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 (PubMed:20667822, PubMed:27259054, PubMed:31534226). Thus, catalyzes the reversible transfer of a two- carbon ketol group from sedoheptulose-7-phosphate to glyceraldehyde-3- phosphate, producing xylulose-5-phosphate and ribose-5-phosphate (PubMed:20667822, PubMed:27259054, PubMed:31534226). The phosphate group of the substrate plays a role in stabilizing the substrate- thiamine intermediate while preventing the formation of the reactive enamine intermediate (Ref.28). Can also use fructose-6-phosphate as a ketose donor (Probable). {ECO:0000269|PubMed:20667822, ECO:0000269|PubMed:27259054, ECO:0000269|PubMed:31534226, ECO:0000269|Ref.28, ECO:0000305|PubMed:31534226}. This is the function of TKT (transketolase, ENSG00000163931).