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Coupled Enzyme Systems in a Vesicular Membrane: Oxidative Phosphorylation as an Example
Terrell L. Hill
Proceedings of the National Academy of Sciences of the United States of America
Vol. 76, No. 1 (Jan., 1979), pp. 232-235
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/69468
Page Count: 4
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We consider a small vesicle whose membrane transports a ligand L into the vesicle through enzymatic units of type A and transports L out of the vesicle through units of type B. Oxidative phosphorylation in mitochondria provides an example, in which L is H+. The kinetics of the two membrane systems (A and B) are coupled through the concentration of L in the vesicle. This interdependence causes the combined membrane system (A plus B) to simulate a single system whenever the net ligand transport into the vesicle is zero. For example, in oxidative phosphorylation, it was thought for some time that ATP was synthesized by the respiratory chain system (via an ``active intermediate''). We give the simplest possible analysis of this kind of coupled system, which is very common, by using two-state enzymes for both A and B above. A numerical example is included that illustrates respiratory control in a qualitative way: although the respiratory chain flux by itself does not depend on ADP concentration, the steady-state flux of the coupled systems (respiratory chain and reverse ATPase) does depend on ADP concentration through the interior ligand (H+) concentration.
Proceedings of the National Academy of Sciences of the United States of America © 1979 National Academy of Sciences