Probing the catalytic roles of n2‐site glutamate residues in Escherichia coli glutamine synthetase by mutagenesis

The contribution of metal ion ligand type and charge to catalysis and regulation at the lower affinity metal ion site (n2 site) of Escherichia coli glutamine synthetase (GS) was tested by mutagenesis and kinetic analysis. The 2 glutamate residues at the n2 site, E129 and E357, were changed to E129D, E129H, E357H, E357Q, and E357D, representing conservative and nonconservative alterations. Unadenylylated and fully adenylylated enzyme forms were studied. The Mn2+‐KD values, UV‐vis, and fluorescence emission properties were similar for all mutants versus WTGS, except E129H. For kinetic determinations with both Mn2+ and Mg2+, nonconservative mutants (E357H, E129H, E357Q) showed lower biosynthetic activities than conservative mutants (E129D, E357D). Relative to WTGS, all the unadenylylated Mn2+‐activated enzymes showed reduced kcar/Km values for ATP (>7‐fold) and for glutamate (> 10‐fold). Of the unadenylylated Mg2+‐activated enzymes, only E129D showed kinetic parameters competitive with WTGS, and adenylylated E129D was a 20‐fold better catalyst than WTGS. We propose the n2‐site metal ion activates ADP for departure in the phosphorylation of glutamate by ATP to generate γ‐glutamyl phosphate. Alteration of the charge density at this metal ion alters the transition‐state energy for phosphoryl group transfer and may affect ATP binding and/or ADP release. Thus, the steady‐state kinetic data suggest that modifying the charge density increases the transition‐state energies for chemical steps. Importantly, the data demonstrate that each ligand position has a specialized spatial environment and the charge of the ligand modulates the catalytic steps occurring at the metal ion. The data are discussed in the context of the known X‐ray structures of GS.