1H-NMR assignments and the dynamics of interconversion of the isomeric forms of cytochrome b5 in solution

Abstract

Cytochrome b 5 reconstituted with specifically deuterated hemins has led to the assignment of the resolved 6,7β-propionate protons and heme meso protons. Freshly reconstituted cytochrome b 5 contains a mixture of two isomers in an approx. 1:1 ratio. As time proceeds the minor isomer decreases in intensity until the equilibrium ratio, approx. 8:1, of the two isomers is reached. The rate of the heme disorder kinetics was investigated for cytochrome b 5 as a function of pH, oxidation state and 2,4 heme substitutents. Comparison of the kinetic data for cytochrome b 5 with that obtained for other b-type heme proteins supports the proposal that the heme disorder arises from a 180° rotation of the heme about the α,γ- meso axis. Computer-difference methods allow the spectra of the two individual isomers to be generated. Comparison of the NMR spectral parameters for the two individual isomers indicates small structural differences for amino acid side-chain orientations.