Bautista James A, Rappaport Fabrice, Guergova-Kuras Mariana, Cohen Rachel O, Golbeck John H, Wang Jamie Yehong, Béal Daniel, Diner Bruce A
Central Research and Development, Experimental Station, E. I. du Pont de Nemours & Co., Wilmington, Delaware 19880-0173, USA.
J Biol Chem. 2005 May 20;280(20):20030-41. doi: 10.1074/jbc.M500809200. Epub 2005 Mar 9.
In photosystem I, oxidation of reduced acceptor A(1)(-) through iron-sulfur cluster F(X) is biphasic with half-times of approximately 5-30 ns ("fast" phase) and approximately 150-300 ns ("slow" phase). Whether these biphasic kinetics reflect unidirectional electron transfer, involving only the PsaA-side phylloquinone or bi-directional electron transfer, involving both the PsaA- and PsaB-side phylloquinones, has been the source of some controversy. Brettel (Brettel, K. (1988) FEBS Lett. 239, 93-98) and Joliot and Joliot (Joliot, P., and Joliot, A. (1999) Biochemistry 38, 11130-11136) have attributed to nearby carotenoids electrochromic band shifts, accompanying A(1) reduction, centered at approximately 450 and 500-510 nm. As a test of these assignments, we separately deleted in Synechocystis sp. PCC 6803 the genes that encode phytoene desaturase (encoded by crtP (pds)) and zeta-carotene desaturase (encoded by crtQ (zds)). The pds(-) and zds(-) strains synthesize phytoene and zeta-carotene, respectively, both of which absorb to shorter wavelength than beta-carotene. Compared with wild type, the mutant A(1)(-) (FeS) - A(1)(FeS)(-) difference spectra, measured in cells and photosystem I complexes, retain the electrochromic band shift centered at 450 nm but show a complete loss of the electrochromic band shifts centered at 500-510 nm. Thus, the latter clearly arise from beta-carotene. In the wild type, the electrochromic band shift of the slow phase (centered at 500 nm) is shifted by 6 nm to the blue compared with the fast phase (centered at 506 nm). Thus, the carotenoid pigments acting as electrochromic markers during the fast and slow phases of A(1)(-) oxidation are different, indicating the involvement of both the PsaA- and the PsaB-side phylloquinones in photosystem I electron transport.
在光系统I中，通过铁硫簇F(X)氧化还原态受体A(1)(-)是双相的，半衰期分别约为5 - 30纳秒（“快”相）和约150 - 300纳秒（“慢”相）。这些双相动力学反映的是仅涉及PsaA侧叶醌的单向电子转移，还是涉及PsaA侧和PsaB侧叶醌的双向电子转移，一直存在一些争议。布雷特尔（Brettel, K. (1988) FEBS Lett. 239, 93 - 98）以及乔利奥和乔利奥（Joliot, P., and Joliot, A. (1999) Biochemistry 38, 11130 - 11136）将伴随A(1)还原、中心位于约450纳米以及500 - 510纳米处的电致变色带位移归因于附近的类胡萝卜素。作为对这些归属的验证，我们在集胞藻PCC 6803中分别删除了编码八氢番茄红素去饱和酶（由crtP（pds）编码）和ζ-胡萝卜素去饱和酶（由crtQ（zds）编码）的基因。pds(-)和zds(-)菌株分别合成八氢番茄红素和ζ-胡萝卜素，它们的吸收波长均比β-胡萝卜素短。与野生型相比，在细胞和光系统I复合物中测量的突变体A(1)(-) (FeS) - A(1)(FeS)(-)差异光谱，保留了中心位于450纳米处的电致变色带位移，但中心位于500 - 510纳米处的电致变色带位移完全消失。因此，后者显然源自β-胡萝卜素。在野生型中，慢相（中心位于500纳米）的电致变色带位移相对于快相（中心位于506纳米）向蓝光方向移动了6纳米。因此，在A(1)(-)氧化的快相和慢相中作为电致变色标记的类胡萝卜素色素是不同的，这表明PsaA侧和PsaB侧叶醌都参与了光系统I的电子传递。
