FERRICYTOCHROME-B5 - ASSIGNMENT OF HEME PROPIONATE RESONANCES ON THE BASIS OF NUCLEAR OVERHAUSER EFFECT MEASUREMENTS AND THE NATURE OF INTERPROTEIN CONTACTS WITH PARTNER REDOX PROTEINS

Nuclear Overhauser effect experiments have lead to the stereospecific assignment of the four methylene proton pairs of the heme propionate side chains in ferricytochrome b5. The pH-sensitive resonances have been assigned to the exposed heme propionate on the heme pyrrole ring III. In addition it has been shown that this group is able to bind extrinsic metal ions and is intimately involved in the binding site for redox partner proteins. The patterns of the .alpha.-methylene proton hyperfine shifts for both propionates are concluded to be inconsistent with the orientation obtained from available X-ray crystal coordinates and suggest specific rotations for each side chain. The nature of the slight changes resulting from complex formation with partner redox proteins such as cytochrome c or myoglobin is shown to be consistent with a decrease of the pK for the exposed 6-propionate carboxylate. This decrease supports a direct participation of this group in a salt bridge to the partner protein.