Protochlorophyllide Oxidoreductase: "Dark" Reactions of a Light-Driven Enzyme
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- 作者：Derren J. Heyes ; Alexander V. Ruban ; and C. Neil Hunter
- 刊名：Biochemistry
- 出版年：2003
- 出版时间：January 21, 2003
- 年：2003
- 卷：42
- 期：2
- 页码：523 - 528
- 全文大小：287K
- 年卷期：v.42,no.2(January 21, 2003)
- ISSN：1520-4995
文摘
The light-driven enzyme NADPH:protochlorophyllide oxidoreductase (POR) catalyzes thereduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), a key regulatory step in the chlorophyllbiosynthesis pathway. As POR is light activated, it allows intermediates in the reaction pathway to beobserved by initiating catalysis with illumination at low temperatures, a technique that has recently beenused to study the initial photochemistry. Here, we use low-temperature spectroscopy to show that thecatalytic mechanism of POR involves two additional steps, which do not require light and have beentermed the "dark" reactions. The first of these involves the conversion of the product of the initial light-driven reaction, a nonfluorescent radical species, into a new intermediate that has an absorbance maximumat 681 nm and a fluorescence peak at 684 nm. During the second dark step this species gradually blueshifts to yield the product, Chlide. The temperature dependence for each of these two processes wasmeasured; the data revealed that these steps could only occur close to or above the "glass transition"temperature of proteins, suggesting that domain movements and/or reorganization of the protein are requiredfor these stages of the catalytic mechanism.