Reaction glucoamylase-catalyzed

Figure 2 (78) illustrates the correspondence found for the first time between condensation and hydrolysis for these glucoamylase-catalyzed reactions. Most important is that the condensations of /3-D-glucopyranose to maltose and isomaltose involve configurational inversion from which we infer that the glucosylic bond is split and the D-glucopyranosyl group transferred. 

Glucoamylases (EC3.2.1.3) are fungal enzymes which hydrolyze maltose to produce glucose (dextrose). These enzymes catalyze hydrolysis of alpha-1,3, alpha-1,6 and beta-1,6 linkages. Their primary reaction is on the 1,4-linked a-D-glucopyranosyl units of... 

In some cases, enzyme-catalyzed reactions can be conducted at higher concentrations of the substrate than are feasible for the acid-catalyzed reaction this is particularly true in the use of glucoamylase, by which starch at 30% concentration can be hydrolyzed to yield 95% D-glucose (on the dry basis). 

Lignophenols having a linear structure were obtained by the surface reaction of a native lignin and phenols in sulfuric acid. Laccase catalyzed the oxidative polymerization of lignocatechol in a mixture of ethanol and phosphate buffer to give the crosslinked polymer [92]. The product showed high affinity for bovine serum albumin and glucoamylase. 

The dissociation constant for pullulan was larger than that for amy-lopectin due to the slow binding (kon) and the fast release (koff) of the enzyme (Table 3). The hydrolysis rate constant in the ES complex (kcat) of pullulan was ca. 30 times slower than that of amylopectin. This reflects the slow reaction of pullulan compared with amylopectin (curve e of Fig. 10a), due to the slow hydrolysis of a-1,6 linkages compared with a-1,4 hnkages catalyzed by glucoamylase [62,63]. 

The modeled transition-state structures for hydrolysis by the a-glucosi-dase and the glucoamylase show significant oxocarbonium ion character with the D-glucosyl unit of each having a flattened 4C conformation consistent with a C-l-O-5 bond order of 1.92, even though opposite o-glucose anomers are formed from the substrate. The transition-state structures show modest differences but they do not predict the stereochemical outcome of the catalyzed reactions.157... 

Because enzymes are polyionic polymers, it is expected that pH will affect most of their properties. In fact, change in pH may change the distribution of charges in the active site and in the whole surface of the protein molecule. Enzymes may present polar amino acid residues at its active site whose charge depends on pH. With respect to enzyme activity, it is a well known fact that rates of enzyme catalyzed reactions tend to decrease at extremes of pH usually exhibiting maxima at some intermediate values as shown in Fig. 3.10 for the case of glucoamylase (Illanes 1983). 

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