Triose Phosphate Isomerase

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TPI

OverviewOverview

Triose Phosphate Isomerase (TPI or TIM) catalyzes the reversible interconversion of the triose phosphate isomers dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde-3-phosphate , an essential process in the glycolytic pathway. More simply, the enzyme catalyzes the isomerization of a ketose (DHAP) to an aldose GAP also referred to as PGA. In regards to the two isomers, at equilibrium, roughly 96% of the triose phosphate is in the DHAP isomer form; however, the isomerization reaction proceeds due to the rapid removal of GAP from the subsequent reactions of glycolysis.

TPI is an example of a catalytically perfect enzyme, indicating that for almost every enzyme substrate encounter, a product is formed and that this interaction is only limited by the substrate diffusion rate. Other catalytically perfect enzymes include carbonic anhydrase, acetylcholinesterase, catalase and fumarase.

MechanismMechanism

TPI catalyzes the transfer of a hydrogen atom from carbon 1 to carbon 2, an intramolecular oxidation-reduction reaction. This isomerization of a ketose to an aldose proceeds through an enediol intermediate.

Acid Base CatalysisAcid Base Catalysis

TPI carries out the isomerization reaction through acid base chemistry involving . First the PGA molecule is held in place by , which provides a positive charge to the active site. , which plays the role of the general base catalyst in a proton abstraction mechanism , abstracts a proton from carbon 1. However, the carboxylate group of Glutamate 165 alone is not basic enough to abstract a proton and requires , the general acid, to donate a proton to C-2 to stabilize the negatively charge C-2 carbonyl group, effectively forming the endediol intermediate. At this point in the reaction, Glutamate 165 acts as a general acid by donating its proton the C-2, while Histidine 95 now acts as a general base by abstracting a proton from the hydroxyl group of C-1. The final step in the reaction is the formation of the GAP isomer product while glutamate and histidine are returned to their original forms.

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Structure & FunctionStructure & Function

Triose Phosphate Isomerase is part of the all alpha and beta(a/b)class of proteins and it is a dimer consisting of two identical subunits. Each subunit contains surrounding 8 interior , which form a structural motif called a closed alpha/beta barrel or more specifically a . Characteristic of most all TIM barrel domains, the active site is located in a similar position, in the loop regions created by the eight loops that connect the C-terminus of the beta strands with the N-terminus of the alpha helices. As mentioned earlier, TPI is a catalytically perfect enzyme and accomplishes this largely due to its ability to suppress or prevent undesired side reactions such as the decomposition of the enediol intermediate into methyl glyoxal and orthophosphate, a process which is 100 fold faster in solution than the desired isomerization. TPI is able to prevent this undesired reaction by trapping the intermediate in the active site through the use of a 10 residue loop. This loop acts as a lid on the active site, which shuts when the enediol is present and reopens when the isomerization is complete.

DiseaseDisease

Triose Phosphate Isomerase Deficiency has been most closely linked to a point mutation at the residue which results in the Glu104Asp mutation. A common marker for TPI deficiency is the increased accumulation of dihydroxyacetone phosphate in erythrocyte extracts as a result in the inability of the mutant enzyme to catalyze the isomerization to D-glyceraldehyde-3-phosphate.

ReferencesReferences

Stryer, L.; Berg, J. M.; Tymoczko, J. L. (2007), Biochemistry (6th ed.), New York: W. H. Freeman, ISBN 0716746840

Introduction to Protein Structure Second Edition.  Carl Branden & John Tooz