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Multiple forms of P700-chlorophyll a-protein complexes from Synechococcus sp.: The iron,quinone and carotenoid contents
Authors:Yuichiro Takahashi  Koitsu Hirota  Sakae Katoh
Institution:(1) Department of Pure and Applied Sciences, College of Arts and Sciences, University of Tokyo, Komaba, Meguro-ku, 153 Tokyo, Japan;(2) Department of Chemistry, Sagami Institute of Technology, Nishikaigan, Tsujido, 251 Fujisawa-shi, Kanagawa, Japan
Abstract:The iron, quinone and carotenoid contents of five P700-chlorophyll a-protein complexes having different subunit structures (CP1-a,-b,-c,-d and-e) from the thermophilic cyanobacterium Synechococcus sp. were determined. CP1-a,-b,-c and-d that commonly have four polypeptides of 62,000, 60,000, 14,000 and 10,000 dalton contained 10–14 iron atoms per P700, whereas CP1-e that lacks the two small polypeptides was totally devoid of iron. All CP1 complexes contained vitamin K1 at the molar ratio of vitamin K1 to P700 of about 2 except CP1-e that had only 0.4 vitamin K1 per P700. No plastoquinone was detected in five CP1 complexes. Out of four major carotenoids, beta-carotene, zeaxanthin, caloxanthin, and myxoxanthophyll, present in the thylakoid membranes, only beta-carotene was found in isolated CP1 complexes; all CP1 complexes contained about 10 beta-carotene molecules per P700. The flourescence excitation spectrum showed that beta-carotene serves as an efficient antenna of photosystem I. It is concluded that all iron atoms and a larger fraction of vitamin K1 molecules present in the photosystem I reaction center complex are associated with the 14,000 and 10,000 dalton polypeptides, whereas beta-carotene exclusively binds to the large polypeptides which carry the functional and antenna chlorophyll a. The possible functions of iron and vitamin K1 as electron carriers and of beta-carotene as the accessary pigment and a photoprotectant in the photosystem I complexes are discussed.
Keywords:beta-carotene" target="_blank">gif" alt="beta" align="MIDDLE" BORDER="0">-carotene  iron  photosystem I reaction center complex  P700  (Synechococcus sp  )  vitamin K1
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