Triose Phosphate Isomerase: Difference between revisions

[[Triose Phosphate Isomerase]] (TPI or TIM) [5.3.1.1] is a ubiquitous dimeric enzyme with a molecular weight of ~54 kD (27 kD per subunit) which catalyzes the reversible interconversion of the triose phosphate isomers dihydroxyacetone phosphate ('''DHAP''') and D-glyceraldehyde-3-phosphate ('''GAP'''), 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 '''PGAL'''. 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.  The TPI structure is shown on the right (PDB entry [[2ypi]]) in complex with GAP ("PGA"), which is bound to each of its two active sites. 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 limited only by the substrate diffusion rate.  In addition to its role in glycolysis, TPI is also involved in several additional metabolic biological processes including gluconeogenesis, the pentose phosphate shunt, and fatty acid biosynthesis.  A point mutation to a glutamate residue (Glu104) of TPI results in triose phosphate isomerase deficiency, an autosomal recessive inherited disorder characterized by an increased accumulation of DHAP in erythrocytes. Structurally, this point mutation abolishes TPI’s ability to dimerize, subsequently inhibiting its catalytic activity.

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 sheets</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>.