Triose Phosphate Isomerase Structure & Mechanism

spinqualitylabelsall models Human triosephosphate isomerase complex with phosphoglycolic acid 1hti Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Ribbon drawing for one chain of the "TIM barrel" fold General Information Triose phosphate isomerase (TIM) (PDB 1wyi and 1hti) is a crucial enzyme in the glycolytic pathway. TIM reversibly converts the aldose Glyceraldehyde-3-phosphate (GAP) to the ketose Dihydroxyacetone phosphate (DHAP). The interconversion proceeds by an enediol intermediate. Triose phosphate isomerase is not directly regulated, but the enzyme two steps before it in the glycolytic pathway, phosphofructokinase, is a heavily regulated, irreversible enzyme. Structural Characteristics The secondary structure consists of 14 alpha helices and 8 beta sheets per monomer, making it fall in the SCOP category of alpha and beta proteins. The tertiary structure is a alpha-beta barrel, and it is the prototypical example of the "TIM barrel" fold (see ribbon drawing). The quaternary structure is a homodimer. The molecular weight of the enzyme is estimated at 57,400 Da. Mechanism The enzyme aids in catalysis by binding tightly to the enediol transition state. To convert GAP to the enediol intermediate, a proton is abstracted from C2 by a base and the carbonyl oxygen atom is protonated by an acid. Glu 165 acts as the base and grabs the C2 proton on glyceraldehyde-3-phosphate, while His 95 is H-bonded to the carbonyl oxygen and acts as the acid by protonating carbonyl oxygen. The enediol intermediate is negatively charged, but is somewhat stabilized by the positively charged side chain of Lys 12. Mutation of Lys 12 to Arg increases Km by a factor of 22 and decreases Vmax by a factor of 180. To convert the enediol intermediate to DHAP, C1 is protonated by Glu 165, with His 95 removing a proton from C2’s OH group. As a result, the catalytic groups are back to their original states, and catalysis is complete. With GAP as a substrate, Km for the reaction is .34 mM and Vmax is 7200 units/mg protein at 25 degrees C and pH 7.5. Mechanism of Triose phosphate isomerase. Created by Christian Krenk using Spartan 08. An interesting part of the enzyme is the flexible loop that stabilizes the enediol-like transition state. The flexible loop (residues 167-176) closes over the active site like a hinged lid when substrate is bound, thus preventing phosphate from leaving. A four-residue segment of the loop H-bonds with the phosphate group of the substrate. Without the loop, the enediol intermediate would eliminate phosphate, with the end products being inorganic phosphate and toxic methylglyoxal. 3D structures of triose phosphate isomerase Triose Phosphate Isomerase Additional Resources For additional information, see: Carbohydrate Metabolism

References

7. Wierenga RK, Kapetaniou EG, Venkatesan R. Triosephosphate isomerase: a highly evolved biocatalyst. Cellular and Molecular Life Sciences. 2010 August 7 67:3961-3982. PMID: 20694739

Proteopedia Page Contributors and Editors (what is this?)

Christian Krenk, Michal Harel, Jane S. Richardson, Diamond B. Reese, David Canner, Alexander Berchansky