In enzymology, a digalactosyldiacylglycerol synthase (EC 2.4.1.241) is an enzyme that catalyzes the chemical reaction

UDP-galactose + 3-(beta-D-galactosyl)-1,2-diacyl-sn-glycerol ${\displaystyle \rightleftharpoons }$ UDP + 3-[alpha-D-galactosyl-(1->6)-beta-D-galactosyl]-1,2-diacyl-sn- glycerol

Thus, the two substrates of this enzyme are UDP-galactose and 3-(beta-D-galactosyl)-1,2-diacyl-sn-glycerol, whereas its 3 products are UDP, [[3-[alpha-D-galactosyl-(1->6)-beta-D-galactosyl]-1,2-diacyl-sn-]], and glycerol.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-galactose:3-(beta-D-galactosyl)-1,2-diacyl-sn-glycerol 6-alpha-galactosyltransferase. Other names in common use include DGD1, DGD2, DGDG synthase (ambiguous), UDP-galactose-dependent DGDG synthase, UDP-galactose-dependent digalactosyldiacylglycerol synthase, and UDP-galactose:MGDG galactosyltransferase. This enzyme participates in glycerolipid metabolism.

• Kelly AA, Dormann P (2002). "DGD2, an arabidopsis gene encoding a UDP-galactose-dependent digalactosyldiacylglycerol synthase is expressed during growth under phosphate-limiting conditions". J. Biol. Chem. 277 (2): 1166–73. doi:10.1074/jbc.M110066200. PMID 11696551.
• Hartel H, Dormann P, Benning C (2000). "DGD1-independent biosynthesis of extraplastidic galactolipids after phosphate deprivation in Arabidopsis". Proc. Natl. Acad. Sci. U.S.A. 97 (19): 10649–54. doi:10.1073/pnas.180320497. PMC 27079. PMID 10973486.
• Kelly AA, Froehlich JE, Dormann P (2003). "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 (11): 2694–706. doi:10.1105/tpc.016675. PMC 280572. PMID 14600212.
• Benning C, Ohta H (2005). "Three enzyme systems for galactoglycerolipid biosynthesis are coordinately regulated in plants". J. Biol. Chem. 280 (4): 2397–400. doi:10.1074/jbc.R400032200. PMID 15590685.