We have investigated the osmotic pressure dependence of the association between ferric cytochrome P450cam and putidaredoxin (Pdx) to gain an insight into the role of water molecules in the P450cam-reduced Pdx complexation amenable to physiological electron transfer. The association constant was evaluated from the electron transfer rates from reduced Pdx to P450cam. The natural logarithm of the association constant Ka was linearly reduced by the osmotic pressure, and osmotic stress yields uptake of 25 waters upon association. In contrast, uptake of only 13 waters is observed from the osmotic pressure dependence of the association in the nonphysiological redox partners P450cam and oxidized Pdx. Although general protein-protein associations proceed through dehydration around the complex interface, the interfacial waters could mediate hydrogen-bonding interactions. Therefore, about 10 more interfacial waters imply an additional water-mediated hydrogen-bonding network in the P450cam-reduced Pdx complex, which does not exist in the complex with oxidized Pdx. It is also possible that the water-mediated hydrogen-bonding interactions support a high P450cam affinity for reduced (Ka = 0.83 μM-1) relative to oxidized (Ka = 0.058 μM-1) Pdx. This study points to a novel role of solvents in assisting redox state-dependent interaction between P450cam and Pdx.
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