Protein Conformational Gating of Enzymatic Activity in Xanthine Oxidoreductase

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• 作者：Hiroshi Ishikita ; Bryan T. Eger ; Ken Okamoto ; Takeshi Nishino ; Emil F. Pai
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：January 18, 2012
• 年：2012
• 卷：134
• 期：2
• 页码：999-1009
• 全文大小：755K
• 年卷期：v.134,no.2(January 18, 2012)
• ISSN：1520-5126

文摘

In mammals, xanthine oxidoreductase can exist as xanthine dehydrogenase (XDH) and xanthine oxidase (XO). The two enzymes possess common redox active cofactors, which form an electron transfer (ET) pathway terminated by a flavin cofactor. In spite of identical protein primary structures, the redox potential difference between XDH and XO for the flavin semiquinone/hydroquinone pair (E_sq/hq) is 170 mV, a striking difference. The former greatly prefers NAD⁺ as ultimate substrate for ET from the iron鈥搒ulfur cluster FeS-II via flavin while the latter only accepts dioxygen. In XDH (without NAD⁺), however, the redox potential of the electron donor FeS-II is 180 mV higher than that for the acceptor flavin, yielding an energetically uphill ET. On the basis of new 1.65, 2.3, 1.9, and 2.2 脜 resolution crystal structures for XDH, XO, the NAD⁺- and NADH-complexed XDH, E_sq/hq were calculated to better understand how the enzyme activates an ET from FeS-II to flavin. The majority of the E_sq/hq difference between XDH and XO originates from a conformational change in the loop at positions 423鈥?33 near the flavin binding site, causing the differences in stability of the semiquinone state. There was no large conformational change observed in response to NAD⁺ binding at XDH. Instead, the positive charge of the NAD⁺ ring, deprotonation of Asp429, and capping of the bulk surface of the flavin by the NAD⁺ molecule all contribute to altering E_sq/hq upon NAD⁺ binding to XDH.