Accessibility of the Distal Heme Face, Rather than Fe-His Bond Strength, Determines the Heme-Nitrosyl Coordination Number of Cytochromes c': Evidence from Spectroscopic Studies

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• 作者：Colin R. Andrew ; Lenord J. Kemper ; Tammy L. Busche ; Arianne M. Tiwari ; Michael C. Kecskes ; James M. Stafford ; Lea C. Croft ; Shen Lu ; Pierre Moë ; nne-Loccoz ; Willa Huston ; James W. B. Moir ; and Robe
• 刊名：Biochemistry
• 出版年：2005
• 出版时间：June 21, 2005
• 年：2005
• 卷：44
• 期：24
• 页码：8664 - 8672
• 全文大小：229K
• 年卷期：v.44,no.24(June 21, 2005)
• ISSN：1520-4995

文摘

The heme coordination chemistry and spectroscopic properties of Rhodobacter capsulatuscytochrome c' (RCCP) have been compared to data from Alcaligenes xylosoxidans (AXCP), with the aimof understanding the basis for their different reactivities with nitric oxide (NO). Whereas ferrous AXCPreacts with NO to form a predominantly five-coordinate heme-nitrosyl complex via a six-coordinateintermediate, RCCP forms an equilibrium mixture of six-coordinate and five-coordinate heme-nitrosylspecies in approximately equal proportions. Ferrous RCCP and AXCP both exhibit high Fe-His stretchingfrequencies (227 and 231 cm^-1, respectively), suggesting that factors other than the Fe-His bond strengthaccount for their differences in heme-nitrosyl coordination number. Resonance Raman spectra of ferrous-nitrosyl RCCP confirm the presence of both five-coordinate and six-coordinate heme-NO complexes.The six-coordinate heme-nitrosyl of RCCP exhibits a fairly typical Fe-NO stretching frequency (569cm^-1), in contrast to the relatively high value (579 cm^-1) of the AXCP six-coordinate heme-nitrosylintermediate. It is proposed that NO experiences greater steric hindrance in binding to the distal face ofAXCP, as compared to RCCP, leading to a more distorted Fe-N-O geometry and an elevated Fe-NOstretching frequency. Evidence that RCCP has a more accessible distal coordination site than in AXCPstems from the fact that ferric RCCP readily forms a heme complex with exogenous imidazole, whereasAXCP does not. A model is proposed in which distal heme-face accessibility, rather than the proximalFe-His bond strength, determines the heme-nitrosyl coordination number in cytochromes c'.