Conformational Substates of the Oxyheme Centers in and mg src="http://pubs.acs.org/images/gifchars/beta2.gif" border="0" align="m

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• 作者：Roman Davydov ; Viktoria Kofman ; Judith M. Nocek ; Robert W. Noble ; Hilda Hui ; and Brian M. Hoffman
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：May 25, 2004
• 年：2004
• 卷：43
• 期：20
• 页码：6330 - 6338
• 全文大小：170K
• 年卷期：v.43,no.20(May 25, 2004)
• ISSN：1520-4995

文摘

Exposure of frozen solutions of oxyhemoglobin to mages/gifchars/gamma.gif" BORDER=0 >-irradiation at 77 K yields EPR- andENDOR-active, one-electron-reduced oxyheme centers which retain the conformation of the diamagneticprecursor. EPR spectra have been collected for the centers produced in human HbO₂ and isolated mages/gifchars/alpha.gif" BORDER=0>O₂and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂ chains, as well as mages/gifchars/alpha.gif" BORDER=0>O₂mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">(Zn), mages/gifchars/alpha.gif" BORDER=0>(Zn)mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂, and mages/gifchars/alpha.gif" BORDER=0>O₂mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">(Fe³⁺) hybrids, each in frozen buffer and infrozen glasses that form in the presence of glycols and sugars and also in the presence of IHP. Thesereveal two spectroscopically distinct classes of such ferriheme centers (g₁ mages/entities/le.gif"> 2.25), denoted A and B.Averaged over many similar sites, the A-center has a rhombic EPR signal with a g-tensor, g^A = [2.248(4),2.146(1), 1.966(1)]; the B-center exhibits a less anisotropic EPR signal, g^B = [2.216(3), 2.118(2), 1.966(1)].Early measurement had suggested that, in the cryoreduced HbO₂ tetramer, the two centers corresponded to the two different chains [Symons, M. C. R., and Petersen, R. L. (1978) Proc. R. Soc. London,Ser. B 201, 285-300]. However, the present EPR and ENDOR results show that the two signals insteadreflect the fact that the parent oxyhemes exist in two major conformational substates and that this is truefor both mages/gifchars/alpha.gif" BORDER=0>O₂ and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂ subunits: mages/gifchars/alpha.gif" BORDER=0>O₂^A (minor species) and mages/gifchars/alpha.gif" BORDER=0>O₂^B (major species); mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂^A(major species)and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂^B (minor species). Similar behavior is seen for MbO₂ [Kappl, R., Höhn-Berlage, M., Hüttermann,J., Bartlett, N., and Symons, M. C. R. (1985) Biochim. Biophys. Acta 827, 327-343]. The A/B g-tensorsof mages/gifchars/alpha.gif" BORDER=0>O₂ and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂ chains vary little with the environment of the chains, while the relative populations ofthe substates depend greatly on glycols and IHP. These results suggest a quaternary influence on theoxyheme distal pocket of mages/gifchars/alpha.gif" BORDER=0> chains and that the glycol-induced changes in the substate populations of theR-state HbO₂ tetramer are largely associated with the mages/gifchars/alpha.gif" BORDER=0>O₂ subunit. ¹H ENDOR spectra from the distalhistidine proton hydrogen-bonded to the peroxo ligand show very different isotropic coupling for the A-and B-centers. Analysis of the spectroscopic data suggests that the A- and B-centers represent differentorientations of the oxyheme O₂ ligand relative to the distal histidine. It is likely that the A and Bconformational substates in the mages/gifchars/alpha.gif" BORDER=0>O₂ and mages/gifchars/beta2.gif" BORDER=0 ALIGN="middle">O₂ subunits differ not only in their tertiary structures but intheir affinities for O₂.