Catalyzes the transfer of a two-carbon ketol group from a ketose donor to an aldose acceptor, via a covalent intermediate with the cofactor thiamine pyrophosphate. D-glyceraldehyde 3-phosphate + D-sedoheptulose 7-phosphate = aldehydo-D-ribose 5-phosphate + D-xylulose 5-phosphate Binds 1 Mg(2+) ion per subunit. Can also utilize other divalent metal cations, such as Ca(2+), Mn(2+) and Co(2+). Binds 1 thiamine pyrophosphate per subunit. Homodimer. Predominantly cytoplasmic and to a lesser extent also nuclear (PubMed:16969476). Additional isoforms seem to exist. Expressed in fetal and adult heart, brain, lung, liver, and kidney, and in adult placenta, skeletal muscle and pancreas. Up-regulated in various epithelial tumors. Strong TKTL1 protein expression has been correlated with a certain type of glucose metabolism (aerobic glycolysis; Warburg effect) and to cells which are affected by chronic complications of diabetic patients. Lacks the N-terminal. Belongs to the transketolase family.