Identification of catalytic residues in human mevalonate kinase

Abstract

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 V(m) = 37 units/mg, K(m(ATP)) = 74 μM, and K(m(DL-MVA))= 24 μM. 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 ~2-fold changes in V(m) and K(m) 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 V(m) and elevated K(m) values for ATP (~20-fold) and mevalonate (~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 K(d) values that are comparable to estimates for wild-type enzyme. The >40,000-fold diminution in k(cat) 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.