Triose Phosphate Isomerase: Difference between revisions

TPI catalyzes the transfer of a hydrogen atom from carbon 1 to carbon 2, an intramolecular oxidation-reduction reaction.[[Image:TPI 2D mechanism2.png|right|thumb|400px| '''Isomerization reaction catalyzed by TPI''']] This isomerization of a ketose to an aldose proceeds through an ''cis''-enediol(ate) intermediate. This isomerization proceeds without any cofactors and the enzyme confers a 10⁹ rate enhancement relative to the nonenzymatic reaction involving a chemical base (acetate ion).<ref>PMID:2043623</ref>.  

[[Image:TPIkinetics.png|thumb|400px| '''Kinetic constants of Triose Phosphate Isomerase''']]

Triose Phosphate Isomerase is a member of the all alpha and beta (α/β) class of proteins and it is a homodimer consisting of two    subunits each comprising 247 amino acids. Each TPI monomer contains the full set of catalytic residues; however, the enzyme is only active in the oligomeric form. <ref>PMID:18562316</ref> Therefore, dimerization is essential for full function of the enzyme even though it is not believed that any cooperativity exists between the two active sites.<ref>PMID: 2065677</ref> Each subunit contains 8 exterior <scene name='Triose_Phosphate_Isomerase/Helix_shaded_sheet_3/1'>alpha helices</scene> surrounding 8 interior <scene name='Triose_Phosphate_Isomerase/Beta_sheet_labelled/1'>beta strands</scene>, which form a conserved structural domain called a closed alpha/beta barrel (αβ) or more specifically a <scene name='Triose_Phosphate_Isomerase/Tim_barrel_2/1'>TIM Barrel</scene>. The TIM barrel was originally named after TPI and is estimated to be present in 10% of all enzymes. Nonpolar amino acids pointing inward from the beta strands contribute to the hydrophobic (<scene name='Triosephosphate_Isomerase/Tim_hydro_polar/3'>hydrophobicity</scene>) core of the structure (grey), whereas residues pointing outward interact with the nonpolar face of the alpha helices on the outer rim (fuchsia).  Characteristic of most all TIM barrel domains is the presence of the enzyme's active site in the lower loop regions created by the eight loops that connect the C-terminus of the beta strands with the N-terminus of the alpha helices. TIM barrel proteins also share a structurally conserved phosphate binding motif, with the phosphate group found in the substrate or cofactors. <ref> http://www.ncbi.nlm.nih.gov/Structure/cdd/cddsrv</ref>.