Komori M, Ghosh R, Takaichi S, Hu Y, Mizoguchi T, Koyama Y, Kuki M
Faculty of Science, Kwansei Gakuin University, Nishinomiya, Japan.
Biochemistry. 1998 Jun 23;37(25):8987-94. doi: 10.1021/bi9730947.
The carotenoids accumulated by a mutant Rhodospirillum rubrum ST4, containing a single Tn5 lesion in the pathway for carotenoid biosynthesis, were analyzed by HPLC, 1H NMR spectroscopy, and field desorption mass spectrometry. The main carotenoid was identified as 3,4,3',4'-tetrahydrospirilloxanthin, and the four minor carotenoids were identified as rhodopin, 3,4-dihydroanhydrorhodovibrin, 3', 4'-dihydrorhodovibrin, and 1,1'-dihydroxylycopene. The C-3,4 and C-3',4' bonds of all 5 carotenoids are saturated, and they have 11 conjugated double bonds. With the exception of rhodopin, which is a normal intermediate of the wild-type pathway, all of the carotenoids are not naturally occurring. The Tn5 lesion was assigned to rhodopin 3,4-desaturase which is proposed to catalyze dehydrogenation at both ends of the symmetrical spirilloxanthin derivative. An unexpected finding was that the enzymes following rhodopin 3,4-desaturase are still able to end-modify the 3,4-, and 3',4'-saturated precursors and that the order of methylation and hydroxylation is not obligatory. It is proposed that the observed nonnatural carotenoids can be explained by the inclusion of a cryptic branch, unmasked by the absence of rhodopin 3,4-desaturase, in the established linear pathway for spirilloxanthin biosynthesis. This is the first example of latent branching of the carotenoid biosynthesis pathway exhibited by a carotenoid mutant of a phototrophic bacterium.
对一株类胡萝卜素生物合成途径中含有单个Tn5损伤的突变型深红红螺菌ST4积累的类胡萝卜素进行了高效液相色谱（HPLC）、¹H核磁共振光谱和场解吸质谱分析。主要类胡萝卜素被鉴定为3,4,3',4'-四氢螺旋藻黄素，四种次要类胡萝卜素被鉴定为玫红菌素、3,4-二氢脱水玫红红素、3',4'-二氢玫红红素和1,1'-二羟基番茄红素。所有5种类胡萝卜素的C-3,4和C-3',4'键均饱和，且它们具有11个共轭双键。除了作为野生型途径正常中间体的玫红菌素外，所有类胡萝卜素均非天然存在。Tn5损伤被定位到玫红菌素3,4-去饱和酶，该酶被认为催化对称螺旋藻黄素衍生物两端的脱氢反应。一个意外的发现是，在玫红菌素3,4-去饱和酶之后的酶仍能够对3,4-和3',4'-饱和前体进行末端修饰，并且甲基化和羟基化的顺序并非是固定不变的。有人提出，观察到的非天然类胡萝卜素可以通过在已确立的螺旋藻黄素生物合成线性途径中包含一个因缺乏玫红菌素3,4-去饱和酶而显现出来的隐蔽分支来解释。这是光养细菌类胡萝卜素突变体所表现出的类胡萝卜素生物合成途径潜在分支的首个实例。
