In continuing studies of the cytochrome P450s using a variety of techniques of theoretical chemistry and known P450cam structures, we shall focus on: elucidation of underlying mechanisms of their monoxygenase activity, the relationship of the spectra and structure of the heme unit to that function, the role of the protein environment in modulating that function, and the role of reversible and irreversible inhibitors in interfering with it. To this end, we shall: (2) calculate the electronic structure and spectra of model Ferrous P450-02 and -CO complexes, as a function of changes in Fe-cysteine geometry and protonated state to probe the role of the cysteine ligand in determining both their unique signature spectra and unique function; (2) continue studies of the path to the bioactive oxygen transfer state by calculating the electronic structure and spin distribution in the putative, transient, unknown P450 (Fe-0x)- and (Fe- OOH)-species, and for the more stable corresponding myoglobin analogs recently isolated and characterized; (3) continue studies of the effects of protein environment of P450cam and of changes in substrate and of amino acids in the binding site of P450cam on product selectivity and substrate efficacy; (4) begin studies of reversible (Type II) inhibition by imidazole-quinoline-, and hydrazine-containing therapeutic agents all known to have adverse in vivo effects because of this inhibition; (5) continue studies of irreversible suicide substrate inhibition of olefins and begin studies of allene, acetylene-, and hydrazine containing therapeutic agents to identify and characterize steric and electronic properties that could modulate relative inhibitor potency, to help design useful P450 inhibitors and safer therapeutic agents; and (6) Perform systematic SAR studies on the known inhibitors and substrates of aromatase, a mammalian P450 involved in steroid biosynthesis. Calculated steric and electronic properties will be examined for their potential role as reliable indicators of recognition of these isozymes. If successful, these criteria will be used in two qualitatively different ways: (a) to design more selective aromatase inhibitors that could be useful therapeutic agents; (b) to engineer the P450cam binding site to transform it into a useful model receptor site mimicking the ability of P450 aromatase to accommodate its substrate and inhibitors.