Transaldolase is an enzyme from the nonoxidative por- tion of the pentose phosphate pathway (pp 110-111). Transaldolase moves a 3-carbon segment of sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate forming erythrose-4-phosphate and fructose-6-phosphate. In this portion of the transaldolase mechanism, an enzyme lysine residue forms a Schiff base with the carbonyl of the sedoheptulose-7-phosphate substrate. Formation of the Schiff base with lysine readies the substrate for cleavage because the 3-carbon segment formed next will be stabilized by imine-enamine tautomerism. (See pg 35 for the complete transaldolase mechanism.)