Triosephosphate isomerase (TIM) catalyzes the isomerization of D-glyceraldehyde 3phosphate to form dihydroxyacetone phosphate, by a proton transfer reaction mechanism through an enediolate reaction intermediate (Scheme 1). Proton transfer at carbon is ubiquitous in biology, and sets enzymes the difficult problem of reducing the large thermodynamic barriers to formation of carbanion intermediates of non-enzymatic reactions. This problem was solved by TIM approximately 3 x 10 9 years ago, through evolution of a catalyst that satisfies two criteria of perfection, as outlined by and which uses the TIM-barrel protein fold present in ca. 10% of all enzymes. These observations have placed TIM at the forefront of enzymes targeted for mechanistic studies, because of the strong possibility that the results of studies on TIM will lead to conclusions that are broadly generalizable to other protein catalysts. Scheme 1. The mechanism of TIM-catalyzed isomerization of D-glyceraldehyde 3-phosphate (GAP) to form dihydroxyacetone phosphate (DHAP).