Kinetic mechanism of Enterococcus faeciumd-aspartate ligase

Enterococcus faeciumd-aspartate ligase (Asl_fm) is a peptide bond-forming enzyme that is involved in the peptidoglycan assembly pathway. It catalyzes the ATP-dependent ligation of the β-carboxylate of D-Asp to the ε-amino group of L-Lys in the nucleotide precursor UDP- MurNAc-pentapeptide. The enzyme is of interest as a target of new, potential, narrow-spectrum antibiotics directed against multiresistant E. faecium. The kinetic mechanism of Asl_fm has not been fully characterized. To determine it, a progress curve analysis of Asl_fm catalytic process using pyruvate kinase/lactate dehydrogenase ATPase detection assay was performed. With an inspection of the shape of measured progress curves and the results of specific qualitative experiments, the Asl_fm reaction mechanism was singled out. The proposed Asl_fm kinetics reaction scheme was evaluated by fitting the parameters of the corresponding differential equations to progress curves using the computer program ENZO. The complete kinetic analysis result is consistent with the substrate binding order 1) ATP, 2) D-Asp, and 3) UDP-MurNAc-pentapeptide. The analysis suggests that slowly establishing non-productive equilibria between the free and ATP-bound enzyme with the participating pentapeptide are responsible for initial reaction burst followed by a steady-state period before the complete depletion of the reactant added in the lowest concentration.