Mutagenesis of the conserved lysine 14 of cytochrome c-550 from Thiobacillus versutus affects the protein structure and the electron self-exchange rate

Abstract

The lysine residue K14 of cytochrome c-550 of Thiobacillus versutus has been mutated to a glutamine (Q) and a glutamate (E) residue. These mutations have a minimal effect on the pK(a) for replacement of the methionine ligand (the ''alkaline transition''), indicating that a presumptive salt bridge between K14 and E11 does not help stabilize the native form. This is in contrast with mitochondrial cytochrome c, where the homologous K13 forms a structurally important salt bridge with glutamate 90. The NMR signals of protons close to the heme iron in wild-type and mutant ferricytochrome c-550 shift considerably with increasing ionic strength. These effects resemble those seen in mitochondrial cytochrome c upon addition of salt and upon complex formation with redox partners. It is likely that electrostatic screening of positive charges near the heme crevice leads to a slight redistribution of the electron density in the heme. At low ionic strength the NMR spectrum of wild-type cytochrome c-550 shows broad peaks. Line widths decrease upon addition of salt up to 200 mM. In K14Q and K14E cytochrome c-550 the line widths are much smaller at low ionic strength. Wild-type cytochrome c-550 may exist in two exchanging conformations, one o which may represent a more open (non-native) form, in analogy with cytochrome c. However, in the case of cytochrome c-550 this non-native form does not show ligand replacement. The electron self-exchange rates of wild type and mutants have been determined as a function of the ionic strength. At zero ionic strength these rates are 2(1) X 10(2) M-1 s-1 (wild type), 7(2) x 10(3) M-1 s-1 (K14Q), and 1.2(0.4) x 10(4) M-1 s-1 (K14E); with increasing ionic strength the differences between these rates decrease. In the presence of 0.55 M NaCl the rate for wild-type cytochrome c-550 is 1.0(0.3) x 10(5) M-1 s-1.