Amino acid replacement studies of human cytochrome c by a complementation system using CYC1 deficient yeast

Various in vitro mutated human cytochrome c genes which encode displaced amino acid residues at the 14th, 17th, 28th, 37th, 38th, 56th, and/or 84th residues were constructed, and their degrees of complementation of yeast CYC1 deficiency were examined. Invariant Cys-17 and Arg-38 could not be replaced by alanine and tryptophan, respectively, without function impairment. Cytochrome c containing Ala-14 instead of conserved Cys-14, Gly-38 or Lys-38 instead of Arg-38, and Ser-84 instead of invariant Gly-84 were partly functional. These results indicate that these invariant or conserved residues are important. Cytochromes c containing Cys-56 instead of native Gly-56 was partly functional. Cytochrome c containing Arg-37 and Gly-38 instead of Gly-37 and Arg-38 was slightly functional. Replacement of variable Thr-28 and Gly-37 by Ile-28 and Arg-37, respectively, produced no effects. Our results are as a whole consistent with the view that conserved residues are important and variable residues are less important for cytochrome c to function.