Role of the amino acid invariants in the active site of MurG as evaluated by site-directed mutagenesis

To evaluate their role in the active site of the MurG enzyme from Escherichia coli, 13 residues conserved in the sequences of 73 MurG orthologues were submitted to site-directed mutagenesis. All these residues lay within, or close to, the active site of MurG as defined by its tridimensional structure [Ha et al., Prot. Sci. 9 (2000) 1045-1052, and Hu et al., Proc. Natl. Acad. Sci. USA 100 (2003) 845-849]. Thirteen mutants proteins, in which residues T15, H18, Y105, H124, E125, N127, N134, S191, N198, R260, E268, Q288 or N291 have been replaced by alanine, were obtained as the C-terminal His-tagged forms. The effects of the mutations on the activity were checked: (i) by functional complementation of an E. coli murG mutant strain by the mutated genes; and (ii) by the determination of the steady-state kinetic parameters of the purified proteins. Most mutations resulted in an important loss of activity and, in the case of N134A, in the production of a highly unstable protein. The results correlated with the assigned or putative functions of the residues based on the tridimensional structure.