Transaldolase

Related Reading

Transaldolase is one of the enzymes in the non‐oxidative branch of the pentose phosphate pathway, catalyzes the reversible transfer of a dihydroxyacetone moiety from fructose 6‐phosphate to erythrose 4‐phosphate, giving sedoheptulose 7‐phosphate and glyceraldehyde 3‐phosphate. The reaction mechanism includes formation of a Schiff base intermediate by an active site lysine, a characteristic of class I aldolases. Amino acid sequence comparisons suggest that transaldolases can be divided into two subclasses: the transaldolase and the MipB/TalC subfamilies. Most enzymes of the transaldolase subfamily consist of around 320–350 amino acids per polypeptide chain. The crystal structure analysis of Escherichia coli transaldolase, a representative of this subfamily, revealed that these enzymes contain an eight‐stranded α/β barrel fold and that they are related to other class I aldolases by a circular permutation. This permutation of the transaldolase gene shifts the catalytic lysine residue from the classical position in β‐strand 6 in class I aldolases to β‐strand 4 of the α/β barrel.

Figure 1. Structure of Transaldolase.

Mechanism

The pentose phosphate pathway has two metabolic functions: (1) the production of nicotinamide adenine dinucleotide phosphate (reduced NADPH) for reductive biosynthesis, and (2) the formation of ribose, which is ATP, DNA and an important part of RNA. Transaldolase links the pentose phosphate pathway to glycolysis. In patients lacking transaldolase, erythritol (from erythrose 4-phosphate), D-arabitol and ribitol will accumulate. The deletion of 3 base pairs in the TALDO1 gene results in a deletion of serine at position 171 of the transaldolase protein, which is part of a highly conserved region, indicating that this mutation causes the transaldolase found in red blood cells and lymphoblasts Enzyme deficiency. In early infancy, the lack of this amino acid can cause liver cirrhosis and hepatosplenomegaly (enlarged spleen and liver). Transaldolase is also an autoimmune target for patients with multiple sclerosis.

Structures

Transaldolase is a single domain composed of 337 amino acids. The active site is located in the center of the barrel and contains three key residues: lysine 142, glutamic acid 106 and aspartic acid 27. Lysine fixes sugar in place, while glutamic acid and aspartic acid act as proton donors and acceptors. The core structure is an α/β barrel, similar to other class I aldolases, consisting of eight parallel β-sheets and seven α-helices. There are seven other alpha helices that are not part of the barrel. Hydrophobic amino acids are located between the β-sheet layer in the barrel and the surrounding α-helix to facilitate accumulation, such as the region containing Leu-168, Phe-170, Phe-189, Gly-311 and Phe-315. In the crystal, human trans aldolase forms a dimer, with two subunits connected by 18 residues in each subunit.

Catalytic

After the lysine 142 residue in the active site of trans-aldolase is deprotonated by another active site glutamate 106, it forms a Schiff base with the ketone group in heptaphosphate heptaphosphate. The reaction mechanism is similar to the reverse reaction catalyzed by aldolase: the bond connecting carbon 3 and 4 is broken, and dihydroxyacetone is connected to the enzyme via Schiff base. This cleavage reaction produces the unusual aldoseerythrose-4-phosphate. Then, transaldolase catalyzes the condensation of glyceraldehyde 3-phosphate with the Schiff base of dihydroxyacetone to produce enzyme-bound fructose 6-phosphate. The hydrolysis of Schiff base releases free fructose 6-phosphate, which is one of the products of the pentose phosphate pathway.