engineering

Sucrose-phosphate synthase

Sucrose-phosphate synthase is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sucrose-phosphate synthase rather than just read about it. In short: Sucrose-phosphate synthase (SPS) is a plant enzyme involved in sucrose biosynthesis. Specifically, this enzyme catalyzes 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.

Key takeaways

Sucrose-phosphate synthase belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sucrose-phosphate synthase to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sucrose-phosphate synthase from memory before moving on to harder problems.

Reference excerpt

Sucrose-phosphate synthase (SPS) is a plant enzyme involved in sucrose biosynthesis. Specifically, this enzyme catalyzes 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 control and reversible phosphorylation. This enzyme participates in starch 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 studies have revealed that the structure 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 binding cleft, where the glucosyl group acceptor binds. Although H. orenii is a non-photosynthetic bacterium, various studies indicate that the structure of its SPS is similar to plant SPS. First, antibodies 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 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 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 suggest that this mechanism is metal ion independent.

Regulatory strategies

Phosphorylation SPS-kinase reversibly phosphorylates a serine residue and subsequently deactivates SPS, In spinach 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 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. Overall, increased rates of photosynthesis will increase SPS activity.

Function SPS plays a major role in partitioning carbon between sucrose and starch 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 that mostly degrade sucrose, participating in futile cycles that allow for large, rapid changes in sucrose flux. 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

Illustrations

Worked examples

Example 1 — a first encounter with Sucrose-phosphate synthase

Start with the simplest possible case. Write down what Sucrose-phosphate synthase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sucrose-phosphate synthase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sucrose-phosphate synthase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sucrose-phosphate synthase

In research: Sucrose-phosphate synthase appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sucrose-phosphate synthase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sucrose-phosphate synthase is common in secondary-school and first-year university syllabi. It links to neighbouring topics EC 2.4.1, Enzymes of unknown structure, so understanding it makes those chapters shorter.
In everyday life: Look for Sucrose-phosphate synthase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Sucrose-phosphate synthase in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sucrose-phosphate synthase means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sucrose-phosphate synthase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sucrose-phosphate synthase in simple terms?

Why does Sucrose-phosphate synthase matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sucrose-phosphate synthase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sucrose-phosphate synthase.