Electron pathways in catalase and peroxidase enzymic catalysis. Metal and macrocycle oxidations of iron porphyrins and chlorins
Charge iterative extended Hueckel calculations are presented for compound II, the one-electron oxidation intermediate of horseradish peroxidase (HRP), and for compounds I, the two-electron oxidation transients of HRP and catalase (CAT) observed in the catalytic cycles of the hydroperoxidase enzymes. Compound II is described in terms of a ferryl configuration (O = Fe/sup IV/), and compounds I are described as ferrylporphyrin ..pi..-cation radicals. The validity of the iron ..pi..-cation calculations is supported by favorable comparison of parallel computations for porphyrin ..pi.. cations of diamagnetic metals with new and previously reported ESR results for radicals of zinc tetrabenz-, meso-tetramethyl, (/sup 14/N and /sup 15/N) tetraphenyl-, and magnesium (/sup 1/H and /sup 2/H) octaethylporphyrins. The calculated electronic configurations and unpaired spin density profiles for the ferryl ..pi.. cations satisfactorily account for the physical properties reported for compounds I of HRP (in the native protoporphyrin IX form or reconstituted with deuteroporphyrin), chloroperoxidase, and CAT. The ground states of the ..pi.. cations, a/sub 1u/ or a/sub 2u/, are determined by peripheral substitution and axial ligation, and the axial ligand of CAT I is predicted to differ from that of HRP I. The combination of model studies and calculations suggests that /sup 2/H, /sup 13/C, and /sup 15/N NMR studies of isotopically substituted proto and deutero HRP I would confirm the electronic profiles predicted. /sup 15/N NMR in particular would clearly discriminate between a/sub 1u/ and a/sub 2u/ configurations. As an additional test of the ferryl ..pi..-cation hypothesis, calculations are presented for a proposed ferrylchlorin ..pi.. cation of Neurospora crassa catalase, which contains an iron chlorin prosthetic group. Compound I of this unusual heme is predicted to occupy an a/sub 2/ ground state with the spin distribution and optical spectra reported here for synthetic chlorin radicals.
- Research Organization:
- Brookhaven National Lab., Upton, NY
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 6875807
- Journal Information:
- J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 103:3; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
CARBON 13
CARBON ISOTOPES
CARBOXYLIC ACIDS
CATALASE
CATALYTIC EFFECTS
CHEMICAL REACTIONS
COMPLEXES
DEUTERIUM
ELECTRONIC STRUCTURE
ENZYMES
EVEN-ODD NUCLEI
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROGEN ISOTOPES
IRON COMPLEXES
ISOTOPES
LIGHT NUCLEI
MAGNETIC RESONANCE
MOLECULAR ORBITAL METHOD
NITROGEN 15
NITROGEN ISOTOPES
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ODD-EVEN NUCLEI
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXIDATION
OXIDOREDUCTASES
PEROXIDASES
PORPHYRINS
RADICALS
RESONANCE
STABLE ISOTOPES
TRANSITION ELEMENT COMPLEXES