D-glyceraldehyde 3-phosphate + D-sedoheptulose 7-phosphate = aldehydo-D-ribose 5-phosphate + D-xylulose 5-phosphate Binds 1 Ca(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Belongs to the transketolase family. Could be the product of a pseudogene. Corresponds to the N-terminal of members of this family.