Involved in the biosynthesis of the L-colitose (3,6-dideoxyl-L-xylo-hexose) present in the O-antigen region of lipopolysaccharides (LPS) where it serves as antigenic determinant and are vital for bacterial defense and survival. Catalyzes the two-step NADP-dependent conversion of GDP-4-keto-3,6-dideoxy-D-mannose to GDP-L-colitose. ColC is a bifunctional enzyme catalyzing the C-5 epimerization of GDP-4-keto-3,6-dideoxy-D-mannose and the subsequent C-4 keto reduction of the resulting L-epimer to give GDP-L-colitose. It can use both NADP(+) and NAD(+) as electron acceptor, with a slight preference for NADP(+). GDP-beta-L-colitose + NAD(+) = GDP-4-dehydro-3,6-dideoxy-alpha-D-mannose + H(+) + NADH GDP-beta-L-colitose + NADP(+) = GDP-4-dehydro-3,6-dideoxy-alpha-D-mannose + H(+) + NADPH Nucleotide-sugar metabolism; GDP-L-colitose biosynthesis. Homodimer. Belongs to the NAD(P)-dependent epimerase/dehydratase family. Fucose synthase subfamily.