The identification of cation‐binding domains on the surface of microsomal cytochrome b 5 using 1 H‐NMR paramagnetic difference spectroscopy

One‐dimensional and two‐dimensional 1 H‐NMR methods and paramagnetic difference spectroscopy have defined cation binding domains on the surface of the tryptic fragment of microsomal cytochrome b 5 . The addition of tris(ethylenediamine) chromium(III) [Cr(en) 3+ 3 ] to solutions of ferricytochrome b 5 reveals at least three distinct sites on the surface of the protein to which highly charged cations may bind (20 mM phosphate pH 7.0, T = 300 K). Surprisingly only one of these sites is located close to the haem edge region of the protein, whilst the remaining two sites are more remote. Site I contains the exposed haem C13 propionate and a series of carboxylate residues that includes glutamates 37, 38, 43, 44, and 48. Sites II and III are located away from the haem edge region and are delineated by the broadening of aromatic resonances of histidines 26 and 80. Further investigation of the interaction between Cr(en) 3+ 3 and cytochrome b 5 using two‐dimensional double‐quantumfiltered correlated spectroscopy shows that resonances assigned to Glu59, Asp60, Glu79, Asp82 and Asp83 are broadened with the distribution of these charged side chains correlating with the relaxation broadening observed from one‐dimensional experiments. In a binary complex with ferricytochrome c , Cr(en) 3+ 3 broadens many cytochrome b 5 resonances including the haem propionates, His26, Ala54, Thr55 and His80. Although the pattern of line‐broadening of resonances at sites II and III is unaltered by complex formation, cytochrome c shields residues at site I, the haem edge site. The results indicate that the interaction between cytochrome b 5 and c in a binary complex involves multiple protein configurations.