sucrose synthase. This model represents sucrose synthase, an enzyme that, despite its name, generally uses rather produces sucrose. Sucrose plus UDP (or ADP) becomes D-fructose plus UDP-glucose (or ADP-glucose), which is then available for cell wall (or starch) biosynthesis. The enzyme is homologous to sucrose phosphate synthase, which catalyzes the penultimate step in sucrose synthesis. Sucrose synthase is found, so far, exclusively in plants and cyanobacteria [Energy metabolism, Biosynthesis and degradation of polysaccharides].

This family is most closely related to the GT1 family of glycosyltransferases and named after YqgM in Bacillus licheniformis about which little is known. Glycosyltransferases catalyze the transfer of sugar moieties from activated donor molecules to specific acceptor molecules, forming glycosidic bonds. The acceptor molecule can be a lipid, a protein, a heterocyclic compound, or another carbohydrate residue. This group of glycosyltransferases is most closely related to the previously defined glycosyltransferase family 1 (GT1). The members of this family may transfer UDP, ADP, GDP, or CMP linked sugars. The diverse enzymatic activities among members of this family reflect a wide range of biological functions. The protein structure available for this family has the GTB topology, one of the two protein topologies observed for nucleotide-sugar-dependent glycosyltransferases. GTB proteins have distinct N- and C- terminal domains each containing a typical Rossmann fold. The two domains have high structural homology despite minimal sequence homology. The large cleft that separates the two domains includes the catalytic center and permits a high degree of flexibility. The members of this family are found mainly in certain bacteria and archaea.

This family is most closely related to the GT1 family of glycosyltransferases. The sucrose-phosphate synthases in this family may be unique to plants and photosynthetic bacteria. This enzyme catalyzes the synthesis of sucrose 6-phosphate from fructose 6-phosphate and uridine 5'-diphosphate-glucose, a key regulatory step of sucrose metabolism. The activity of this enzyme is regulated by phosphorylation and moderated by the concentration of various metabolites and light.

sucrose-phosphate synthase, putative, glycosyltransferase domain. This family consists of the N-terminal regions, or in some cases the entirety, of bacterial proteins closely related to plant sucrose-phosphate synthases (SPS). The C-terminal domain (TIGR02471), found with most members of this family, resembles both bona fide plant sucrose-phosphate phosphatases (SPP) and the SPP-like domain of plant SPS. At least two members of this family lack the SPP-like domain, which may have binding or regulatory rather than enzymatic activity by analogy to plant SPS. This enzyme produces sucrose 6-phosphate and UDP from UDP-glucose and D-fructose 6-phosphate, and may be encoded near the gene for fructokinase.

sucrose phosphate synthase/possible sucrose phosphate phosphatase, plant. Members of this family are sucrose-phosphate synthases of plants. This enzyme is known to exist in multigene families in several species of both monocots and dicots. The N-terminal domain is the glucosyltransferase domain. Members of this family also have a variable linker region and a C-terminal domain that resembles sucrose phosphate phosphatase (SPP) (EC 3.1.3.24) (see TIGR01485), the next and final enzyme of sucrose biosynthesis. The SPP-like domain likely serves a binding and not a catalytic function, as the reported SPP is always encoded by a distinct protein.