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In organic chemistry, a tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde () functional group in position 1 ( aldotetroses) or a ketone () group in position 2 ( ketotetroses).
The aldotetroses have two (asymmetric carbon atoms) and so 4 different stereoisomers are possible. There are two naturally occurring stereoisomers, the enantiomers of erythrose and threose having the D but not the L enantiomers. The ketotetroses have one chiral center and, therefore, two possible stereoisomers: erythrulose (L- and D-form). Again, only the D enantiomer is naturally occurring.
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.
D-erythrose 4-phosphate was found to be an inhibitor of phosphoglucose isomerase. Phosphoglucose isomerase is the second enzyme in the glycolysis pathway, and its role is to convert glucose 6-phosphate into fructose 6-phosphate.
In both of these cases, the tetrose is an inhibitor of an enzyme in the glycolysis pathway, preventing it from proceeding onward.
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