van Besouw A, Wintermans J F
Biochim Biophys Acta. 1978 Apr 28;529(1):44-53. doi: 10.1016/0005-2760(78)90102-9.
Two different enzymes for galactosylation occur in isolated chloroplast envelopes of spinach leaves, UDPgalactose-diglyceride galactosyltransferase and galactolipid-galactolipid galactosyltransferase. The first enzyme is responsible for the biosynthesis of monogalactosyldiglyceride, UDPgalactose being donor of the galactosyl moiety. The second enzyme is responsible for the biosynthesis of digalactosyldiglyceride and higher homologues. The optimum pH for monogalactosyldiglyceride synthesis was found to be 7.5, for digalactosyldiglyceride synthesis 6.5. After incubation at pH 7.4 a Lineweaver-Burk plot indicated two binding sites for UDPgalactose on the UDPgalactose-diglyceride galactosyltransferase with different affinities for UDPgalactose and different activities. Indirectly the galactolipid-galactolipid galactosyltransferase was shown to respond similarly to various UDPgalactose concentrations. However, the second reaction also proceeds in absence of UDPgalactose. It was concluded that the second enzyme does not require the presence of UDPgalactose, but that galactosyl transfer proceeds by direct exchange of galactosyl groups between molecules of galactolipids, or via unknown lipid intermediates, not detected in our system. Results of other investigations will be discussed in the light of the present data.
在菠菜叶分离的叶绿体被膜中存在两种不同的半乳糖基化酶，即UDP - 半乳糖 - 二甘油酯半乳糖基转移酶和半乳糖脂 - 半乳糖脂半乳糖基转移酶。第一种酶负责单半乳糖基二甘油酯的生物合成，UDP - 半乳糖是半乳糖基部分的供体。第二种酶负责二半乳糖基二甘油酯及更高同系物的生物合成。发现单半乳糖基二甘油酯合成的最适pH为7.5，二半乳糖基二甘油酯合成的最适pH为6.5。在pH 7.4孵育后，Lineweaver - Burk图表明UDP - 半乳糖 - 二甘油酯半乳糖基转移酶上有两个UDP - 半乳糖结合位点，对UDP - 半乳糖具有不同的亲和力和不同的活性。间接表明半乳糖脂 - 半乳糖脂半乳糖基转移酶对各种UDP - 半乳糖浓度的反应类似。然而，第二个反应在没有UDP - 半乳糖的情况下也能进行。得出的结论是，第二种酶不需要UDP - 半乳糖的存在，而是半乳糖基转移通过半乳糖脂分子之间半乳糖基的直接交换，或通过在我们的系统中未检测到的未知脂质中间体进行。将根据目前的数据讨论其他研究的结果。
