In enzymology, an alpha,alpha-trehalose-phosphate synthase (GDP-forming) (EC 2.4.1.36) is an enzyme that catalyzes the chemical reaction

GDP-glucose + glucose 6-phosphate ${\displaystyle \rightleftharpoons }$ GDP + alpha,alpha-trehalose 6-phosphate

Thus, the two substrates of this enzyme are GDP-glucose and glucose 6-phosphate, whereas its two products are GDP and alpha,alpha'-trehalose 6-phosphate.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is GDP-glucose:D-glucose-6-phosphate 1-alpha-D-glucosyltransferase. Other names in common use include GDP-glucose-glucose-phosphate glucosyltransferase, guanosine diphosphoglucose-glucose phosphate glucosyltransferase, and trehalose phosphate synthase (GDP-forming).

• Elbein AD (February 1967). "Carbohydrate metabolism in Streptomyces hygroscopicus. I. Enzymatic synthesis of trehalose phosphate from guanosine diphosphate D-glucose-14C". The Journal of Biological Chemistry. 242 (3): 403–6. PMID 6022837.