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A comparison of the reaction mechanisms of iron- and manganese-containing 2,3-HPCD: an important spin transition for manganese
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. (Theoretical Biochemistry)
Polish Academy of Sciences.
Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. (Theoretical Biochemistry)
(Theoretical Biochemistry)
2008 (engelsk)Inngår i: Journal of Biological Inorganic Chemistry, ISSN 0949-8257, E-ISSN 1432-1327, Vol. 13, nr 6, s. 929-40Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Homoprotocatechuate (HPCA) dioxygenases are enzymes that take part in the catabolism of aromatic compounds in the environment. They use molecular oxygen to perform the ring cleavage of ortho-dihydroxylated aromatic compounds. A theoretical investigation of the catalytic cycle for HPCA 2,3-dioxygenase isolated from Brevibacterium fuscum (Bf 2,3-HPCD) was performed using hybrid DFT with the B3LYP functional, and a reaction mechanism is suggested. Models of different sizes were built from the crystal structure of the enzyme and were used in the search for intermediates and transition states. It was found that the enzyme follows a reaction pathway similar to that for other non-heme iron dioxygenases, and for the manganese-dependent analog MndD, although with different energetics. The computational results suggest that the rate-limiting step for the whole reaction of Bf 2,3-HPCD is the protonation of the activated oxygen, with an energy barrier of 17.4 kcal/mol, in good agreement with the experimental value of 16 kcal/mol obtained from the overall rate of the reaction. Surprisingly, a very low barrier was found for the O-O bond cleavage step, 11.3 kcal/mol, as compared to 21.8 kcal/mol for MndD (sextet spin state). This result motivated additional studies of the manganese-dependent enzyme. Different spin coupling between the unpaired electrons on the metal and on the evolving substrate radical was observed for the two enzymes, and therefore the quartet spin state potential energy surface of the MndD reaction was studied. The calculations show a crossing between the sextet and the quartet surfaces, and it was concluded that a spin transition occurs and determines a barrier of 14.4 kcal/mol for the O-O bond cleavage, which is found to be the rate-limiting step in MndD. Thus the two 83% identical enzymes, using different metal ions as co-factors, were found to have similar activation energies (in agreement with experiment), but different rate-limiting steps.

sted, utgiver, år, opplag, sider
Berlin: Springer , 2008. Vol. 13, nr 6, s. 929-40
HSV kategori
Identifikatorer
URN: urn:nbn:se:su:diva-30694DOI: 10.1007/s00775-008-0380-9ISI: 000257934500009PubMedID: 18458966OAI: oai:DiVA.org:su-30694DiVA, id: diva2:273631
Tilgjengelig fra: 2009-10-26 Laget: 2009-10-22 Sist oppdatert: 2017-12-12bibliografisk kontrollert
Inngår i avhandling
1. Reaction Mechanisms of Metalloenzymes and Synthetic Model Complexes Activating Dioxygen: A Computational study
Åpne denne publikasjonen i ny fane eller vindu >>Reaction Mechanisms of Metalloenzymes and Synthetic Model Complexes Activating Dioxygen: A Computational study
2009 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Quantum chemistry has nowadays become a powerful and efficient tool that can be successfully used for studies of biosystems. It is therefore possibleto model the enzyme active-site and the reactions undergoing into it, as well as obtaining quite accurate energetic profiles. Important conclusions can be drawn from such profiles about the  plausibility of different putative mechanisms.

Density Functional Theory is used in the present thesis for investigation of the catalytic mechanism of dioxygenase metallo-enzymes and synthetic model complexes. Three enzymes were studied – Homoprotocatechuate 2,3-dioxygenase isolated from Brevibacterium fuscum (Bf 2,3-HPCD), Manganese-Dependent Homoprotocatechuate 2,3-Dioxygenase (MndD) and Homogentisate Dioxygenase (HGD). Models consisting of 55 to 208 atoms have been built from X-ray crystal structures and used in the calculations. The computed energies were put in energy curves and were used for estimation of the feasibility of the suggested reaction mechanisms. A non-heme [(L4Me4)Fe(III)]+3 complex that mimics the reactivity of intradiol dioxygenases, and a heme [T(o-Cl)PPFe] complex catalyzing the stepwise oxidation of cyclohexane to adipic acid, were also studied.

For the enzymes and the non-heme biomimetic complex the reaction was found to follow a mechanism that was previously suggested for extradiol and intradiol dioxygenases – ordered substrates binding and formation of peroxo species, which further undergoes homolytic O-O bond cleavage. Different reaction steps appear to be rate limiting in the particular cases: proton transfer from the substrate to the peroxide in Bf 2,3-HPCD, the formation of the peroxo bridge in HGD and the biomimetic complex, and notably, spin transition in MndD.

The catalytic oxidation of cyclohexane to adipic acid in the presence of molecular oxygen as oxidant was studied, a reaction of great importance for the chemical industry. Reaction mechanism is suggested, involving several consecutive oxidative steps. The highest calculated enthalpy of activation is 17.8 kcal/mol for the second oxidative step.

sted, utgiver, år, opplag, sider
Stockholm: Department of Physics, Stockholm University, 2009. s. 104
Emneord
catalysis, density functional theory, extradiol, intradiol, dioxygenases, oxygen, biomimetic complexes, heme, spin transition, adipic acid
HSV kategori
Forskningsprogram
kvantkemi
Identifikatorer
urn:nbn:se:su:diva-30706 (URN)978-91-7155-965-4 (ISBN)
Disputas
2009-11-20, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, Sweden, 10:00 (engelsk)
Opponent
Veileder
Merknad
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: In progress, Paper 5: In progressTilgjengelig fra: 2009-10-28 Laget: 2009-10-23 Sist oppdatert: 2010-04-09bibliografisk kontrollert

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