ELECTRON-TUNNELING AND AB-INITIO CALCULATIONS RELATED TO THE ONE-ELECTRON OXIDATION OF NAD(P)H BOUND TO CATALASE

Models for NAD(P)H 1e(-) oxidation in bovine catalase were studied usi ng Hartree-Fock ab initio calculations, along with information taken f rom the published X-ray structure of the enzyme. Geometries and energi es of ground states and transition states were calculated at the 6-31G () level for N-methyl-1,4-dihydropyridine and N-methyl-1,4-dihydronic otinamide undergoing the pathway (i) 1e(-) oxidation to yield the radi cal cation, (ii) general-base-catalyzed (hydroxide and/or imidazole) d eprotonation of the radical cation to yield the neutral radical, and ( iii) 1e(-) oxidation of the neutral radical to the N-methylpyridinium or N-methylnicotinamide cation. Barrier heights for deprotonation of t he radical cation intermediates were calculated to be 7-11 kcal/mol. K inetic isotope effects were calculated for general-base-catalyzed depr otonation of the N-methyl-1,4-dihydropyridine radical cation and the 4 ,4-dideuterio species and found to be k(H)/k(D2) = 5.38 (hydroxide) or 3.64 (imidazole), in qualitative agreement with published experimenta l isotope effects for the analogous deprotonation of N-methyl-1,10-dih ydroacridan or the N-methyl-1,10-dideuterioacridan radical cation. In the calculated transition state for imidazole deprotonation of the N-m ethyl-1,4-dihydronicotinamide radical cation, an unusual short contact was calculated and interpreted as a hydrogen bond (2.35 Angstrom) bet ween the amide oxygen and the hydrogen attached to C2 of imidazole. Si milar hydrogen bonds were also observed and calculated at the 3-21G an d 6-31G() levels between His234 of catalase and the amide oxygen of b ound NAD(P)H and complexes of N,N'-dimethyl-1,4-dihydronicotinamide or cis-N-methylformamide with N-methylimidazole. Comparison of these res ults to the X-ray structure of bovine catalase allows for further inte rpretation of the possible roles of the imidazole bases His234 and His 304 and the hydrogen-bonded contacts in the NAD(P)H binding site. Elec tron tunneling pathways between NAD(P)H and the iron protoporphyrin IX (PP-M) axial tyrosinate ligand Tyr357 in molecular dynamics and X-ray crystal structures of bovine catalase were calculated using PATHWAYS II (version 2.01). The pathways which were calculated included those i nvolving the amino acid residue Tyr214, which is near the NAD(P)H bind ing site. Coupling involving Tyr357 was not particularly efficient; ho wever, strong coupling between Tyr214 and iron-protoporphyrin IX was o bserved. These pathways may be important if electron transfer is stepw ise; i.e., Tyr214 oxidized first, followed by NAD(P)H.