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How O-2 binds to he...
How O-2 binds to heme - Reasons for rapid binding and spin inversion
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- Jensen, Kasper (författare)
- Lund University,Lunds universitet,Beräkningskemi,Enheten för fysikalisk och teoretisk kemi,Kemiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Computational Chemistry,Physical and theoretical chemistry,Department of Chemistry,Departments at LTH,Faculty of Engineering, LTH
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- Ryde, Ulf (författare)
- Lund University,Lunds universitet,Beräkningskemi,Enheten för fysikalisk och teoretisk kemi,Kemiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Computational Chemistry,Physical and theoretical chemistry,Department of Chemistry,Departments at LTH,Faculty of Engineering, LTH
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(creator_code:org_t)
- 2004
- 2004
- Engelska.
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Ingår i: Journal of Biological Chemistry. - 1083-351X. ; 279:15, s. 14561-14569
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Abstract
Ämnesord
Stäng
- We have used density functional methods to calculate fully relaxed potential energy curves of the seven lowest electronic states during the binding of O-2 to a realistic model of ferrous deoxyheme. Beyond a Fe-O distance of similar to 2.5 Angstrom, we find a broad crossing region with five electronic states within 15 kJ/mol. The almost parallel surfaces strongly facilitate spin inversion, which is necessary in the reaction of O-2 with heme ( deoxyheme is a quintet and O-2 a triplet, whereas oxyheme is a singlet). Thus, despite a small spin-orbit coupling in heme, the transition probability approaches unity. Using reasonable parameters, we estimate a transition probability of 0.06-1, which is at least 15 times larger than for the nonbiological Fe-O+ system. Spin crossing is anticipated between the singlet ground state of bound oxyheme, the triplet and septet dissociation states, and a quintet intermediate state. The fact that the quintet state is close in energy to the dissociation couple is of biological importance, because it explains how both spin states of O-2 may bind to heme, thereby increasing the overall efficiency of oxygen binding. The activation barrier is estimated to be < 15 kJ/mol based on our results and Mossbauer experiments. Our results indicate that both the activation energy and the spin-transition probability are tuned by the porphyrin as well as by the choice of the proximal heme ligand, which is a histidine in the globins. Together, they may accelerate O-2 binding to iron by &SIM;10(11) compared with the Fe-O+ system. A similar near degeneracy between spin states is observed in a ferric deoxyheme model with the histidine ligand hydrogen bonded to a carboxylate group, i.e. a model of heme peroxidases, which bind H2O2 in this oxidation state.
Ämnesord
- NATURVETENSKAP -- Kemi -- Teoretisk kemi (hsv//swe)
- NATURAL SCIENCES -- Chemical Sciences -- Theoretical Chemistry (hsv//eng)
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