Investigation of the Functional Contributions of Invariant Serine Residues in Yeast Mevalonate Diphosphate Decarboxylase

Abstract

Alignment of more than 20 deduced sequences for mevalonate diphosphate decarboxylase (MDD) indicates that serines 34, 36, 120,121, 153, and 155 are invariant residues that map within a proposed interdomain active site cleft. To test possible active site roles for these invariant serines, each has been mutated to alanine. S34A exhibits limited solubility and impaired binding of the fluorescent ATP analogue, trinitrophenyl-ATP (TNP-ATP), suggesting that Ser-34 substitution destabilizes proper enzyme folding. All other serine mutants retain structural integrity, as indicated by their ability to bind TNP-ATP at levels comparable to wild-type enzyme. S153A exhibits a 18-fold inflation in K(d) for Mg-ATP, as indicated by competitive displacement of TNP-ATP; the enzyme also is characterized by a 35-fold inflation in K(m) for Mg-ATP. S155A exhibits a 26-fold inflation in K(m) for Mg-ATP, but competitive displacement of TNP-ATP indicates only a 2-fold inflation in K(d) for this substrate. S155A exhibits both a 16-fold inflation in K(m) for mevalonate diphosphate and a 14-fold inflation in K(i(slope)) for the substrate analogue, diphosphoglycolylproline. These observations suggest roles for Ser-153 and Ser-155 in substrate binding. Catalytic consequences of mutating invariant serines 36, 120, 153, and 155 are modest (<8-fold diminution in k(cat)). In contrast, S121A, which exhibits only modest changes in K(d) for Mg-ATP and K(m) for mevalonate diphosphate, is characterized by a >42,000-fold diminution in k(cat), indicating the critical involvement of Ser-121 in reaction catalysis. The selective involvement of the latter of two tandem serine residues (Ser-120, Ser-121) in a conserved sequence motif suggests mechanistic similarities within the GHMP kinase superfamily of proteins.