C5-epimerase

The HS C5-epimerase that catalyzes the conversion of GlcA to IdoA has been cloned from bovine lung [41]. This enzyme is distinct from the epimerase involved in dermatan sulfate biosynthesis [42]. IdoA has a more flexible conformation than GlcA [43], and the formation of IdoA in GAGs is therefore believed to generally promote binding of the polysaccharides to proteins. The GlcA C5-epimerization is the only modification reaction in heparin/HS biosynthesis that cannot be reproduced without an enzyme catalyst [44]. 

Polysaccharide Lyases. The results on the modification of 4-0-methylglucuronic acid indicate that C5 epimerisation and elimination of 04 are comparably facile processes in vitro. Natural selection therefore predicts that where enzymes have evolved in the same organism, working on the same substrate, to perform both epimerisation and elimination, they should be structurally and evolutionarily related. A case in point is the alginate lyases and epimerases, although the sequences of modern lyases and epimerases are not similar, possibly because of the different stereochemical requirements of the... 

Different from most other HS modification enzymes, only a single isoform of C5-epi is present in almost all species examined (with the exception of fish). C5-epi is a fairly large protein with over 600 amino acids. The epimerase catalytic domain has been tentatively assigned to the 220-230 residues of the C-terminus based on alignment to homologous proteins and biochemical studies. ... 

Figure 6 The chemical mechanism of the C5-mannuronan epimerase reaction. The exchange of solvent deuterium into the product is indicated. Note the conformation change in the polymeric substrate that is induced by the epimerization reaction.

Figure 7 Schematic of the AlgE family of C5-mannuronan epimerases. The active site for epimerization is in the A module. Fgg is the fraction of product containing GG diads, and Fmg, Fqm is the fraction of product containing MG (or GM) diads data are taken from El. Ertesvag El. K. Eloidal El. Schjerven B. I. G. Svanem S. Valla, Metab. Eng. 1999, 1, 262-269.

There is no experimentally determined structure available for a complete C5-mannuronan epimerase. However, the structure of the R module from A. vindandii AlgE4 epimerase has been determined by NMR spectroscopy (PDB code 2agm), and the A module has been determined by X-ray crystallography (PDB codes 2pyh and 2pyg). Both domains contain CASH domains. 

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