Mutational variants of the Neurospora crassa NADP-specific glutamate dehydrogenase altered in a conformational equilibrium

Abstract

Seven defective variants of the NADP-specific glutamate dehydrogenase of Neurospora crassa, resulting from missense mutations in the am gene, are quantitatively different from the wild type enzyme in the allosteric equilibrium between enzymically active A and inactive I conformations, and in the kinetics of conformational transitions between these states. These abnormalities have been defined using measurements of enzymic activity and of the intrinsic tryptophan fluorescence emission of the proteins. The protein from am 1(Ser336 → Phe) is hyperstable in the A conformation but this state is enzymically inactive because it fails to bind coenzyme. The other six variants are potentially active but are, to different extents, hyperstable in the I conformation. They form a series of analogues, those of am 131 (substitution not determined), am 130(Pro75 → Ser), am 3(Glu393 → Gly), am 2(His142 → Gln), am 19(Lys141 → Met) in order of increasing abnormality of the equilibrium position. am 122(Trp389 changed to an undetermined residue) resembles am 19. The hyperstability is sufficient to explain the auxotrophy of am The proteins of am 131 and am 130 are, in addition, abnormally prone to denaturation. These hyperstabilities of the I state are small in free energy terms, consistent with the fact that the defects of some variants may be corrected or partially corrected by second site substitutions or by complementation in hybrid hexamers with am 1 protein. Five out of seven amino acid substitutions known to affect this equilibrium (including Gln391 → Arg of revertant am 1924) involve charged residues clustered around positions 141 and 391. Interactions between these two parts of the polypeptide are implicated in stabilizing the A state of the enzyme, possibly by providing protonatable groups or part of the dicarboxylate binding site, and in affecting the environment of a tryptophan residue responsible for the fluorescence difference of the two conformations.