In enzymology, a NDP-glucose—starch glucosyltransferase (EC 2.4.1.242) is an enzyme that catalyzes the chemical reaction

NDP-glucose + (1,4-alpha-D-glucosyl)n ${\displaystyle \rightleftharpoons }$ NDP + (1,4-alpha-D-glucosyl)n⁺1

Thus, the two substrates of this enzyme are NDP-glucose and (1,4-alpha-D-glucosyl)n, whereas its two products are NDP and (1,4-alpha-D-glucosyl)n+1.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is NDP-glucose:1,4-alpha-D-glucan 4-alpha-D-glucosyltransferase. Other names in common use include granule-bound starch synthase, starch synthase II (ambiguous), waxy protein, starch granule-bound nucleoside diphosphate glucose-starch, glucosyltransferase, granule-bound starch synthase I, GBSSI, granule-bound starch synthase II, GBSSII, GBSS, and NDPglucose-starch glucosyltransferase.

• Tsai CY (1974). "The function of the waxy locus in starch synthesis in maize endosperm" (PDF). Biochem. Genet. 11 (2): 83–96. doi:10.1007/BF00485766. PMID 4824506. S2CID 23024343.
• Nakamura T, Vrinten P, Hayakawa K, Ikeda J (1998). "Characterization of a granule-bound starch synthase isoform found in the pericarp of wheat". Plant Physiol. 118 (2): 451–9. doi:10.1104/pp.118.2.451. PMC 34820. PMID 9765530.
• Fujita N, Taira T (1998). "A 56-kDa protein is a novel granule-bound starch synthase existing in the pericarps, aleurone layers, and embryos of immature seed in diploid wheat (Triticum monococcum L.)". Planta. 207 (1): 125–32. doi:10.1007/s004250050464. PMID 9951718. S2CID 7146307.
• Murai J, Taira T, Ohta D (1999). "Isolation and characterization of the three Waxy genes encoding the granule-bound starch synthase in hexaploid wheat". Gene. 234 (1): 71–9. doi:10.1016/S0378-1119(99)00178-X. PMID 10393240.
• Nelson OE (1968). "The waxy locus in maize. II The location of the controlling element alleles". Genetics. 60 (3): 507–524. doi:10.1093/genetics/60.3.507. PMC 1212058. PMID 17248421.