One of the metabolic pathways that a tetrose is involved in is the Pentose Phosphate Pathway. In the Pentose Phosphate Pathway, there is an oxidative stage and a non-oxidative stage. A tetrose sugar, D-erythrose, is utilized in the non-oxidative stage, where D-ribulose 5-phosphate is generated into a 6 carbon sugar (fructose 6-phosphate) and a 3 carbon sugar (glyceraldehyde 3-phosphate). Both of these molecules can be used elsewhere in the body.
D-erythrose 4-phosphate is generated as a product of a reaction called transaldolation. In the Pentose Phosphate Pathway, a transaldolase removes the first 3 carbon molecules of sedoheptulose 7-phosphate and places them onto a glyceraldehyde 3-phosphate. The transaldolase utilizes a Schiff base to perform a reverse aldol reaction and a forward aldol reaction in its mechanism, generating an erythrose 4-phosphate and fructose 6-phosphate. The erythrose 4-phosphate is an important intermediate in the Pentose Phosphate Pathway because it is then used in the final non-oxidative step of the pathway.
The final non-oxidative step of the pathway is a transketolase reaction. A transketolase utilizes a thiamine pyrophosphate, or TPP cofactor, to break the unfavorable bond between the carbon in a carbonyl and the alpha carbon. TPP attacks a xylulose 5-phosphate molecule and facilitates the cleavage of the bond between the C2 (carbonyl carbon) and the C3 (alpha carbon), where glyceraldehyde 3-phosphate is released. Then, C2 can attack erythrose 4-phosphate, which forms fructose 6-phosphate. Both of the products of this reaction can enter the gluconeogenesis pathway to regenerate glucose.