Pathways formed of delocalized pi-electron systems and polar groups of polypeptide chains bridged by hydrogen bonds are referred as pi-H pathways. Suitable for electron transfer, these pathways in cytochrome c oxidase connect CuA, the source of electrons distributed in cytochrome c oxidase, with the metal centers, heme a, heme a(3), CuB, the constituents of the catalytic binuclear center. The unusually rapid electron transfer between heme a and heme a(3) would have been facilitated by the link pathway of a long sequence of alternate peptide unit and hydrogen bond spanning Pro336-Val374, referred as suprahelix, between these hemes. Two pathways between CuA center and zinc center, share some portions with purported proton-translocating channels, designated "K" and "D". (C) 2019 Elsevier Inc. All rights reserved.