Mutarotation enzymic catalysis

The original experimental evidence for concerted acid-base catalysis of the mutarotation in benzene is now considered unsound133 134 and concerted acid-base catalysis has been difficult to prove for nonenzy-matic reactions in aqueous solution. However, measurements of kinetic isotope effects seem to support Swain and Brown s interpretation.135 Concerted acid-base catalysis by acetic acid and acetate ions may have been observed for the enolization of acetone136 and it may be employed by enzymes.1363... 

Mutarotation of 0.3% solutions of the freshly dissolved sugars in 12 ml of 5 mM EDTA, pH 7.4 was followed. Significant differences in mutarotation rates (AK) in the presence and absence of 100 units of bovine kidney enzyme were expressed as the ratio AK/Ksp. Differences of less than 5% in these rate constants were not considered significant. Of the 18 sugars listed, nine have been tested previously as substrates for other mammalian mutarotases with essentially the same pattern as described here. The pattern of specificity indicates that a 3-point attachment of enzyme and substrate is necessary for catalysis of mutarotation. b Data from 72). 

D2O has the further advantage that the solvent isotope effect on mutarotation is usually greater than that on glycosidase catalysis. The temperature coefficient of most enzymes, moreover, is less than that of mutarotation, so it is often advantageous to cool the sample. Finally, the more powerful the magnet, the fewer the FIDs and hence the shorter the time that is required to obtain a good spectrum. Tours de force of NMR-based glycosidase stereochemistry... 

Fig. 1 Properties of SXA-catalyzed hydrolysis of 4-nitrophenyl- 3-D-xylopyranoside (4NPX). a Stereochemistry. SXA catalyzes hydrolysis of 4NPX with inversion of anomeric configuration. Mutarotation, off the enzyme, converts a-D-xylose to its equilibrium mixture (a P ratio of 1 2.5) with a half-life of 1 h [3]. R = 4-nitrophenyl. b Diprotic model. pX,s 5 and 7, assigned to catalytic base (D14) and catalytic acid (El 86), respectively, govern catalysis and binding of ligands [3]...

Micellar catalysis in water is not striking rate enhancements greater than 10 have seldom been obtained, and micelles exhibit only limited substrate specificity (in contrast to enzymes). Of particular interest, therefore, is the remarkable rate enhancement which has been observed for the mutarotation of 2,3,4,6-tetramethyl-a-D-glucose (8 in Figure 7) catalysed by alkylammonium carboxylate micelles... 

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