Triosephosphate Isomerase - Structure

Structure

Triose phosphate isomerase is a dimer of identical subunits, each of which is made up of about 250 amino acid residues. The three-dimensional structure of a subunit contains eight α-helices on the outside and eight parallel β-strands on the inside. In the illustration, the ribbon backbone of each subunit is colored in blue to red from N-terminus to C-terminus. This structural motif is called an αβ-barrel, or a TIM-barrel, and is by far the most commonly observed protein fold. The active site of this enzyme is in the center of the barrel. A glutamic acid residue and a histidine are involved in the catalytic mechanism. The sequence around the active site residues is conserved in all known triose phosphate isomerases.

The structure of triose phosphate isomerase contributes to its function. Besides the precisely placed glutamate and histidine residues to form the enediol, a ten- or eleven-amino acid chain of TPI acts as a loop to stabilize the intermediate. The loop, formed by residues 166 to 176, closes and forms a hydrogen bond to the phosphate group of the substrate. This action stabilizes the enediol intermediate and the other transition states on the reaction pathway.

In addition to making the reaction kinetically feasible, the TPI loop sequesters the reactive enediol intermediate to prevent decomposition to methylglyoxal and inorganic phosphate. The hydrogen bond between the enzyme and the phosphate group of the substrate makes such decomposition stereoelectronically unfavorable. Methylglyoxal is a toxin and, if formed, is removed through the glyoxalase system. The loss of a high-energy phosphate bond and the substrate for the rest of glycolysis makes formation of methylglyoxal inefficient.

Studies suggest that a lysine close to the active site (at position 12) is also crucial for enzyme function. The lysine, protonated at physiological pH, may help neutralize the negative charge of the phosphate group. When this lysine is mutated to a neutral amino acid, TPI loses all function, but mutants with a different positively charged amino acid retain some function.