The role of entropy in the discrimination between CO and O2 in myoglobin

Cryoradiolysis and cryospectroscopy for studies of heme-oxygen intermediates in cytochromes p450

Cryogenic radiolytic reduction is one of the most straightforward and convenient methods of generation and stabilization of reactive iron-oxygen intermediates for mechanistic studies in chemistry and biochemistry. The method is based on one-electron reduction of the precursor complex in frozen solution via exposure to the ionizing radiation at cryogenic temperatures. Such approach allows for accumulation of the fleeting reactive complexes which otherwise could not be generated at sufficient amount for structural and mechanistic studies. Application of this method allowed for characterizing of peroxo-ferric and hydroperoxo-ferric intermediates, which are common for the oxygen activation mechanism in cytochromes P450, heme oxygenases, and nitric oxide synthases, as well as for the peroxide metabolism by peroxidases and catalases.

Protein relaxation in the photodissociation of myoglobin-CO complexes

Laser-induced optoacoustic spectroscopy has been applied to the study of the photodissociation of myoglobin-CO complexes. Time-resolved optoacoustic signals have been measured from aqueous solutions of horse myoglobin-CO complex (hMbCO) at pH 3.5 and 8, and of sperm whale myoglobin-CO complex (swMbCO) at pH 8, in the temperature range 273-300 K. The signal of hMbCO at pH 8 exhibits three components. The first, which is faster than 20 ns and is associated with a reaction enthalpy of 61 kJ mol(-1), corresponds to Fe-CO bond breakage. The second component has a decay time of 80 ns at 293 K and is associated with an exothermic protein relaxation (-13 kJ mol(-1)) and a volume change of -3 ml mol(-1). The relaxation, which involves a state where the photo-dissociated CO is still in a protein docking site, is thermally activated, with an activation enthalpy of 51 kJ mol(-1). The third component has a decay time of 800 ns at 293 K and an activation enthalpy of 39 kJ mol(-1), and is associated with an endothermic process (26 kJ mol(-1)) and an expansion of 19 ml mol(-1). This process is ascribed to the migration of the photodissociated CO to the bulk solvent. At acidic pH, the latter process becomes faster (230 ns) and the volume change decreases. These features are correlated with the presence of an open form of the protein. swMbCO exhibits two components only, due to the overlap of the two fastest processes. The first involves a reaction enthalpy of 49 kJ mol(-1) and a volume contraction of -4.9 ml mol(-1). The second component (900 ns at 293 K, activation enthalpy 45 kJ mol(-1)) is associated with a reaction enthalpy of 38 kJ mol(-1) and a volume expansion of 15.3 ml mol(-1). These experimental findings have been interpreted by means of a new model, which also takes into account both laser flash photolysis results and structural information. The model is based on a two-dimensional scheme which describes both protein relaxation and the CO pathway following photodissociation.

Harmonic and anharmonic dynamics of Fe-CO and Fe-O(2) in heme models

We present density-functional molecular dynamics simulations of FeP(Im)(AB) heme models (AB = CO, O(2), Im = imidazole) as a way of sketching the dynamic motion of the axial ligands at room temperature. The FeP(Im)(CO) model is characterized by an essentially upright FeCO unit, undergoing small deviations with respect to its linear equilibrium structure (bending and tilting up to 10 degrees and 7 degrees, often occur). The motion of the carbon monoxide ligand is found to be quite complex and fast, its projection on the porphyrin plane sampling all the porphyrin quadrants in a short time ( approximately 0.5 ps). Simultaneously, the imidazole ligand rotates slowly around the Fe-N(epsilon) bond. In contrast to carbon monoxide, the oxygen ligand in FeP(Im)(O(2)) prefers a conformation where the projection of the O-O axis on the porphyrin plane bisects one of the porphyrin quadrants. A transition to other quadrants takes place through an O-O/Fe-N(p) overlapping conformation, within 4-6 ps. Further details of these mechanisms and their implications are discussed.