Triose Phosphate Isomerase: Difference between revisions
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=== Acid Base Catalysis === | === Acid Base Catalysis === | ||
The mechanism of TPI has been extensively studied by prominent enzymologists for several decades leading to several different proposed mechanisms of catalysis, a few of which will be discussed | The mechanism of TPI has been extensively studied by prominent enzymologists for several decades leading to several different proposed mechanisms of catalysis, a few of which will be discussed in the following section. The original "Classic" mechanism put forth by Knowles and co-workers is provided below.<ref>PMID:9398185</ref> | ||
[[Image:picture.png|left|thumb|650px| '''Classic Mechanism proposed by Knowles and co-workers'''. Harris ''et al''. ''Biochemistry'' 1997, 26, 14661-14675]] | [[Image:picture.png|left|thumb|650px| '''Classic Mechanism proposed by Knowles and co-workers'''. Harris ''et al''. ''Biochemistry'' 1997, 26, 14661-14675]] | ||
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'>Lysine 12</scene>, with the resulting interaction stabilizing the substrate.<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 [http://en.wikipedia.org/wiki/Carboxylate carboxylate group]of Glutamate 165 alone does not possess the basicity to abstract a proton and requires <scene name='Triose_Phosphate_Isomerase/His95/6'>histidine 95</scene>, the general acid, to donate a proton to stabilize the negatively charged C-2 carbonyl oxygen, effectively stabilizing the planar endediol(ate) intermediate. The formation of the enediol(ate) intermediate is the rate-limiting step in the reaction. At this point in the mechanism, Glutamate 165 acts as a general acid by donating its proton to C-2, while Histidine 95 now acts as a general base by abstracting a proton from the [http://en.wikipedia.org/wiki/Hydroxyl 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. 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> | 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'>Lysine 12</scene>, with the resulting interaction stabilizing the substrate.<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 [http://en.wikipedia.org/wiki/Carboxylate carboxylate group]of Glutamate 165 alone does not possess the basicity to abstract a proton and requires <scene name='Triose_Phosphate_Isomerase/His95/6'>histidine 95</scene>, the general acid, to donate a proton to stabilize the negatively charged C-2 carbonyl oxygen, effectively stabilizing the planar endediol(ate) intermediate. The formation of the enediol(ate) intermediate is the rate-limiting step in the reaction. At this point in the mechanism, Glutamate 165 acts as a general acid by donating its proton to C-2, while Histidine 95 now acts as a general base by abstracting a proton from the [http://en.wikipedia.org/wiki/Hydroxyl 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. 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> | ||