PubMed · 10847847
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M Kanzaki, M Nagasawa, I Kojima, C Sato, K Naruse, M Sokabe, H Iida. 2000-05-26. Report clarification.. https://doi.org/10.1126/science.288.5470.1347
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A unique feature of P450 enzymes is in the presence of a thiolate ligand heme but its exact function in catalysis is a matter of debate. For P450 dependent monooxygenases the "active oxygen" complex seems to exist only as a transition state in which the thiolate ligand provides electron density in order to prevent pi-backbonding of the oxygen to the iron (-S-Fe-O(z.rad;)). The corresponding ground state (Compound I) would be a ferryl species (Fe(IV)z.dbnd6;O) with an electron hole either at the porphyrin or at the sulfur. Apart from this role we postulate that a second function is related to the electronic structure of Compound II as an electron acceptor and this property is shared among monooxygenases, thromboxane synthase, prostacyclin synthase, allene oxide synthase, P450(NOR(-)) and chloroperoxidase. As a common step in all P450 enzymes an extremely rapid electron uptake by Compound II allows that the primary substrate radicals are oxidized to cations which immediately combine with a neighbouring nucleophile. Thus "electron transfer" may substitute for "oxygen rebound" as the final step leading to product formation. The same principle also applies methane monooxygenases in which the role of the thiyl sulfur is replaced by a ferryl-oxyl entity.
The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)33+ (bpy = 2,2'-bipyridine). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d2) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH*+ were determined from the kinetic analysis of the electron-transfer reactions.
The relative stabilities of thymidine-3',5'-diphosphoric acid (1) derived isomeric cations and radicals were calculated and key geometric parameters were thoroughly analyzed. The most probable sites of a hydride-ion (1', 2', 5-Me) and H-atom (4', 5', 5-Me) abstraction were identified, thus allowing prediction of the regioselectivity of potential damage to the deoxyribose ring and thymine moiety caused by carcinogenic agents of electrophilic and radical nature. [structure: see text]