Uncovering Biphasic Catalytic Mode of C5-epimerase in Heparan Sulfate Biosynthesis

Heparan sulfate (HS), a highly sulfated polysaccharide, is biosynthesized through a pathway involving several enzymes. C5-epimerase (C5-epi) is a key enzyme in this pathway. C5-epi is known for being a two-way catalytic enzyme, displaying a “reversible” catalytic mode by converting a glucuronic acid to an iduronic acid residue, and vice versa. Here, we discovered that C5-epi can also serve as a one-way catalyst to convert a glucuronic acid to an iduronic acid residue, displaying an “irreversible” catalytic mode. Our data indicated that the reversible or irreversible catalytic mode strictly depends on the saccharide substrate structures. The biphasic mode of C5-epi offers a novel mechanism to regulate the biosynthesis of HS with the desired biological functions. C5-epimerase converts a glucuronic acid to an iduronic acid residue in the heparan sulfate biosynthetic pathway. C5-epimerase displays both “reversible” and “irreversible” catalytic modes. C5-epimerase recognizes the saccharide sequence of the substrate to position the iduronic acid. The biphasic catalytic mode of C5-epimerase reveals a unique control mechanism in the biosynthesis of heparan sulfate.