The bridging element for electron transfer in proteins is the hydrogen bond according to the new experimental perspective in preference to carbon-carbon sigma-bond presently used. The purpose of this study is to identify an alternative pathway linked by hydrogen bonds suitable for electron transfer from heme-Fe of cytochrome c to subunit II-CuA of cytochrome a. A pathway consisting of 15 delocalized electron systems including peptide bonds, 5 polar groups of side chains of amino acid residues and 8 water molecules, linked by 27 hydrogen bonds, exists between the two metal electron centers of heme-Fe of cytochrome c, cytochrome c and of subunit II-CuA of cytochrome a. Pathways built of delocalized pi-electron systems, polar groups and water molecules linked by hydrogen bonds may be considered for intramolecular and intermolecular electron transfer in proteins. (C) 2018 Elsevier Inc. All rights reserved.