* Synthesizes GDP-mannose from mannose 1 phosphate and GTP.
* Knowledge of GMPPA and B is subsumed based on porcupine enzyme activity and the homology of porcupine and GMPPA and B structures.
* GDP-mannose is a critical reactant for sugar nucleotide synthesis and glycosylation, as it can be directly used in the synthesis of N-linked glycans and can be metabolized into other sugar nucleotide donors.