Importance of a conserved phenylalanine-35 of cytochrome b5 to the protein's stability and redox potential

Phenylalanine-35, which is a residue of the hydrophobic patch on the surface of cytochrome b5, has been mutated into Tyr35, His35 and Leu35 to elucidate the functions of the Phe35 and give further insight into the roles of the hydrophobic patch and/or aromatic network. The effects of these mutations on the heme environment, denaturation towards heating and the denaturant urea, redox potential and stability of protein were studied. The relative stability of cytochrome b5 and its mutants towards heating has the order Phe35Tyr > wild type > Phe35Leu > Phe35His in the oxidized state and wild type > Phe35Tyr > Phe35Leu > Phe35His in the reduced state. All the mutants exhibit decreased reduction potentials: Phe35Tyr -66 mV, Phe35His -51 mV and Phe35Leu -28 mV, which are more negative than that of the wild type. The order of redox potential reflects the relative stability in the oxidized and reduced states. A method of producing multiple mutants at a single site of a gene is also described for the first time.

The influence of Glu44 and Glu56 of cytochrome b5 on the protein structure and interaction with cytochrome c

The gene encoding trypsin-solubilized bovine liver microsomal cytochrome b5 (82 residues in length) has been mutated, in which the codons of Glu44 and Glu56 were changed to those of Ala. The mutated genes were expressed in Escherichia coli successfully and three mutant proteins (E44A, E56A and E44/56A) were obtained. The UV-visible, CD and 1H NMR spectra of proteins have been studied. The results show that the mutagenesis at surface residues does not alter the secondary and tertiary structures of cytochrome b5 significantly. The interactions between recombinant cytochrome b5 and its mutants with cytochrome c were studied by using optical difference spectra. The results demonstrated that both Glu44 and Glu56 of cytochrome b5 participate in the formation of a complex between cytochrome b5 and cytochrome c.