Inhibition of uridine 5'-diphosphate-N-acetylmuramyl-L-alanine synthetase by beta-chloro-L-alanine in Escherichia coli

Biochemical evidence for the formation of a covalent acyl-phosphate linkage between UDP-N-acetylmuramate and ATP in the Escherichia coli UDP-N-acetylmuramate:L-alanine ligase-catalyzed reaction

In the peptidoglycan biosynthesis pathway in Escherichia coli, UDP-N-acetylmuramate:L-alanine ligase (MurC) catalyzes the formation of UDP-N-acetylmuramyl-L-alanine. A peptide bond is formed in this reaction and an ATP molecule is hydrolyzed concomitantly to produce ADP and orthophosphate. A biochemical approach was devised to elucidate the role of ATP in this reaction. A fusion construct pMAL::murC was prepared and the maltose binding protein--UDP-N-acetylmuramyl:L-alanine ligase fusion protein was overproduced in E. coli/pMal::murC upon isopropyl beta-thiogalactoside induction. The fusion protein was purified to > or = 90% homogeneity by a single-step affinity chromatography. Subsequently, the ligase was released from the maltose binding protein by proteolytic cleavage and was purified to > or = 95% homogeneity by an ion-exchange chromatographic step. The kinetic parameters of the regenerated ligase are comparable to those of the purified native enzyme. This ligase was used to investigate the role that ATP plays in the formation of UDP-N-acetylmuramyl-L-alanine. UDP-N-acetyl[18O]muramate (with 18O located at the carboxylate function only) was prepared by a combination of chemical and enzymatic processes and was used as the substrate of the ligase to probe the reaction mechanism. All reaction products were purified and subjected to liquid chromatographic-mass spectrometric analysis. A single [18O]oxygen was transferred from UDP-N-acetyl[18O]muramate to the orthophosphate produced in the reaction. No [18O]oxygen was detected in the adenosine nucleotides recovered from the reaction. These results strongly suggest that this ligase-catalyzed peptide formation proceeds through an activated acyl-phosphate linkage during the reaction process. ATP therefore assists in the process of the peptide bond formation by donating its gamma-phosphoryl group to activate the carboxyl group of UDP-N-acetylmuramic acid.

Overexpression, purification, and characterization of UDP-N-acetylmuramyl:L-alanine ligase from Escherichia coli

UDP-N-acetylmuramyl:L-alanine ligase from Escherichia coli was overexpressed more than 600-fold and purified to near homogeneity. The purified enzyme was found to ligate L-alanine, L-serine, and glycine, as well as the nonnatural amino acid beta-chloro-L-alanine, to UDP-N-acetylmuramic acid. On the basis of (i) the specificity constants of the enzyme determined for L-alanine, L-serine, and glycine and (ii) the levels of these amino acids in the intracellular pool, it was calculated that the rates of incorporation of L-serine and glycine into peptidoglycan precursor metabolites could maximally amount to 0.1 and 0.5%, respectively, of the rate of L-alanine incorporation.

murH, a new genetic locus in Escherichia coli involved in cell wall peptidoglycan biosynthesis

A temperature-sensitive mutant of Escherichia coli defective in peptidoglycan synthesis was characterized. The incorporation of radiolabeled meso-diaminopimelate into peptidoglycan by the mutant was inhibited at the restrictive growth temperature, resulting in autolysis. The defective step appeared to be part of the terminal stage in peptidoglycan synthesis involving the incorporation of disaccharide peptide units into the wall peptidoglycan. The mutation was assigned to a new locus, designated murH, at 99.2 min on the E. coli linkage map.