Replacement of cysteine-107 of Saccharomyces cerevisiae iso-1-cytochrome c with threonine: improved stability of the mutant protein.

Site-directed mutagenesis has been used to change the codon for cysteine-107 of Saccharomyces cerevisiae iso-1-cytochrome c to a threonine codon. The resulting protein is active in vivo, is methylated as in the wild-type protein and has optical properties indistinguishable from those of the wild-type protein. The threonine-107 iso-1-cytochrome c demonstrated fully reversible electrochemical behaviour and a mid-point reduction potential of 272 mV versus NHE. In addition, this mutant does not demonstrate a tendency to autoreduce or to dimerize as does the wild-type protein. These properties of the threonine-107 mutant establish that it will provide a useful background in which to make subsequent mutations for mechanistic and physical studies of yeast iso-1-cytochrome c.