Electrochemical Aspects of Electron Transfer in Mitochondrial Respiratory Chains

Oxidation of NADH and succinate by O2 in respiratory particles involves various electron carriers. The kinetics of their reduction and reoxidation is still a very unsettled question. A complete electron-transfer chain is made up by electron carriers and proteins serving structural and coupling functions (oxidative phosphorylation and others). It seems that a complete electron-transfer chain can be broken into four complexes or blocks, the redox proteins being distributed among them. The flow of electrons in each block may involve the formation of charge-transfer complexes between the prosthetic groups of the redox proteins, but it requires a molecular rearrangement within the coenzymes or within the associated proteins in order to bring close enough the redox prosthetic groups. The existence of an alternative mechanism to explain the NADH oxidation in block I, based on a direct transfer of hydride H− ions, is discussed. The concerted action of the redox systems constituting this block permits such a mechanism. Although all along the respiratory chain the electron transport is expressed in terms of one-electron steps, in block IV (cytochrome oxidase) this mechanism must be coupled to a multi-electron step mechanism to account for the reduction of O2 to H2O without formation of undesirable intermediates.