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The surprising similarity of SPS structures: Why are the SPSs of plants and bacteria so similar? Amazing structural secrets!
In the biological community, studying the structural and functional similarities of enzymes has always been one of the focuses of in-depth exploration by scientists. Recent studies have shown that sucrose phosphate synthase (SPS) in plants has striking structural similarities to SPS in some bacteria, which has triggered many thoughts about gene evolution, function, and biological adaptation. This article will deeply explore the structural characteristics, mechanisms and important regulatory strategies of SPS to uncover its mystery.
What is sucrose phosphate synthetase (SPS)?
SPS is an enzyme present in plants and is mainly involved in the biosynthesis process of sucrose. This enzyme catalyzes a key reaction: transferring the six-carbon sugar group of uridine diphosphate glucose (UDP-glucose) to D-fructose-6-phosphate to form UDP and D-sucrose-6-phosphate. This reversible step is a critical control point in sucrose biosynthesis and is an excellent example of a variety of key enzyme regulatory strategies, such as allosteric control and reversible phosphorylation. In addition, SPS also plays an important role in starch and sucrose metabolism.
SPS structure
According to X-ray diffraction studies, the SPS structure of Halothermothrix orenii belongs to the GT-B folding family. SPS contains two Rossman folding domains, namely A domain and B domain. The structures of these two domains are similar in that they both contain a central β-sheet surrounded by α-helices. However, domain A consists of eight parallel beta strands and seven alpha helices, while domain B consists of six parallel beta strands and nine alpha helices. These domains are connected by a loop of residues, forming a matrix-binding groove where the glucose-based acceptor binds.
Although H. orenii is a non-photosynthetic bacterium, various studies have shown that the structure of its SPS is very similar to that of plant SPS.
Mechanism characteristics
In the open conformation of H. orenii, fructose-6-phosphate forms hydrogen bonds with Gly-33 and Gln-35 residues within the A domain, while UDP-glucose interacts with the B domain. Crystal structure studies show that upon binding, the two domains twist, narrowing the entrance to the substrate-binding groove from 20 Å to 6 Å. This closed conformation allows the Gly-34 residue of the A domain to interact with UDP-glucose, forcing the substrate to adapt to the folded structure and promoting the donation of the six-carbon sugar moiety.
Once bound, fructose-6-phosphate interacts with UDP via hydrogen bonds, lowering the activation energy of the reaction and stabilizing the transition state.
Adjustment strategy
Phosphorylation
SPS-kinase can reversibly phosphorylate a serine residue, thereby inactivating SPS. Studies have shown that in spinach and corn, the phosphorylation regulatory sites are Ser158 and Ser162. This regulatory approach not only controls intracellular sucrose levels, but also helps cells adapt to high osmotic pressure environments and manage the carbon flow produced by photosynthesis.
Allosteric regulation
The binding of glucose-6-phosphate at the allosteric site will cause a conformational change in SPS, increasing the affinity of the enzyme to the glucose-accepting substrate. Inorganic phosphate will prevent the activation of SPS by glucose-6-phosphate. This strategy is closely related to photosynthesis. As photosynthesis increases, the concentration of inorganic phosphoric acid will decrease, further promoting the activity of SPS.
Function of SPS
SPS plays an important role in the distribution of carbon in photosynthetic and non-photosynthetic tissues, affecting the growth and development of plants. In ripe fruits, SPS is responsible for converting starch into sucrose and other soluble sugars. In addition, SPS is also active in cells that store sucrose, allowing plants to quickly respond to environmental changes.
In low temperature environments, the activity of SPS and the rate of sucrose biosynthesis will be significantly increased, helping plants to survive at low temperatures.
Whether it is plants or bacteria, the amazing similarities of sucrose phosphate synthetase reveal the mysteries of biology. Does this mean that similar enzyme structures can facilitate functional adaptation among different organisms during evolution?
