Site-directed Mutational Analysis of Active Site Residues in the Acetate Kinase from Methanosarcina thermophila

Acetate kinase catalyzes the magnesium-dependent transfer of the γ-phosphate of ATP to acetate. The recently determined crystal structure of the Methanosarcina thermophila enzyme identifies it as a member of the sugar kinase/Hsc70/actin superfamily based on the fold and the presence of five putative nucleotide and metal binding motifs that characterize the superfamily. Residues from four of these motifs in M. thermophila acetate kinase were selected for site-directed replacement and analysis of the variants. Replacement of Asp¹⁴⁸ and Asn⁷ resulted in variants with catalytic efficiencies less than 1% of that of the wild-type enzyme, indicating that these residues are essential for activity. Glu³⁸⁴ was also found to be essential for catalysis. A 30-fold increase in the magnesium concentration required for half-maximal activity of the E384A variant relative to that of the wild type implicated Glu³⁸⁴ in magnesium binding. The kinetic analysis of variants and structural data is consistent with nonessential roles for active site residues Ser¹⁰, Ser¹², and Lys¹⁴ in catalysis. The results are discussed with respect to the acetate kinase catalytic mechanism and the relationship to other sugar kinase/Hsc70/actin superfamily members.