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:
- DE-AC02-76CH00016
- OSTI ID:
- 6875807
- Journal Information:
- J. Am. Chem. Soc.; (United States), Vol. 103:3
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CATALASE
PEROXIDASES
PORPHYRINS
CARBON 13
CATALYTIC EFFECTS
DEUTERIUM
ELECTRONIC STRUCTURE
IRON COMPLEXES
MOLECULAR ORBITAL METHOD
NITROGEN 15
NUCLEAR MAGNETIC RESONANCE
OXIDATION
RADICALS
CARBON ISOTOPES
CARBOXYLIC ACIDS
CHEMICAL REACTIONS
COMPLEXES
ENZYMES
EVEN-ODD NUCLEI
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROGEN ISOTOPES
ISOTOPES
LIGHT NUCLEI
MAGNETIC RESONANCE
NITROGEN ISOTOPES
NUCLEI
ODD-EVEN NUCLEI
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXIDOREDUCTASES
RESONANCE
STABLE ISOTOPES
TRANSITION ELEMENT COMPLEXES
400301* - Organic Chemistry- Chemical & Physicochemical Properties- (-1987)