B4GALT1 (beta-1,4-galactosyltransferase 1) in the Golgi membrane associates with LALBA (alpha-lactalbumin) to form a heterodimer. This binding reaction is positively regulated by Mn2+ ions and UDP-galactose. This heterodimer has been observed both in solution in vitro and in crystalline form (Andrews 1970; Ramakrishnan & Qasba 2001). The galactosyltransferase enzyme is found in the Golgi membrane of many cell types. LALBA is expressed specifically in the lactating mammary gland. Binding LALBA modulates the specificity of the galactosyltransferase enzyme. Alone, B4GALT1 transfers galactose moieties to glycolipids and N-linked oligosaccharides of glycoproteins. Complexed with LALBA, it transfers galactose to free glucose to form lactose (Brew and Hill 1975).
In the Golgi membrane, the B4GALT1 subunit of the B4GALT1:LALBA (beta-1,4-galactosyltransferase 1 : alpha-lactalbumin) complex catalyzes the transfer of galactose from UDP-galactose to free glucose to form galactose and UDP (Andrews 1970; Ramakrishnan & Qasba 2001). The representation here of a stable protein complex catalyzing the transformation of small molecule substrates provides a simplified view of the true reaction mechanism (Brew and Hill 1975), but is sufficient to identify the key participating molecules and their roles.
Synthesis of the disaccharide lactose takes place within the Golgi apparatus of epithelial cells of the lactating mammary gland. The synthesis itself is a single chemical reaction of free glucose and UDP-galactose to form lactose and UDP. For this reaction to occur, glucose is transported from the cytosol into the Golgi lumen, and B4GALT1 interacts with LALBA (alpha-lactalbumin) to modulate its substrate specificity (Brew and Hill 1975).