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Function

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

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 normally a homotetrameric enzyme, but it can also exist in a dimer form ^[3]. There are 6 SUS isoforms in Arabidopsis thaliana and all of them are structurally similar to sucrose phosphate synthases and glycogen synthases. ^[4]. Each monomer is a chain of 808 residues which possesses four specific domains:

• 1-127: N-Terminal regulatory domain involved in targeting ^[5]. On this sequence, two serines can be phosphorylated, which enable a control of enzyme location ^[6].

• 1-127: CTD: Cellular targeting domain.

• 157-276: EPBD: ENOD40 peptide-binding domain. This domain has a role in the regulation of the enzyme. It is able to bind a potassium ion.

• 277-776 : GT-B glycosyltransferase domain. It contains the catlytic site and presents a characteristic Rossman-folding

^[7]

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

The SUS1 tetramer is flat, with two types of subunit interfaces, the A:B and A:D interfaces. The A:D interface is an interaction between the C-terminal extension and the linker, whereas the A:B interface is created by the interaction of adjascents EPBD domains.

The active site of SUS1 is able to bind both UDP-fructose and UDP-glucose.

The GT-B domain is highly conserved in other isoforms and in the sucrose-phosphate synthase. This conservation reinforce the evolutionary relationship of those enzymes. Furthermore, this domain is also conserved in other species.

Regulation

ENOD40-A is a small hormon-like peptide able to specifically thiolates the Cys-266 of AtSUS1. It also inhibit the phosphorylation of Ser-167.