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 studi ed several varieties of cytochrome P-450: CYP11A1 (scc), CYP2B4 (LM2), CYP3A6 (LM3c), and Cyp2a (7 alpha), as well as chloroperoxidase and l actoperoxidase, and compared the results to data reported for other he moproteins. Whereas the CO binding activation enthalpy Delta H double dagger and entropy Delta S double dagger (correlated through a compens ation effect) varied greatly between the hemoproteins, with no apparen t relation to structural features, the pressure effect depended on the nature of the proximal axial heme ligand: the activation volume was v ery small for cysteine (S-) ligand hemoproteins (Delta V double dagger = +1 to +6 mt mol(-1)), and markedly negative for histidine (N) ligan d hemoproteins (Delta V double dagger = -3 to -36 ml mol(-1)). Further more, the transition state volume of the histidine ligand class enzyme s, but not that of the cysteine ligand enzymes, depended on the solven t composition. These results suggest that the CO-binding transition st ate of the S-ligand class has a molecular conformation similar to the ground state. In the histidine class, however, the transition state ap pears to involve protein conformational changes and/or solvation proce sses.