Epimerases, C5 uronyl residue

Both the glucuronyl epimerases involved in the biosynthesis of heparin and the mannuronyl epimerases involved in the biosynthesis of alginate exchange H5 with solvent. The enzymes have no cofactors, so mechanisms involving simple deprotonation of substrate and protonation of carbanions from both directions are likely. 

The R modules are not carbohydrate-binding modules, but appear to be the opposite. Direct atomic force microscopy (AFM) measurements found that polymannuronan bound more tightly to the AlgE4 A subunit than to the AR holoenzyme, and that the isolated R module did not bind polymannuronate at all. From the various force-extension curves it is possible to calculate the 

The structure of the R module from AlgE 4 from A. vinelandii has been solved by NMR spectroscopy it is an elongated molecule consisting of right-handed parallel p-roll, a similar to several other polyuronate-binding pro- 

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