In enzymology, a cellulose synthase (GDP-forming) is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are GDP-glucose and (1,4-beta-D-glucosyl)n, whereas its two products are GDP and (1,4-beta-D-glucosyl)n+1. This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is GDP-glucose:1,4-beta-D-glucan 4-beta-D-glucosyltransferase. Other names in common use include cellulose synthase (guanosine diphosphate-forming), cellulose synthetase, guanosine diphosphoglucose-1,4-beta-glucan glucosyltransferase, and guanosine diphosphoglucose-cellulose glucosyltransferase. This enzyme participates in starch and sucrose metabolism. , no proteins with this activity are known in the UniProt/NiceZYme or the gene ontology database.