shown previously for the lytic transglycosylases Slt70 and MltB.
机译：来自大肠杆菌的裂解性转糖基化酶MltE是一种周质的，外膜附着的酶，可裂解细胞壁肽聚糖中N-乙酰基尿酸和N-乙酰基葡糖胺残基之间的β-1,4-糖苷键，从而产生1,6-脱水多肽。在这里，我们报告MltE的三个晶体结构：在无底物的状态下，与壳五糖的二元复合物，以及与糖肽抑制剂bulgecin A和多肽肽N-乙酰葡糖胺基-N-乙酰村酰胺-1-Ala-d的三元复合物。 -胶底物结合结构允许对肽聚糖结合槽的六个亚位点（亚位点-4至+2）中的糖结合相互作用进行详细分析，并结合定点诱变分析，证实了Glu64作为催化酸的作用。 /基础。该结构允许对8个糖残基的短聚糖链进行精确建模，为另外两个亚位点（+3和+4）提供证据，并揭示底物在亚位点-1和+1的糖基键的生产构象状态被劈开。肽聚糖结合槽的完全可及性和亚位点+2处〜4C _1椅子构象中N-乙酰基尿酸残基的优先结合说明了为什么MltE仅对糖链显示内切而不对胞外特异性活性。结果进一步表明，MltE催化的糖苷键裂解过程是通过向亚位点-1处的N-乙酰基尿酸糖环的沙发样构象变形以及糖的N-乙酰基的邻氨基苯甲酸辅助进行的，如前所述。裂解转糖基酶Slt70和MltB。

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