Sucrose-phosphate synthase is a plant enzyme involved in

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

biosynthesis Biosynthesis is a multi-step, enzyme-catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecules. ...

. Specifically, this enzyme

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the transfer of a hexosyl group from uridine diphosphate glucose ( UDP-glucose) to D- fructose 6-phosphate to form UDP and D-sucrose-6-phosphate. This reversible step acts as the key regulatory control point in sucrose biosynthesis, and is an excellent example of various key enzyme regulation strategies such as

allosteric In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...

control and reversible

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

. This enzyme participates in

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

and sucrose metabolism.

Nomenclature

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-glucose:D-fructose 6-phosphate 2-alpha-D-glucosyltransferase. Other names in common use include UDP-glucose-fructose-phosphate glucosyltransferase, sucrosephosphate-UDP glucosyltransferase, UDP-glucose-fructose-phosphate glucosyltransferase, SPS, uridine diphosphoglucose-fructose phosphate glucosyltransferase, sucrose 6-phosphate synthase, sucrose phosphate synthetase, and sucrose phosphate-uridine diphosphate glucosyltransferase.

Structure

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

studies have revealed that the

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

of ''Halothermothrix orenii'' SPS belongs to the GT-B fold family. Like other GT-B proteins, SPS contains two Rossmann fold domains that are named the A domain and the B domain. Generally, the structure of these domains are somewhat similar, as both contain central beta sheets that are surrounded by alpha helices. However, the A domain consists of eight parallel beta strands and seven alpha helices while the B domain contains six parallel beta strands and nine alpha helices. These domains are joined by residue loops to form a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

binding cleft, where the glucosyl group acceptor binds. Although ''H. orenii'' is a non-

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

bacterium, various studies indicate that the structure of its SPS is similar to plant SPS. First,

antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

with high specificities for plant SPS also target the bacterial SPS, indicating the structure is conserved enough for the antibody to recognize the enzyme as an antigen. Furthermore,

genomic Genomics is an interdisciplinary field of biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, three-dim ...

studies reveal that closely related plant homologues exhibit up to 54% sequence identities.

Mechanism

In the open conformation of ''H. orenii'' SPS, fructose 6-phosphate forms

hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

with Gly-33 and Gln-35 residues in the A domain while UDP-glucose interacts with the B-domain. Crystal structures studies reveal that after binding, the two domains twist to narrow the entrance of the substrate binding cleft from 20 Å to 6 Å. In this closed conformation, the Gly-34 residue of the A domain interacts with UDP-glucose and forces the substrate to adapt a folded structure, facilitating its donation of the hexosyl group. After binding, fructose 6-phosphate will interact with UDP via a hydrogen bond, which lowers the activation energy of the reaction and stabilizes the transition state. Finally, the C1 atom of UDP-glucose undergoes nucleophilic attack by an oxygen atom in fructose 6-phosphate, resulting in glucosyl group transfer to fructose 6-phosphate. Whether or not this mechanism requires a divalent ion is currently unclear, but failed attempts to trap and detect the presence of the magnesium

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

suggest that this mechanism is metal ion independent.

Regulatory strategies

Phosphorylation

SPS-

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

reversibly phosphorylates a

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

residue and subsequently deactivates SPS, In

spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...

and maize, the site of phosphorylation regulation has been identified as Ser158 and Ser162 respectively. While it is currently unclear if this seryl residue homolog in other plant SPSes is phosphorylated to suppress SPS activity, conservation of the neighboring residues has been observed in other plant species. This conserved sequence may potentially aid in recognition of a regulatory SPS-kinase. Once phosphorylated, the inactivated enzyme can be dephosphorylated and reactivated by SPS-phosphatase. Aside from controlling the levels of sucrose in the cell, regulation via phosphorylation can help the cell adapt to hyperosmotic conditions; in times of osmotic stress, the seryl residue is phosphorylated and enzyme activity decreases. This regulation strategy also controls carbon flux from photosynthesis, as studies indicate the signal transduction pathway responsible for SPS activation responds to light stimulus.

Allostery

Glucose 6-phosphate binds to an allosteric site, resulting in conformational changes to SPS that increase the enzyme's

affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Partn ...

for the glucosyl accepting substrate. Inorganic phosphate can also bind to this allosteric site, preventing glucose 6-phosphate activation of SPS. Like regulation via phosphorylation, this regulation strategy is also closely related to photosynthesis, as high rates of photosynthesis will deplete levels of inorganic phosphate and increase concentrations of glucose 6-phosphate in the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

. Overall, increased rates of photosynthesis will increase SPS activity.

Function

SPS plays a major role in partitioning carbon between sucrose and

in photosynthetic and non-photosynthetic tissues, affecting the growth and development of the plant. In ripening fruits, SPS is responsible for converting starch to sucrose and other soluble sugars. Additionally, SPS is also active in

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

that mostly degrade sucrose, participating in futile cycles that allow for large, rapid changes in sucrose

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...

. At low temperature, SPS activity and sucrose biosynthesis rates are increased. Sucrose accumulation is advantageous at low temperature, as sucrose is a form of energy storage that can be rapidly metabolized for respiratory purposes. Furthermore, increased amounts of sucrose can help the plant withstand freezing.

References

{{Portal bar, Biology, border=no EC 2.4.1 Enzymes of unknown structure