Identification of catalytic residues in human mevalonate kinase

cDNA encoding human mevalonate kinase has been overexpressed and the recombinant Enzyme isolated. This stable Enzyme is a dimer of 42-kDa subunits and exhibits a Vm = 37 units/mg, Km(ATP) = 74 microM, and Km(DL-MVA) = 24 microM. The sensitivity of Enzyme to water-soluble carbodiimide modification of carboxyl groups prompted evaluation of four invariant acidic Amino acids (Glu-19, Glu-193, Asp-204, and Glu-296) by site-directed mutagenesis. Elimination of Glu-19's carboxyl group (E19A, E19Q) destabilizes the Enzyme, whereas E19D is stable but exhibits only approximately 2-fold changes in Vm and Km values. E296Q is a stable Enzyme, which exhibits kinetic parameters comparable to those measured for wild-type Enzyme. E193A is a labile protein, whereas E193Q is stable, exhibiting >50-fold diminution in Vm and elevated Km values for ATP (approximately 20-fold) and mevalonate (approximately 40-fold). Such effects would be compatible with a role for Glu-193 in interacting with the cation of the MgATP substrate. D204A and D204N are stable Enzymes lacking substantial mevalonate kinase activity. The active sites of these Asp-204 mutants are intact, based on their ability to bind a spin-labeled ATP analog with stoichiometries and equilibrium binding constants that are comparable to those determined for wild-type Enzyme. Competitive displacement experiments demonstrate that the Asp-204 mutants can bind ATP with Kd values that are comparable to estimates for wild-type Enzyme. The >40,000-fold diminution in kcat for the Asp-204 mutants and the demonstration that they contain an otherwise intact active site support assignment of a crucial catalytic role to Asp-204. The assignment of Asp-204 as the catalytic base that facilitates deprotonation of the C-5 hydroxyl of mevalonic acid would be compatible with the experimental observations.