Serine carbohydrate esterases and transacylases

Physically safer as a mechanistic criterion is reversible inhibition by boronic acids, RB(0H)2, which add the active site serine to form tetrahedral species [RB(0H)20Ser] , which mimic the tetrahedral intermediate/transition state. They also mimic the tetrahedral intermediates in aspartic protease action, however, and are therefore not as definitive. 

CE Family 1 is very large and contains members which do not act on carbohydrate-derived substrates. The crystal structure of a CE 1 domain of XynlOB modular enzyme from Clostridium thermocellum has been solved. The CE 1 domain is a feruloyl esterase which hydrolyses the feruloyl groups attached to some arabinofuranosyl 05 groups in native xylan. (The Xyn lOB protein as a whole consists of two CBM 22 domains, a dockerin domain, and a GH 20 xylanase domain, and forms part of a cellulosome - see Section 5.10.) The enzyme has the common a/p hydrolase fold. Studies of ferulic acid complexes of the inactive alanine mutant of the active site serine revealed the classic catalytic triad, and two main-chain peptide NH bonds are in place to form an oxyanion hole . A remarkable feature is that the enzyme as repeatedly isolated was esterilied on the active site serine by phosphate or sulfate. 

As families containing the double displacement glycohydrolases can also contain transglycosylases, so CE 1, which contains double-displacement carboxyl esterases, also contains transacylases. Trehalose [a-D-Glcp-(l - l)-a-D-Glcp] esterified on 06 with mycolic acid plays a role in maintaining the hydrophobic exteriors of the tubercle bacterium Mycobacterium tuberculosis, and the crystal structure of three related mycoloyl transesterases are 

These transfer mycoloyl residues between the 6-positions of 

CE 6 apparently contains only acetyl xylan esterases, but catalytic functions of many of the proteins in it have not been experimentally confirmed. One such protein from Arabidopsis thaliana proved to have a catalytic triad,and as isolated was found to have the catalytic serine covalently modified by PMSF. This enabled an oxyanion hole formed by two main-chain NH and a side-chain carboxamide to be identified. 

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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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