ON THE MECHANISM OF PROTON TRANSLOCATION BY RESPIRATORY ENZYME

The protonmotive function of the respiratory heme-copper oxidases is o ften described as the sum of two separate mechanisms: a proton pump pl us an incomplete Mitchellian redox loop. However, these two functions may be mechanistically intertwined so that the uptake of protons to fo rm water during the reduction of O-2 is a crucial part of the proton p ump mechanism itself. This principle can be deduced from thermodynamic , kinetic, mechanistic, as well as from structural considerations, and was first proposed in conjunction with a histidine cycle model of pro ton translocation [Morgan, J. E., Verkhovsky, M. I., and Wikstrom, M. (1994). J. Bioenerg. Biomembr. 26, 599-608]. However, histidine cycle models go much further to suggest chemical details of how this princip le might be applied.