Ferricytochrome c

The structure of crystalline horse heart ferricytochrome c has been determined by x-ray methods to a resolution of 2.8 A, and the results have been extended to obtain the structure of bonito cytochrome c as well. There is no difference between the horse and bonito structures, other than the expected changes in side chains where the amino acid sequences differ. The tertiary folding of cytochrome c has been maintained constant since the ancestors of mammals and fish diverged 400 million years ago, and probably for much longer. The polypeptide chain of 104 amino acids is wrapped around the heme in two halves; residues 1 to 47 to the right and 48 to 91 to the left of the heme, which sits in the resultant heme crevice with one edge exposed to solvent. Residues 92 to 104 form an α helical strap that rises over the top rear of the molecule and back across the right side again. Cysteines 14 and 17 and histidine 18 extend to the heme from the right wall of the heme crevice, and methionine 80 extends from the left wall. The heme is tightly enveloped in hydrophobic groups. Two channels filled with hydrophobic side chains lead to the right and left from the heme to the surface of the molecule. Each channel contains at least two aromatic rings in roughly parallel orientation, and each channel is surrounded by a cluster of positively charged lysines where it meets the surface. At the back rear of the molecule, between the two positive regions, is a cluster of nine negatively charged acidic groups. It is proposed that these surface features may be involved in binding to other macromolecular complexes, including the cytochrome oxidase system. The principal folding influence on the molecule appears to be the heme group itself. Only residues 92 to 102 are genuinely α helical, although several other areas might have been were it not for interference from the heme. Methods currently proposed for predicting α helical regions are promising but insufficiently discriminating. The abrupt 310 bend, occasionally observed in other globular proteins, occurs six times in cytochrome c, at locations where a sharp chain reversal is needed in wrapping around the heme. The x-ray structure shows the reasons for many of the unchanging features apparent from comparison of amino acid sequences from over 30 different species. Hydrophobic groups are invariant because they play an essential role in providing the proper heme environment and causing the protein to fold around the heme. Acidic and basic chains are necessary for the preservation of segregated regions of charge on the surface. Glycines often occur where there is no room for a side chain, and serines and theronines play important hydrogen-bonding roles in maintaining the folding of the protein.