Triose Phosphate Isomerase: Difference between revisions

<StructureSection load='2ypi' size='450' frame='true' align='right'

caption='TPI (yeast) at 2.5 &Aring; [[resolution]] ([[2ypi]]) dimer. The two identical chains are different colors. Ligand is the inhibitor 2-phosphoglycolic acid (PGA). '

[[Triose Phosphate Isomerase]] (TPI or TIM) 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 <scene name='Triose_Phosphate_Isomerase/Morph/1'>dihydroxyacetone phosphate (DHAP)</scene> and <scene name='Triose_Phosphate_Isomerase/Morph/2'>D-glyceraldehyde-3-phosphate (GAP)</scene>, an essential process in the glycolytic pathway. More simply, the enzyme catalyzes the <scene name='Triose_Phosphate_Isomerase/Morph/3'>isomerization of a ketose (DHAP) to an aldose (GAP)</scene>, 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 tertiary structure is the classic example of the "TIM barrel" fold (see image at left).  The TPI structure is shown on the right (PDB entry [[2ypi]]) in complex with the inhibitor 2-phosphoglycolic acid (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.  More details in [[ Triose Phosphate Isomerase Structure & Mechanism]] and [[Triosephosphate Isomerase]].

[[Image:LBHB2_Glu.png|left|thumb|500x250px|'''LBHB between Glu165 and DHAP''']] The formation of the LBHB between Glu165 and O1 of the inhibitor PGH is due to the matching of p''K''as and the alternative mechanism suggests that Glu-165, in addition to its role in initially abstracting the proton from the substrate, may also shuttle protons to and from the oxygens in the intermediate.  Also, the "criss-cross" mechanism implies that, by donating a normal hydrogen bond, the role of His95 is to polarize the carbonyl oxygen and lower the p''K''a of PGH in order to facilitate subsequent proton abstraction<ref>PMID:9748211</ref>. It has been argued that that the LBHB formed between Glu165 and PGH is a consequence of using the inhibitor PGH, whose hydroxamate p''K''a of 9 better matches Glu165 then His95, and that the biological reaction would instead see the enediol forming a LHBH with His95, as mentioned above. Overall, the mechanism employed by TPI has yet to be completely solved and recent NMR studies involving both WT and mutant TPI enzymes have revealed contributions from both the "classic" and "criss-cross" mechanisms.  

'''Triose Phosphate Isomerase Deficiency''': Initially described in 1965,  TPI deficiency is an autosomal recessive inherited disorder with characteristics ranging from chronic haemolytic anaemia, increased susceptibility to infections, severe neurological dysfunction, and often times death in early childhood.<ref>PMID:10916682</ref> The effects of TPI deficiency are most closely linked to a point mutation at the <scene name='Triose_Phosphate_Isomerase/Glu_104_1/7'>Glu104</scene> residue which results in the <scene name='Triose_Phosphate_Isomerase/Glu104asp3/2'>Glu104Asp</scene> mutation. A common marker for TPI deficiency is the increased accumulation of DHAP in erythrocyte extracts as a result in the inability of the mutant enzyme to catalyze the isomerization to GAP. Recent evidence has indicated that the point mutation does not prove detrimental to the rate of catalysis of the enzyme, but rather effects the ability of the enzyme to dimerize.<ref>PMID:17183658</ref>

'''Role in Alzheimer's Disease''': Recent discoveries in Alzheimer Disease research have indicated that amyloid beta-peptide induced nitro-oxidative damage promotes the nitrotyrosination of TPI in human neuroblastoma cells.<ref>PMID:19251756</ref> Nitrosylated TPI was found to be present in brain slides from double transgenic mice over-expressing human amyloid precursor protein as well as in Alzheimer's disease patients. Specifically, the nitrotyrosination occurs on <scene name='Triose_Phosphate_Isomerase/Two_tyrosines_shaded1/1'>Tyr164 and Tyr208</scene> , which are located in close proximity to the catalytic center, and this modification correlates with reduced isomerization activity. Additionally, Francesc Guix and colleagues have shown nitrosylated TPI contributed to the formation of large beta-sheet aggregates ''in vitro'' and ''in vivo''.

TPI has a roughly 50% sequence conservation from bacteria to humans.<ref>PMID:8130194</ref> The <scene name='Triose_Phosphate_Isomerase/Conserved1/1'>3D pattern of amino acid conservation</scene> ([[2ypi]]) shows dramatic conservation around the catalytic site. Glu104 is also highly conserved, as are several residues in the [[#Why is the enzyme an obligate dimer?|interdigitating loop]]. Curiously, two Arg residues on the surface, distant from the dimer contact and the catalytic side, are also highly conserved. (See note about the conservation calculation<ref>The conservation pattern shown was calculated by [[ConSurfDB_vs._ConSurf|ConSurfDB]] and might obscure some conservation due to [[Evolutionary_Conservation#ConSurf-DB_Often_Obscures_Some_Functional_Sites|inclusion of proteins of different functions]]. However in the case of [[2ypi]], all sequences used in the multiple sequence alignment were TPI sequences. A manual run at the ConSurf Server, using 500 TPI sequences, gave a nearly identical result. Both runs gave an average pairwise distance close to 1.0. Hence, the conservation pattern shown is correct for TPI.</ref>.)