Electron transfer tunneling pathways in bovine heart cytochrome c oxidase

Results of a study of internal electron transfer in bovine heart cytochrome c oxidase with the method of tunneling currents are presented. Electronic structure of the protein complex is treated at the semiempirical extended H uckel level. Two distinct pathways connecting Cu-A and heme a are found, on e of them is similar to proposed earlier in the literature, the other is ne w. The pathway connecting heme a and heme a(3) is also identified. This pat hway differs from those proposed before. The calculated reaction rates betw een Cu-A and Fe-a and between Fe-a and Fe-a3 are in reasonable agreement wi th experimental data. The tunneling matrix element for electron transfer fr om Cu-A to the binuclear site is found to be very small, which is consisten t with experimental evidence of the absence of this reaction. With the assu mption that evolution places constraints on functionally important amino ac ids, we suggest that amino acids implicated in the electron-transfer pathwa ys will show a high degree of conservation in different organisms. Sequence analysis performed on subunit I and II revealed that this is indeed the ca se: amino acids of the identified tunneling pathways showed very little evo lutionary variability.