SITE-DIRECTED MUTAGENESIS OF CYSTEINE RESIDUES IN BILIVERDIN REDUCTASE - ROLES IN SUBSTRATE AND COFACTOR BINDING

Biliverdin reductase is unique among all enzymes described to date in having two pH optima, 6.75 and 8.7, at which NADH or NADPH, respectively, are required for activity. The enzyme converts biliverdin to bilirubin in mammals. The mature enzyme, which is 293 amino acids long, has 3 cysteine residues, and is sulfhydryl dependent. To understand the role of the cysteine residues in enzyme activity, we examined the effects of the neutral substitution with alanine of each of three residues, individually and in combination, by site-directed mutagenesis. These residues in the pre dieted amino acid sequence of rat biliverdin reductase correspond to amino acids 73, 280 and 291. The modification of the amino-proximal cysteine (Cys73), which is flanked by a tyrosine residue, completely inactivated the enzyme with NADH at pH 6.75 and NADPH at pH 8.7. The loss of reductase activity was not due to changes in three-dimensional characteristics of the protein as suggested by its mobility in a non-denaturing gel. Although modification of either of the two cysteines located near the C-terminus (Cys280 and Cys291) significantly reduced activity with both cofactors, these mutations did not inactivate the enzyme. Comparison of K-m values for the Cys280-->Ala and Cys291-->Ala mutants with the wild type protein, at pH 8.7, suggests that Cys280 principally functions in substrate binding while Cys291 is predominantly involved in cofactor binding. This assignment probably also applies at pH 6.75. Comparison of k(cat) of the mutants with wild type shows that mutation of Cys280 decreases V-max of the enzyme. Mutation of both C-terminal cysteines caused inactivation of the enzyme, comparable to that produced by mutation of Cys73. Analysis by circular dichroism at far-ultraviolet wavelengths suggests that the alterations in activity are not the result of changes in the secondary structure of these mutants. These results are consistent with Cys73 having a central role in substrate/cofactor binding while biliverdin reductase can function, albeit at a reduced rate, with only one of the near C-terminus cysteines. The results are further consistent with the suggestion that although the two C-terminal cysteines have preferential affinities, they can serve similar functions in the interaction with substrate/cofactor.