Hydration as a control mechanism in membrane proteins: the case of cytochrome c oxidase

Variable hydration and volume exclusion are postulated for the deviations from osmotic ideality of macromolecules. Interactions between different macromolecules [e.g. polyethyleneglycols (PEGs) and proteins] in solution may involve both hydration-dehydration and direct binding. Membrane enzymes such as cytochrome c oxidase undergo functionally significant conformational changes with osmotic consequences during their catalytic cycles and in reactions with inhibitors such as cyanide. Osmotic stress identifies three processes in the oxidase involving water binding and release: (i) uptake of water on binding of the substrate cytochrome c when bulk osmolarity is adjusted with sucrose. With glucose or glycerol, a smaller number of water molecules are taken up. Dextran increases cytochrome c binding, suggesting multiple regions in the cytochrome c binding cleft on the oxidase with differing osmolyte accessibilities. (ii) diminution of maximal enzyme activity by high osmolarity: water-stressed states are less active than the fully hydrated state. (iii) modulation of the transitions from resting to pulsed states and from free to cyanide-ligated states by high osmolarity, indicating a role for hydration in conformational changes. In each type of process water may play a mechanistically distinct role.