Triose Phosphate Isomerase Structure & Mechanism

Ribbon drawing for one chain of the "TIM barrel" fold

General Information

Triose phosphate isomerase (TIM)^[1][2] (PDB 1wyi and 1hti) is a crucial enzyme in the glycolytic pathway. 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 , 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.^[3]

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.^[4] The enediol intermediate is negatively charged, but is somewhat ^[5] Mutation of Lys 12 to Arg increases Km by a factor of 22 and decreases Vmax by a factor of 180.^[5] 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.^[3]

Mechanism of Triose phosphate isomerase. Created by Christian Krenk using Spartan 08.

An interesting part of the enzyme is the that stabilizes the enediol-like transition state. The flexible loop (residues 167-176)^[6] 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.^[4] Without the loop, the enediol intermediate would eliminate phosphate, with the end products being inorganic phosphate and toxic methylglyoxal.^[4]

3D structures of triose phosphate isomerase

Triose Phosphate Isomerase

Additional Resources

For additional information, see: Carbohydrate Metabolism