Catalyzes the transfer of a two-carbon ketol group from a ketose donor to an aldose acceptor, likely via a covalent intermediate with the cofactor thiamine pyrophosphate. Can use L-erythrulose as donor and D-ribose-5-phosphate as acceptor substrates, forming glycolaldehyde and D-sedoheptulose-7-phosphate. For synthetic purposes, is able to use hydroxypyruvate (HPA) as donor substrate, making the reaction irreversible due to the release of carbon dioxide, and various aldehydes as acceptor substrates, which leads to the corresponding ketoses. Thus, using hydroxypyruvate as donor and three different aldehydes as acceptors, i.e. glycolaldehyde, D-glyceraldehyde and butyraldehyde, the enzyme stereoselectively forms the corresponding products L-erythrulose, D-xylulose and (3S)-1,3-dihydroxyhexan-2-one, respectively. D-glyceraldehyde 3-phosphate + D-sedoheptulose 7-phosphate = aldehydo-D-ribose 5-phosphate + D-xylulose 5-phosphate Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Optimum pH is 7-8. Optimum temperature is around 60-70 degrees Celsius. Thermostable. Retains 100% activity for one week at 50 degrees Celsius and for 3 days at 65 degrees Celsius. Activity decreases rapidly at 75 degrees Celsius (half-life is about 15 minutes), and at 85 degrees Celsius loses its activity immediately. Homodimer. The thermostability, stereoselectivity and wide substrate specificity of this enzyme offers good opportunities for applications in biocatalysis for ketose and chiral threo-diols synthesis. Belongs to the transketolase family.