Mechanistic studies on N-acetylmuramic acid 6-phosphate hydrolase (MurQ): an etherase involved in peptidoglycan recycling.
Peptidoglycan recycling is a process in which bacteria import cell wall degradation products and incorporate them back into either peptidoglycan biosynthesis or basic metabolic pathways. The enzyme MurQ is an N-acetylmuramic acid 6-phosphate (MurNAc 6-phosphate) hydrolase (or etherase) that hydrolyzes the lactyl side chain from MurNAc 6-phosphate and generates GlcNAc 6-phosphate. This study supports a mechanism involving the syn elimination of lactate to give an alpha,beta-unsaturated aldehyde with (E)-stereochemistry, followed by the syn addition of water to give product. The observation of both a kinetic isotope effect slowing the reaction of [2-(2)H]MurNAc 6-phosphate and the incorporation of solvent-derived deuterium into C2 of the product indicates that the C2-H bond is cleaved during catalysis. The observation that the solvent-derived (18)O isotope is incorporated into the C3 position of the product, but not the C1 position, provides evidence of the cleavage of the C3-O bond and argues against imine formation. The finding that 3-chloro-3-deoxy-GlcNAc 6-phosphate serves as an alternate substrate is also consistent with an elimination-addition mechanism. Upon extended incubations of MurQ with GlcNAc 6-phosphate, the alpha,beta-unsaturated aldehydic intermediate accumulates in solution, and (1)H NMR analysis indicates it exists as the ring-closed form of the (E)-alkene. A structural model is developed for the Escherichia coli MurQ and is compared to that of the structural homologue glucosamine-6-phosphate synthase. Putative active site acid/base residues are probed by mutagenesis, and Glu83 and Glu114 are found to be crucial for catalysis. The Glu83Ala mutant is essentially inactive as an etherase yet is capable of exchanging the C2 proton of substrate with solvent-derived deuterium. This suggests that Glu83 may function as the acidic residue that protonates the departing lactate.
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
Timin
T
Hadi
Ulrike
U
Dahl
Christoph
C
Mayer
Martin E
ME
Tanner
D000595:Amino Acid Sequence; D001483:Base Sequence; D020134:Catalytic Domain / Q000235:genetics; D017931:DNA Primers / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008954:Models, Biological; D008969:Molecular Sequence Data; D009112:Muramic Acids / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D010457:Peptidoglycan / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
D017931:DNA Primers; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D009112:Muramic Acids; D010457:Peptidoglycan; D011994:Recombinant Proteins; D006026:Glycoside Hydrolases; C504247:MurQ protein, E coli