TPI catalyzes interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) (see here).
Standard state conditions favor formation of DHAP, but the intracellular concentration of glyceraldehyde-3-phosphate is low in the cell, drawing the reaction toward G3P. The reaction, which is common to glycolysis and gluconeogenesis, is also useful in glycerol metabolism.
The enzyme is a dimer of two identical subunits,
each possessing a parallel 
barrel with 
helices in
the interconnecting loops. The active site lies near the top of
the barrel and can accommodate either glyceraldehyde-3-phosphate
or dihydroxyacetone phosphate. The active site of TPI includes
a glutamate residue (Glu
165) that has been found essential for the function of the enzyme.
This glutamate is negatively charged (therefore basic) at physiological
pH and helps to extract the proton from carbon 2. At the same
time an acidic, proton-donating group (HA) shuttles protons between
the substrate's carbonyl group and the -OH on C-2. Thus the reaction
seems to proceed via an enediol intermediate.
Other groups surrounding the substrate help to stabilize the enediol intermediate. These include Lys 12 and His 95.
Previous step of glycolysis; Next step of glycolysis
1. RasMol Image of TPI (slow)
= +7.6 kJ/mol)