Two Enzymes with a Common Function but Different Heme Ligands in the Forms as Isolated. Optical and Magnetic Properties of the Heme Groups in the Oxidized Forms of Nitrite Reductase, Cytochrome cd<

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• 作者：Myles R. Cheesman ; Stuart J. Ferguson ; James W. B. Moir ; David J. Richardson ; Walter G. Zumft ; and Andrew J. Thomson
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：December 23, 1997
• 年：1997
• 卷：36
• 期：51
• 页码：16267 - 16276
• 全文大小：240K
• 年卷期：v.36,no.51(December 23, 1997)
• ISSN：1520-4995

文摘

It is shown that, in the oxidized state, hemec of Pseudomonas stutzeri (ZoBellstrain)cytochrome cd₁ has histidine-methionine ligationas observed for cytochrome cd₁ fromPseudomonasaeruginosa [Sutherland, J., Greenwood, C., Peterson, J., andThomson, A. J. (1986) Biochem. J.233,893-898]. However, the X-ray structure of Thiosphaerapantotropha cytochrome cd₁ revealsbis-histidineligation for heme c. It is confirmed by EPR andnear-infrared (NIR) MCD measurements that the bis-histidine coordination remains unaltered in the solution phase.Hence, the difference between the hemec ligation states defines two distinct classes of oxidizedcytochromes cd₁ as isolated. A weak featureinthe T. pantotropha NIR MCD at 1900 nm suggeststhat a small population of heme c has histidine-methionine coordination. The ligation state of hemed₁ cannot be defined with the same level ofconfidence,because the porphyrin-to-Fe(III) charge-transfer (CT) bands areless well characterized for this class ofpartially reduced porphyrin ring. However, variable temperatureabsorption and MCD spectra show that,in the T. pantotropha enzyme, hemed₁ exists in a thermal low-spin/high-spinmixture with the low-spinas the ground state, whereas in P. stutzericytochrome cd₁, and d₁heme is low-spin at all temperatures.A weak band, assigned as the heme d₁porphyrin-ges/gifchars/pi.gif" BORDER=0 >(a_1u,a_2u)-to-ferric(d)charge-transfer transition has beenidentified for the first time at 2170 nm. Its magnetic propertiesshow the heme d₁ to have an unusual(d_xz,yz)⁴(d_xy)¹electronic ground state as is found for low-spin Fe(III) chlorins[Cheesman, M. R., and Walker,F. A. (1996) J. Am. Chem.Soc. 118, 7373-7380]. It is proposed thatthe localization of the Fe(III)unpaired d-electron in an orbital lying in the heme plane may decreasethe affinity of the Fe(III) heme forunsaturated ligands such as NO. Although hemed₁ in the enzymes from P.stutzeri and T.pantotrophashows different temperature-dependent spin properties, the positions ofthe low-spin Fe(III) ges/gifchars/alpha.gif" BORDER=0>-absorptionband, at ~640 nm, are very similar to those observed for cytochromescd₁ from eight other sources,suggesting that all have similar strength fields from the axial ligandsand, hence, that all have the samecoordination, namely histidine-tyrosine or possibly histidine-hydroxideat the heme.