Colleagues -
Here is a relevant message at last (perhaps). We recently had a discussion
with a colleague in the cytochrome oxidase business about control of the
enzyme. Transmembrane helix analysis of cytochrome c oxidase - the
'structure' argument - indicates that the two haem groups, a and a3, are
approximately level in the membrane perhaps one third of the way down from
the P (external) face of the inner mitochondrial or bacterial plasma
membrane. This is the structural evidence based upon transmembrane helix
assignments and observations on conserved histidines and the effects seen
when they are mutated. Redox potential studies, however, suggested a long
time ago that the a haem saw about half the membrane potential and the a3
haem saw essentially all that potential when reduced from the P side.
Secondly, we find that if the membrane potential is converted to delta pH
by valinomycin addition that the turnover of the enzyme is slowed;
indicating that delta pH is more effective than delta psi as a flux
controller and that the change in internal pH is seen by the enzyme. If the
a3 haem were connected to the N (internal) face of the membrane by a proton
well that dipped 2/3 of the way thru the membrane then the membrane
potential that it sees should already have been converted to a pH gradient
by the accumulation of hydroxyl ions and exclusion of protons from the
well. 'Thermodynamics' therefore says that the a3 haem sees the bulk
conditions on the inside of the mitochondrion or bacterium and that a large
part of the dielectric lies between the a and a3 haems, as in the original
Mitchell model of the oxidase as transmembrane wire. Which should we
believe and/or can the apparent discrepancy be resolved?
Peter Nicholls.
