Reaction: UDP-α-D-galactose + 1,2-diacyl-3-O-(β-D-galactosyl)-sn-glycerol = UDP + 1,2-diacyl-3-O-[α-D-galactosyl-(1→6)-β-D-galactosyl]-sn-glycerol
For diagram click here.
Other name(s): DGD1; DGD2; DGDG synthase (ambiguous); UDP-galactose-dependent DGDG synthase; UDP-galactose-dependent digalactosyldiacylglycerol synthase; UDP-galactose:MGDG galactosyltransferase; UDP-galactose:3-(β-D-galactosyl)-1,2-diacyl-sn-glycerol 6-α-galactosyltransferase
Systematic name: UDP-α-D-galactose:3-(β-D-galactosyl)-1,2-diacyl-sn-glycerol 6-α-galactosyltransferase
Comments: Requires Mg2+. Diacylglycerol cannot serve as an acceptor molecule for galactosylation as in the reaction catalysed by EC 2.4.1.46, monogalactosyldiacylglyerol synthase. When phosphate is limiting, phospholipids in plant membranes are reduced but these are replaced, at least in part, by the glycolipids digalactosyldiacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol [3]. While both DGD1 and DGD2 are increased under phosphate-limiting conditions, DGD2 does not contribute significantly under optimal growth conditions. DGD2 is responsible for the synthesis of DGDG molecular species that are rich in C16 fatty acids at sn-1 of diacylglycerol whereas DGD1 leads to molecular species rich in C18 fatty acids [3]. The enzyme has been localized to the outer side of chloroplast envelope membranes.
Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, PDB, CAS registry number: 125752-89-8
References:
1. Kelly, A.A. and Dörmann, P. DGD2, an Arabidopsis gene encoding a UDP-galactose-dependent digalactosyldiacylglycerol synthase is expressed during growth under phosphate-limiting conditions. J. Biol. Chem. 277 (2002) 1166-1173. [PMID: 11696551]
2. Härtel, H., Dörmann, P. and Benning, C. DGD1-independent biosynthesis of extraplastidic galactolipids after phosphate deprivation in Arabidopsis. Proc. Natl. Acad. Sci. USA 97 (2000) 10649-10654. [PMID: 10973486]
3. Kelly, A.A., Froehlich, J.E. and Dörmann, P. Disruption of the two digalactosyldiacylglycerol synthase genes DGD1 and DGD2 in Arabidopsis reveals the existence of an additional enzyme of galactolipid synthesis. Plant Cell 15 (2003) 2694-2706. [PMID: 14600212]
4. Benning, C. and Ohta, H. Three enzyme systems for galactoglycerolipid biosynthesis are coordinately regulated in plants. J. Biol. Chem. 280 (2005) 2397-2400. [PMID: 15590685]