Sandbox Reserved 1136

Sucrose Synthase 1 (EC:2.4.1.13), also known as the Sucrose-UDP glucolsyltransferase 1 or simply Susy, is a reversible enzyme allowing the synthesis or the degradation of Sucrose in Arabidopsis thaliana. It belongs to the Glycosyltransferase family 4 (GT4).

Function

The Sucrose Synthase is able to catalyse the following reaction in both directions:

sucrose = glucose + fructose

It has a fundamental role in the Sucrose flow regulation since it is able to produce Sucrose (used for transportation and stockage) and both ADP-Glucose and UDP-Glucose. It is one of the only four proteins able to synthesize or cleave the Sucrose, and the only one able to catalyze it in both direction ^[1], thereby Sucrose synthase is implied in many pathways including cell wall, starch and glycoprotein synthesis ^[2].

Sucrose synthase catalyses the cleavage reaction at pH around 7 and the synthesis reaction at pH above 9. Therefore, it is a linker between all the pathways that use or synthesize sucrose. SUS is involved in biomass production, nitrogen fixation, fruit and seed maturation and can response to stress (cold, anaerobic conditions, drought or gene induction). However, the binding to target mechanism is unknown at the molecular level.

Structural highlights

Sucrose synthase is a homotetrameric enzyme. There are 6 SUS isoforms in Arabidopsis thaliana and all of them are structurally similar to sucrose phosphate synthases and glycogen synthases. ^[3]. Each monomer is a chain of 808 residues which possesses four specific domains:

• 1-127: N-Terminal regulatory domain involved in targeting ^[4]. On this sequence, two serines can be phosphorylated, which enable a control of enzyme location ^[5].

• 157-276: ENOD40 peptide-binding domain.

• 277-776 : GT-B glycosyltransferase domain.

• 776-808 : C-terminal domain. The length of this domain is variable depending of the SUS isoform.