Triose Phosphate Isomerase: Difference between revisions

TPI carries out the isomerization reaction through an acid base mediated mechanism involving <scene name='Triose_Phosphate_Isomerase/Three_catalytic_residues/3'>three catalytic residues</scene>.  First the DHAP or GAP subtrate is initially attracted to the enzyme active site through electrostatic interactions between the negatively charged substrate phosphate group and the positively charged <scene name='Triose_Phosphate_Isomerase/Lys12_shaded/1'>Lys12</scene>, with the resulting interaction stabilizing the substrate. According to the "classic" mechanism, <scene name='Triose_Phosphate_Isomerase/Glu165/3'>Glu165</scene> plays the role of the general base catalyst by abstracting a proton from the pro(''R'') position of carbon 1 of DHAP or the C-2 proton of GAP. However, the carboxylate group of Glutamate 165 alone does not possess the basicity to abstract a proton and requires <scene name='Triose_Phosphate_Isomerase/His95/6'>His95</scene>, the general acid, to donate a proton to stabilize the negative charge building up on C-2 carbonyl oxygen, effectively stabilizing the planar endediol(ate) intermediate,.  Lys12 and Asn11 also function to stabilize the negative charge which builds up on this intermediate. At this point in the mechanism, Glutamate 165 acts as a general acid by donating its proton to the neighboring 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, regenerating the enzyme.  In studies using tritium labeled DHAP, Knowles observed only ~ 6% intramolecular transfer of the ³H label to the GAP product.  In explaining this result, Knowles argued that the hydrogen bound to the Glu165 was in equilibrium with those in bulk solvent.  Additionally, the reaction mechanism of the methylglyoxal forming enzyme [http://en.wikipedia.org/wiki/Methylglyoxal_synthase methylglyoxal synthase (MGS)] is believed to be similar to that of triosephosphate isomerase. Both enzymes utilize DHAP to form an enediol(ate) phosphate intermediate as the first step of their reaction pathways; however, the second catalytic step in the MGS reaction pathway features the elimination of phosphate and collapse of the enediol(ate) to form methylglyoxal rather then reprotonation to form the isomer glyceraldehyde 3-phosphate as seen in TPI.<ref>PMID:10368300</ref>

Although a highly studied enzyme, there are relatively few effective inhibitors of TPI.  From a pharmaceutical perspective, if TPI structures differ greatly between humans and microorganisms such as ''Plasmodium'' or ''Trypanosoma'', whose growth rely heavily or entirely on glycolysis, inhibition may be a strong therapeutic target.<ref>PMID:15911278</ref> Two irreversible inhibitors, halo-acetone phosphate and glycidol phosphate (1,2-epoxypropanol-3-P), act by labeling active site residues. Early biochemical studies involving glycidol phosphate have revealed the labeled residue to be the active site glutamate. There are several weak reversible inhibitors of TPI including 3-Phosphoglycerate, glycerol phosphate and phosphoenol pyruvate, with ''K''_i values ranging from 0.2-1.3 mM.<ref>PMID:15911278</ref>  Additionally, several transition state analogues have been used to study the mechanism of TPI, including phosphoglycolohydroxamate <scene name='Triose_Phosphate_Isomerase/Inhibitors_active_site/1'>(PGH)</scene> (''K''_i = 6-14 μM) and the phosphoglycolic acid