Hydrogen bond-linked pathways of peptide units and polar groups of amino acid residues suitable for electron transfer in cytochrome c proteins.

Electron transfer occurs through heme-Fe across the cytochrome c protein. The current models of long range electron transfer pathways in proteins include covalent σ-bonds, van der Waals forces, and through space jump. Hydrogen-bond-linked pathways of delocalized electron units in peptide units and polar side chains of amino acid residues in proteins and internal water molecules are better suited for intramolecular atom-to-atom electron transfer in proteins. Crystal structures of cytochrome c proteins from horse (1HRC), tuna (3CYT), rice (1CCR), and yeast (3CX5) were analyzed using pymol software for 'Hydrogen Bonds' marking the polar atoms within the distance of 2.6-3.3 Å and tracing the atom-to-atom pathways linked by hydrogen bonds. Pathways of hydrogen-bond-linked peptide units, polar side chains of the amino acid residues, and buried water molecules connect heme-Fe through axially coordinated Met80-S and His18-N have been traced in cytochrome c proteins obtained from horse, tuna, rice and yeast with an identical hydrogen-bonded sequence around the heme-Fe: Asn-N-water-O-Tyr-O-Met-S-heme-Fe-His (HN-C=N)-Pro-Asn-Pro-Gly (peptide unit, HN-C=O)-water-O. More than half of the amino acid residues in these pathways are among the conserved list and delocalized electron units, internal water molecules and hydrogen bonds are conspicuous by their presence.