Involved in catabolism of D-apiose. Catalyzes the transfer of the glycolaldehyde group from apulose-4-phosphate to D-glyceraldehyde 3-phosphate, generating dihydroxyacetone phosphate and D-xylulose-5-phosphate. apulose 4-phosphate + D-glyceraldehyde 3-phosphate = D-xylulose 5-phosphate + dihydroxyacetone phosphate Carbohydrate metabolism. Probable heterodimer composed of AptA and AptB. Belongs to the transketolase family.