FIGURE 1.

Chemical reactions catalyzed by C₅-epi and 2OST. A, HS C₅-epi-catalyzed reaction. HS C₅-epi catalyzes the conversion of a glucuronic acid unit in HS to iduronic acid and the reverse reaction, namely from the iduronic acid to glucuronic acid. B, 2OST WT-catalyzed reaction. 2OST WT catalyzes the 2-O-sulfation of both GlcUA and IdoUA units in the polysaccharide. C, strategy for determining C₅-epi activity by coupling C₅-epi and 2OST mutant. By coupling these two enzymes together, the IdoUA produced by the epimerase-catalyzed reaction is converted to IdoUA2S, which can be detected by the level of [³⁵S]sulfate incorporation to the polysaccharide.