Solution structure of the B form of oxidized rat microsomal cytochrome b(5) and backbone dynamics via N-15 rotating-frame NMR-relaxation measurements- Biological implications

Cytochrome b(5) in solution has two isomers (A and B) differing by a 180 de grees rotation of the protoporphyrin IX plane around the axis defined by th e alpha and gamma meso protons. Homonuclear and heteronuclear NMR spectrosc opy has been employed in order to solve the solution structure of the minor (B) form of the oxidized state of the protein and to probe its backbone dy namics in the mu s-ms timescale in both oxidation states. A family of 40 co nformers has been obtained using 1302 meaningful NOEs and 220 pseudocontact shifts and is characterized by high quality and good resolution (rmsd to t he mean structure of 0.055 +/- 0.009 nm and 0.103 +/- 0.011 nm for backbone and heavy atoms, respectively). Extensive comparisons of the structural an d dynamics changes associated with the A-to-B form interconversion for both oxidation states were subsequently performed. Propionate 6 experiences a r edox-state-dependent reorientation as does propionate 7 in the A form. Sign ificant insights are obtained into the role of the protein frame for effici ent biological function and backbone mobility is proposed to be one of the factors that could control the reduction potential of the heme.