Orphan Enzymes in the Mammalian L-Fucose Degradation Pathway

Orphan enzymes are recognized and classified enzymatic activities that lack associated amino acid sequences. Since the term was coined in the mid-2000s, the proportion of orphan enzymes has substantially decreased; however, it is estimated that at least ≈900 enzymatic activities remain devoid of molecular identity to date. The putative mammalian metabolic pathway for L-fucose degradation represents a system that long consisted exclusively of orphan enzymes, with only a few recently "deorphaned" and biochemically characterized. L-Fucose is a unique monosaccharide frequently found in various glycolipids and glycoproteins synthesized by mammalian cells, such as the ABO blood group antigens in humans. While the importance of the biosynthetic pathways for its active form (GDP-L-fucose) is well established in diverse biological processes, the enzymology and physiological role of L-fucose catabolism remain largely enigmatic. In this review, we summarize the current knowledge regarding the enzymological and physiological aspects of L-fucose catabolism in mammals.