RESPONSES OF 2 PROTEIN-PROTEIN COMPLEXES TO SOLVENT STRESS - DOES WATER PLAY A ROLE AT THE INTERFACE

We have analyzed the stability of the cytochrome c-cytochrome b5 and c ytochrome c-cytochrome c oxidase complexes as a function of solvent st ress. High concentrations of glycerol were used to displace the two eq uilibria. Glycerol promotes complex formation between cytochrome c and cytochrome b5 but inhibits that between cytochrome c and cytochrome c oxidase. The results with cytochrome b5 and cytochrome c were expecte d; the association of this complex is largely entropy driven. Our inte rpretation is that the cytochrome c-cytochrome b5, complex excludes wa ter. The results with the cytochrome c oxidase and cytochrome c couple were not expected. We interpret them to mean that either glycerol is binding to the oxidase, thereby displacing the cytochrome c, or that w ater is required at this protein-protein interface. A requirement for substantial quantities of water at the interface of some protein compl exes is logical but has been reported only once.