The synthesis of galactosydiacylglycerols by chloroplast envelopes

Lipid requirement and kinetic studies of solubilized UDP-galactose:diacylglycerol galactosyltransferase activity from spinach chloroplast envelope membranes

We have demonstrated a lipid requirement for the UDPgalactose:1,2-diacylglycerol 3-beta-D-galactosyl-transferase (or monogalactosyldiacylglycerol synthase; EC 2.4.1.46), an enzyme involved in the biosynthesis of monogalactosyldiacylglycerol, solubilized from chloroplast envelope membranes and partially purified by hydroxyapatite chromatography. The enzyme fraction was highly delipidated (<0.1 mg of lipid per mg of protein), and addition of lipids extracted from chloroplast membranes was necessary to reveal the activity. Acidic glycerolipids, and especially phosphatidylglycerol, were the best activators of the enzyme. The preparation of a delipidated enzyme fraction and the development of optimal assay conditions were prerequisites for the determination of the kinetic parameters for the hydrophobic substrate of the enzyme, diacylglycerol. In addition, we have demonstrated the existence of two substrate-binding sites: a hydrophobic one for diacylglycerol and a hydrophilic one for UDP-galactose.

The catalytic site of monogalactosyldiacylglycerol synthase from spinach chloroplast envelope membranes. Biochemical analysis of the structure and of the metal content

We have analyzed the structure of the active site of monogalactosyldiacylglycerol (MGDG) synthase from spinach chloroplast envelope. Since purification of this membrane-embedded enzyme yielded such low amounts of protein that analyses of the amino acid sequence were so far impossible, we used indirect strategies. Analyses of the inhibition of MGDG synthase by UDP and of its inactivation by citraconic anhydride first indicated that the enzyme contained two functionally independent and topologically distinct binding sites for each substrate. Whereas MGDG synthase binds both the nucleotidic part of UDP-Gal and the acyl chains of 1,2-diacylglycerol, UDP is a competitive inhibitor relatively to UDP-Gal, while it does not compete with 1,2-diacylglycerol for binding on the enzyme. The UDP-Gal-binding site contains lysine residues, as demonstrated for UDP-Gal-binding sites from all galactosyltransferases studied so far. Radiolabeling of MGDG synthase by sulfur labeling reagent, a 35S-labeled lysine-blocking reagent, confirmed that MGDG synthase was a polypeptide with a low molecular mass (around 20 kDa). The 1,2-diacylglycerol-binding site contains reduced cysteines and at least one metal. The divalent cation(s) associated to apo-MGDG synthase was not unambiguously identified, but the results suggest that it could be zinc. Therefore, MGDG synthase presents some structural features in common with diacylglycerol-manipulating enzymes, such as protein kinase C and 1,2-diacylglycerol kinase, which are characterized by the presence of a ubiquitous Cys6His2 domain involved in zinc coordination in their 1,2-diacylglycerol-binding domains.

Galactolipid formation in chloroplast envelopes. II. Isolation-induced changes in galactolipid composition

Spinach envelopes were isolated at pH 8.5, which is inhibitory to interlipid galactosyl transfer, and at pH 7.2, which is close to values used in most recent work on chloroplast envelopes. Peptide patterns were identical, but galactolipid compositions differed markedly. These changes are such as could be explained by interlipid galactosyl transfer during isolation at pH 7.2, i.e. a strong decrease in monogalactolipid and a corresponding increase in tri- and tetragalactolipids and diacylglycerol. Digalactolipid was little affected. It was further observed that the interlipid galactosyl transfer was restricted to lipids rich in hexadecatrienoic acid. Arguments are presented that the lipid composition of envelopes in vivo is better represented by the results obtained at high pH. The results will be discussed in relation to previous data on galactolipid composition and biosynthesis in envelopes.

Galactolipid formation in chloroplast envelopes. III. Some observations on galactose incorporation by envelopes with high and low content of diacylglycerol

Chloroplast envelopes from spinach, isolated at two different pH values, were incubated with UDP[14C]galactose at pH values of 6.0, 7.2 and 8.5 and rates and patterns of galactolipid synthesis were measured. Envelopes isolated at pH 8.5 and considered to be poor in initial diacylglycerol content were generally inhibited, especially at high pH. At this pH, where interlipid galactosyl transfer is very slow, incorporation rates reflect diacylglycerol content. At lower ph values in both types of envelope, interlipid galactosyl transferase is active. The highest rates of interlipid galactosyl transferase are seen at pH 6.0 in diacylglycerol-poor envelopes and at pH 7.2 in diacylglycerol-rich envelopes. The latter type of envelope shows considerable activity of monogalactolipid acylase at low pH 6.0, which seems to compete with interlipid galactosyl transferase for monogalactolipid. After removal of UDPGal and transfer of the envelopes to higher pH, acylmonogalactolipid can be transformed again into monogalactolipid and this regenerated lipid can again be transformed into digalactolipid and into higher-homologous galactolipids. Intact spinach chloroplast in similar experiments behave essentially as isolated envelopes. Results are compared with patterns of galactolipid synthesis in vivo and considered as an indication that interlipid galactosyl transferase may be regulated not only by pH but also by a factor originating in the cytoplasm. Results are also discussed in the light of recent suggestions that galactolipids are synthesized in a multi-enzyme complex in the envelope and also considered as a contribution towards understanding the regulation of the mono-/digalactolipid ratio. The observations on the behaviour of acyltransferase and the reversibility of its reaction may constitute a first step to an understanding of a physiological role for this enzyme.