Site-directed mutagenesis for cysteine residues of cobalt-containing nitrile hydratase

Three cysteine residues, which are completely conserved among alpha-subunits in all nitrile hydratases, are thought to be the ligands of a metal ion in the catalytic center of this enzyme. These cysteine residues (i.e. alpha C102, alpha C105 and alpha C107) in the high-molecular-mass nitrile hydratase (H-NHase) of Rhodococcus rhodochrous J1 were replaced with alanine by site-directed mutagenesis using the R. rhodochrous ATCC12674 host-vector system, and the resultant transformants were investigated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) for the cell-free extracts of each mutant transformant revealed that four mutant transformants (i.e. alpha C105A, alpha C107A, alpha C102A/C105A and alpha C105A/C107A) showed predominant alpha- and beta-subunit protein bands with a mobility identical to those of the native H-NHase, while three mutant transformants (i.e. alpha C102A, alpha C102A/C107A and alpha C102A/C105A/C107A) did not produce the corresponding proteins. The purified former four mutant enzymes showed neither enzymatic activity nor the maximum absorption at 410 nm which was detected in the wild type H-NHase. They also did not contain cobalt ions. Based upon these findings, these three cysteine residues were found to be essential for the active expression of H-NHase.