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    首頁> 外文期刊>Biochimica et Biophysica Acta. Molecular and cell biology of Lipids >Catalytic properties of the expressed acyclic carotenoid 2-ketolases from Rhodobacter capsulatus and Rubrivivax gelatinosus.
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    Catalytic properties of the expressed acyclic carotenoid 2-ketolases from Rhodobacter capsulatus and Rubrivivax gelatinosus.

    機譯：表達的無環類胡蘿卜素2-酮酶的莢膜紅細菌和Rubrivivax gelatinosus的催化性能。

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    Purple photosynthetic bacteria synthesize the acyclic carotenoids spheroidene and spirilloxanthin which are ketolated to spheroidenone and 2,2'-diketospirilloxanthin under aerobic growth. For the studies of the catalytic reaction of the ketolating enzyme, the crtA genes from Rubrivivax gelatinosus and Rhodobacter capsulatus encoding acyclic carotenoid 2-ketolases were expressed in Escherichia coli to functional enzymes. With the purified enzyme from the latter, the requirement of molecular oxygen and reduced ferredoxin for the catalytic activity was determined. Furthermore, the putative intermediate 2-HO-spheroidene was in vitro converted to the corresponding 2-keto product. Therefore, a monooxygenase mechanism involving two consecutive hydroxylation steps at C-2 were proposed for this enzyme. By functional pathway complementation studies in E. coli and enzyme kinetic studies, the product specificity of both enzymes were investigated. It appears that the ketolases could catalyze most intermediates and products of the spheroidene and spirilloxanthin pathway. This was also the case for the enzyme from Rba. capsulatus from which spirilloxanthin synthesis is absent. In general, the ketolase of Rvi. gelatinosus had a better specificity for spheroidene, HO-spheroidene and spirilloxanthin as substrates than the ketolase from Rba. capsulatus.
    機譯：紫色的光合細菌合成了無環類胡蘿卜素和螺黃嘌呤，它們在需氧生長下被酮化為橢球酮和2,2'-二螺螺黃嘌呤。為了研究酮化酶的催化反應，在大腸桿菌中將來自Rubrivivax gelatinosus和Rhodobacter莢膜的crtA基因編碼無環類胡蘿卜素2-酮酶表達為功能性酶。使用來自后者的純化酶，確定了分子氧和還原鐵氧還蛋白對催化活性的需求。此外，將假定的中間體2-HO-橢球體外轉化為相應的2-酮產物。因此，針對這種酶提出了涉及在C-2處兩個連續的羥基化步驟的單加氧酶機理。通過大腸桿菌中的功能途徑互補研究和酶動力學研究，研究了兩種酶的產物特異性?？磥硗济缚梢源呋蠖鄶档闹虚g體和橢球黃素和螺并黃嘌呤途徑的產物。來自Rba的酶也是如此。缺少螺黃嘌呤合成的莢膜。通常，Rvi的酮醇酶。與Rba的酮醇酶相比，明膠酶對球體，HO-球體和螺菌黃質具有更好的特異性。莢膜。

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