Triosephosphate Isomerase

Triose phosphate isomerase is the last enzyme reaction of the first phase of glycolysis. It converts dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde-3-phosphate (G3P), so that G3P can then proceed through the second phase of glycolysis.

Triose phosphate isomerase is a dimer of identical subunits (MW of each is 26 kD).

Its structure is an excellent example of a parallel beta-barrel protein. It is easier to see the structure when focusing only on one subunit.

The eight beta strands are wound together in such a way that they form a barrel. Since each beta-strand runs in the same direction, it is a parallel beta-barrel. Each beta-strand is flanked by an antiparallel alpha helix—forming a larger cylinder of alpha helices (on the outside) concentric with the Beta-barrel (on the inside). Both of these layers have a right handed twist. Hydrophobic residues are buried between these two concentric layers

To convert DHAP to G3P, one of the enzyme's basic residues accepts a hydroxyl proton from an enediol intermediate and thereby creates either DHAP or G3P. This occurs via a histidine residue, a leucine residue, and an alpha helix . The dipole moment of each of these groups induces a positive electric field and thereby polarizes the carbonyl oxygens of the substrate. This polarization weakens the C-H bonds of the substrate and makes it very easy for one of the enzyme's basic residues to abstract a proton and transfer it from one carbon to another. The residue used to perform this action is Glu165.

The protonated Glu165 is also where Phosphoglycolate (PGA) binds. PGA is believed to be a transition-state analog of triose phosphate isomerase. Thus, by studying how PGA binds, one can learn more about the binding of the transition state. Specifically PGA forms hydrogen bonds with His95 and Glu165. (When bound in this manner, PGA binds over 100 times tighter than either DHAP or G3P.)

In the absence of PGA, Glu165 is hydrogen bonded to Ser96 and His95. However when PGA binds, these hydrogen bonds are broken, and Glu 165 moves more than 2 Angstroms to form a hydrogen bond with the inhibitor. A more dramatic conformational change occurs with a 10 residue loop consisting of two type 3 turns and one type 2 turn. Upon the binding of PGA, this loop moves from an open to a closed state and closes over the active site. It most likely does this to shield the active site from bulk water.

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For more information on this protein, consult:
Lolis, E and Petsko, G. (1990) Biochemistry 29: 2213-2219.