Activation volume and energetic properties of the binding of CO to hemoproteins

We have investigated the CO binding to various reduced hemoproteins by stopped-flow rapid mixing as a function of pressure (from 0.1 to 200 MPa) and temperature (from 4 to 35 degrees C). In particular, we studied several varieties of cytochrome P-450: CYP11A1 (scc), CYP2B4 (LM2), CYP3A6 (LM3c), and Cyp2a (7 alpha), as well as chloroperoxidase and lactoperoxidase, and compared the results to data reported for other hemoproteins. Whereas the CO binding activation enthalpy delta H++ and entropy delta S++ (correlated through a compensation effect) varied greatly between the hemoproteins, with no apparent relation to structural features, the pressure effect depended on the nature of the proximal axial heme ligand: the activation volume was very small for cysteine (S-) ligand hemoproteins (delta V++ = +1 to +6 ml mol-1), and markedly negative for histidine (N) ligand hemoproteins (delta V++ = -3 to -36 ml mol-1). Furthermore, the transition state volume of the histidine ligand class enzymes, but not that of the cysteine ligand enzymes, depended on the solvent composition. These results suggest that the CO-binding transition state of the S-ligand class has a molecular conformation similar to the ground state. In the histidine class, however, the transition state appears to involve protein conformational changes and/or solvation processes.