Guanosine diphosphate mannose or GDP-mannose is a nucleotide sugar that is a substrate for glycosyltransferase reactions in metabolism. This compound is a substrate for enzymes called mannosyltransferases. Guanosine diphosphate mannose or GDP-mannose is a nucleotide sugar that is a substrate for glycosyltransferase reactions in metabolism. This compound is a substrate for enzymes called mannosyltransferases. GDP-mannose is produced from GTP and mannose-6-phosphate by the enzyme mannose-1-phosphate guanylyltransferase.