N-acetylmuramic acid 6-phosphate lyases (MurNAc etherases): role in cell wall metabolism, distribution, structure, and mechanism

MurNAc etherases cleave the uniqued-lactyl ether bond of the bacterial cell wall sugar N-acetylmuramic acid (MurNAc). Members of this newly discovered family of enzymes are widely distributed among bacteria and are required to utilize peptidoglycan fragments obtained either from the environment or from the endogenous cell wall (i.e., recycling). MurNAc etherases are strictly dependent on the substrate MurNAc possessing a free reducing end and a phosphoryl group at C6. They carry a single conserved sugar phosphate isomerase/sugar phosphate- binding (SIS) domain to which MurNAc 6-phosphate is bound. Two subunits form an enzymatically active homodimer that structurally resembles the isomerase module of the double-SIS domain protein GlmS, the glucosamine 6-phosphate synthase. Structural comparison provides insights into the two-step lyase-type reaction mechanism of MurNAc etherases: β-elimination of the D-lactic acid substituent proceeds through a 2,3-unsaturated sugar intermediate to which water is subsequently added. Received 31 August 2007; received after revision 12 October 2007; accepted 1 November 2007     

乳酸杆菌通过定植和提供免疫信号调节宿主免疫反应

提取了Lactobacillussp.的肽聚糖,用AffymetrixMOE430A基因芯片研究了肽聚糖对机体免疫细胞基因表达的影响。结果发现,乳酸杆菌肽聚糖可以刺激免疫细胞Th1型免疫反应相关基因的表达,可能激活TLR-NF-κB相关信号通路,同时,不同剂量的肽聚糖可能通过调节Tollip的表达,影响TLR-NF-κB信号通路的激活,从而调节细胞因子的表达。     

The peptidoglycan recognition proteins LCa and LCx

Infection of bacteria triggers innate immune defense reactions in Drosophila. So far, the only bacterial component known to be recognized by the insect innate immune system is peptidoglycan, one of the most abundant constituents of the bacterial cell wall. Insects use peptidoglycan recognition proteins to detect peptidoglycan and to activate innate immune responses. Such specialized peptidoglycan receptors appear to have evolved from phage enzymes that hydrolyze bacterial cell walls. They are able to bind specific peptidoglycan molecules with distinct chemical moieties and activate innate immune pathways by interacting with other signaling proteins. Recent X-ray crystallographic studies of the peptidoglycan recognition proteins LCa, and LCx bound to peptidoglycan have provided structural insights into recognition of peptidoglycan and activation of innate immunity in insects. Received 28 December 2006; received after revision 2 February 2007; accepted 21 February 2007     

<Emphasis Type="Italic">Drosophila melanogaster</Emphasis> innate immunity: an emerging role for peptidoglycan recognition proteins in bacteria detection

Over the past years, parallel studies conducted in mammals and flies have emphasized the existence of common mechanisms regulating the vertebrate and invertebrate innate immune systems. This culminated in the discovery of the central role of the Toll pathway in Drosophila immunity and in the implication of Toll-like receptors (TLRs)/interleukin-1(IL-1) in the mammalian innate immune response. In spite of clear similarities, such as shared intracellular pathway components, important divergences are expected between the two groups, whose last common ancestor lived more than half a billion years ago. The most obvious discrepancies lie in the mode of activation of the signalling receptors by microorganisms. In mammals, TLRs are part of protein complexes which directly recognize microbe-associated patterns, whereas Drosophila Toll functions like a classical cytokine receptor rather than a pattern recognition receptor. Recent studies demonstrate that members of the evolutionarily conserved peptidoglycan recognition protein family play an essential role in microbial sensing during immune response of Drosophila.Received 26 June 2003; received after revision 29 July 2003; accepted 25 August 2003     

细菌肽聚精的研究

本文介绍了肽聚糖的化学组成、结构及其生物合成，并讨论了抗生素对肽聚糖合成的抑制作用．     

The 2.4-A crystal structure of the penicillin-resistant penicillin-binding protein PBP5fm from Enterococcus faecium in complex with benzylpenicillin

Penicillin-binding proteins (PBPs) are membrane proteins involved in the final stages of peptidoglycan synthesis and represent the targets of beta-lactam antibiotics. Enterococci are naturally resistant to these antibiotics because they produce a PBP, named PBP5fm in Enterococcus faecium, with low-level affinity for beta-lactams. We report here the crystal structure of the acyl-enzyme complex of PBP5fm with benzylpenicillin at a resolution of 2.4 A. A characteristic of the active site, which distinguishes PBP5fm from other PBPs of known structure, is the topology of the loop 451-465 defining the left edge of the cavity. The residue Arg464, involved in a salt bridge with the residue Asp481, confers a greater rigidity to the PBP5fm active site. In addition, the presence of the Val465 residue, which points into the active site, reducing its accessibility, could account for the low affinity of PBP5fm for beta-lactam. This loop is common to PBPs of low affinity, such as PBP2a from Staphylococcus aureus and PBP3 from Bacillus subtilis. Moreover, the insertion of a serine after residue 466 in the most resistant strains underlines even more the determining role of this loop in the recognition of the substrates.     

细菌细胞壁肽聚糖的研究

肽聚糖是真细菌细胞壁中特有的成分,因其在细菌细胞壁的生理功能中发挥着主要作用,而成为目前细菌细胞壁方面研究的主题.介绍了细菌细胞壁肽聚糖的化学组成、结构特点、分类、分离纯化、生物合成、生物学功能及应用前景等.     

植物乳杆菌细胞壁肽聚糖的分离及鉴定

采用三氯乙酸法提取了植物乳杆菌细胞壁肽聚糖,对提取物进行了成分分析和结构验证.结果表明:经氨基酸组分分析,溶菌酶实验,糖类、脂肪、蛋白质含量测定证明了该物质主要成分确系肽聚糖,其得率为(15.29±1.28)%,其中总糖含量为(39.10±1.95)%,脂肪含量为(1.41±0.04)%,蛋白质含量为(58.00±2....     

革兰氏染色原理

从染色剂三苯甲烷特有的分子结构、三苯甲烷染色剂在不同pH溶液中有不同的电离状态,当pH>3．2溶液时它将电离为带一个正电荷的离子的特质;以及不同细菌细胞壁的物质组成差异,构成细胞壁特征性物质——肽聚糖分子组成和空间结构存在的差异,不同细菌酸式解离以及等电点的差异;由此引发染色剂与细菌之间相互作用的角度论述了革兰氏染色的原理。使该理论更加完善,对相应操作和结果分析更具指导意义。