The oxidized and semiquinone anion radical forms of flavin mononucleotide carried by flavocytochrome b2 and L-lactate monooxygenase have been studied by resonance Raman (RR) spectroscopy. The RR spectra of their oxidized forms are compared with previously published RR data on various flavins and flavoproteins. Taking as a support available X-ray crystallographic data on flavoproteins, we have found correlations between the frequencies of RR bands II (1575-1588 cm-1), III (1534-1557 cm-1), and X (1244-1266 cm-1) and the H-bonding environment and/or the structure of the flavin ring. The present RR data provide strong evidence that the electron density, the conformation, and the H-bonding environment of the oxidized flavin mononucleotide of flavocytochrome b2 and L-lactate monooxygenase are different. As far as the anionic semiquinone form of flavoproteins is concerned, the behavior of two bands observed at 1280-1300 and 1320-1350 cm-1 suggests that they have vibrational origins similar to those of RR bands II and III of oxidized compounds. On this basis, the differences in conformation and H-bonding environment of the isoalloxazine ring, observed for the oxidized form of flavocytochrome b2 and L-lactate monooxygenase, appear to be preserved upon one-electron reduction of the flavin. For both flavoproteins, the RR spectra of the semiquinone form are affected by pyruvate binding. The data are interpreted in the frame of a change in H-bonding interaction of the C4&dbd;O carbonyl group of the flavin without significant alteration of the isoalloxazine conformation. This modification in electrostatic interaction quantitatively accounts for the pyruvate-induced changes of the oxidized/semiquinone and semiquinone/reduced redox potentials of the flavoproteins. Considering the high homology in the flavin catalytic sites of flavocytochrome b2 and L-lactate monooxygenase, the observed differences in H-bonding environment and conformation of the FMN ring are related to the different biological functions of the two flavoproteins.

译文

通过共振拉曼光谱研究了黄素细胞色素b2和L-乳酸单加氧酶携带的黄素单核苷酸的氧化和半醌阴离子自由基形式。将其氧化形式的RR光谱与先前发表的有关各种黄素和黄素蛋白的RR数据进行比较。作为对黄素蛋白的可用X射线晶体学数据的支持,我们发现RR波段II(1575-1588 cm-1),III(1534-1557 cm-1)和X(1244-1266 cm)的频率之间存在相关性-1)和H键合环境和/或黄素环的结构。目前的RR数据提供了有力的证据,证明黄素细胞色素b2和L-乳酸单加氧酶的氧化黄素单核苷酸的电子密度,构象和H键环境不同。就黄素蛋白的阴离子半醌形式而言,在1280-1300和1320-1350 cm-1处观察到的两个谱带的行为表明,它们的振动起源与氧化化合物的RR谱带II和III相似。在此基础上,对黄素细胞色素b2和L-乳酸单加氧酶的氧化形式观察到的异恶嗪环的构象和H键环境的差异似乎在黄素的单电子还原中得以保留。对于两种黄素蛋白,丙酮酸结合都会影响半醌形式的RR光谱。在黄素的C 4 -O羰基的H键相互作用变化的框架内解释了数据,而异四恶嗪构象没有显着改变。静电相互作用的这种改变定量地解释了黄酮蛋白的丙酮酸诱导的氧化/半醌和半醌/还原的氧化还原电势的变化。考虑到黄素细胞色素b2和L-乳酸单加氧酶在黄素催化位点上的高度同源性,观察到的H键环境和FMN环构象的差异与两种黄素蛋白的不同生物学功能有关。

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