Osmotic Pressure Effects Reveal a Specific Dehydration-Induced Hydrophobic Electron Transfer Structure Comprising Cytochrome C and Cytochrome C Oxidase

In the mitochondrial respiratory chain, Cytochrome c oxidase (CcO) accepts electrons from Cytochrome c (Cyt c) to reduce molecular oxygen. Previous studies showed that the complex formation of Cyt c with CcO is characterized by the positive entropy change, suggesting the dehydration from hydrophobic residues, but no experimental evidence of such dehydration has been reported. To confirm the dehydration during the complex formation between Cyt c and CcO, we focused on the partial molar volume change of protein (ΔV_W) for the complex formation, which reflects the volume of dehydrated water molecules. ΔV_W is correlated with the osmotic pressure (π) dependence of the K_d.  

Osmotic pressure was increased by addition of sugars or polyols having no specific interactions with proteins and K_d was estimated by the Michaelis-Menten analysis of the steady state kinetics for the electron transfer from Cyt c to CcO.

As Figure 1 shows, the K_d value is decreased with increasing the osmotic pressure, corresponding to the negative ΔV_W of – 300 ± 78 mL mol^-1. Assuming that the partial molar volume change of one hydrated water molecule around hydrophobic residues would be－17.5 mL mol^-1, the volume change we obtained here corresponds to the dehydration of 17 ± 4 water molecules. Considering that the entropy change from the dehydration (^dehydΔS) of one water molecule was estimated to be 5.5 JK^-1mol^-1, ^dehydΔS of 17 water molecules corresponds to ~90 JK^-1mol^-1, which is about 75% of total entropy change for the Cyt c-CcO complex formation. The dehydration is, therefore, one of the major factors to increase the entropy for the complex formation.

To identify the dehydration sites from hydrophobic residues in the interaction site for CcO on Cyt c, we mutated Ile11 or Ile81, hydrophobic amino acid residues in the interaction site for CcO [1], to Ala, an amino acid residue having a smaller side chain and less number of the hydrated water molecules. As clearly illustrated in Figure 1, the slope of the osmotic pressure dependence of K_d for the Ile mutant is less steep than that for the wild type protein. The estimated numbers of the dehydrated water molecules were reduced to 13 ± 3 and 8 ± 2 for the Ile11 and Ile81 mutants, respectively. The reduced numbers of the dehydrated water molecules in these Ile mutants indicate that the hydrated water molecules around Ile11 and Ile81 were expelled to the solvent in the complex formation between Cyt c and CcO.

In conclusion, we successfully identified that about 17 water molecules are dehydrated in the Cyt c-CcO complex formation and the primary dehydration sites are Ile11 and Ile81. Such dehydration from the hydrophobic residues can be a major factor to increase the entropy, facilitating the complex formation and the electron transfer from Cyt c to CcO.

References

[1] Sakamoto, K. et al., PNAS., 108, (2011), 12271