<p>This family represent GDP-mannose 4,6-dehydratase, also known as GDP-D-mannose dehydratase. This enzyme converts GDP-mannose to GDP-4-dehydro-6-deoxy-D-mannose, the first of three steps for the conversion of GDP-mannose to GDP-fucose in animals, plants, and bacteria. In bacteria, GDP-L-fucose acts as a precursor of surface antigens such as the extracellular polysaccharide colanic acid of <taxon tax_id="562">Escherichia coli</taxon>. Excluded from this family are members of the clade that are poorly related because of highly dervied (phylogenetically long-branch) sequences, e.g. <taxon tax_id="143495">Aneurinibacillus thermoaerophilus</taxon> Gmd, described as a bifunctional GDP-mannose 4,6-dehydratase/GDP-6-deoxy-D-lyxo-4-hexulose reductase [<cite idref="PUB00009601"/>].</p>