Structural Biochemistry/Enzyme/Isomerases/Triose Phosphate Isomerase

Definition
Triose Phosphate Isomerase (TPI) is an isomerase that catalyzes the isomerization of dihydroxyacetone phosphate to and from D-glyceraldehyde 3-phosphate. It takes part in the glycolytic pathway, which is a biochemical pathway employed by many organisms. In the pathway, TPI's action takes its place directly after the splitting of fructose 1,6-biphosphate by aldolase. The objective of the glycolytic pathway is to metabolize glucose into two pyruvate molecules, also producing two ATP molecules. Hence, TPI is an enzyme that contributes to the production of ATP, the molecules used as an energy source by all organisms.

TPI is an example of a "kinetically perfect enzyme," which means that it catalyzes isomerization so quickly that the rate of reaction is determined by the diffusion rate of the substrate. This means it isomerizes essentially every molecule of TPI specificity that it encounters. TPI increases the rate of isomerization by ten degrees of magnitude. Part of TPI's kinetic perfection comes from it being an isomerase; the enzyme does not have to wait for multiple substrates to bind to the active site. Also, the mechanism (see below) involves few steps and involves the transfer of protons only.

=Mechanism= The catalytic mechanism of TPI begins with the glutamate residue removing a hydrogen from one of the substrate's carbon atoms (see image), while the carbonyl oxygen deprotonates the nearby histidine residue, forming an enediol intermediate. The negatively charged histidine then deprotonates the original hydroxyl group, which yields an enolate-like product. The glutamic acid, now acting as an acid, adds a proton to the middle carbon to form the product, glyceraldehyde 3-phosphate. The net result is the original carbon-hydroxyl bond and carbon-oxygen double bond switching places.

One important aspect of the catalytic behavior is the restraining of the enediol intermediate shown in the upper-right portion of the image below. Under normal conditions this molecule loses its phosphate group, and this degradation occurs at a rate two orders of magnitude faster than the isomerization of the substrate. To counteract this undesirable decomposition, a small loop of residues closes over the active site while the reaction takes place. The reactive intermediate is enclosed in a conformation that does not favor its spontaneous decomposition.