Aspartic carbohydrate esterases

23 Aspartic carbohydrate esterases. CE 8 contains only pectin methyl esterases, which demethylate methyl esters of a-(l- 4)-polygalacturonan. Two X-ray crystal structures - from the bacterial plant pathogen Erwinia chry-santhemi and from carrot (Daucus carota), have been solved.The enzyme 

The structure of the carrot CE 8 pectin methyl esterase was very similar to the bacterial enzyme, although for reasons which are not clear a mechanism involving one of the aspartates as a nucleophile, rather than a general base, was preferred to the aspartate protease mechanism. Such mechanisms have been previously proposed for the aspartic proteinases, but were thoroughly disproved when one of this class of enzymes, the HIV proteinase, was found to 

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Serine carbohydrate esterases and transacylases. The commonest reaction mechanism is the standard serine esterase /protease mechanism, demonstrated paradigmally for chymotrypsin, involving an acyl-enzyme intermediate. The enzyme nucleophile is a serine hydroxyl, which is hydrogen bonded the imidazole of a histidine residue, whose other nitrogen is hydrogen bonded to a buried, but ionised, aspartate residue (Figure 6.28),... 

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