Influence of molecular structures of substrates and analogues on Taka-amylase A catalyzed hydrolyses. 3. Inhibition by 2-deoxy-D-glucose and methyl -D-glucoside: change in inhibition type with substrate chain length

Experimental evidence for a 9-binding subsite of Bacillus licheniformis thermostable α-amylase

The action pattern of Bacillus licheniformis thermostable α-amylase (BLA) was analyzed using a series of (14)C-labeled and non-labeled maltooligosaccharides from maltose (G2) to maltododecaose (G12). Maltononaose (G9) was the preferred substrate, and yielded the smallest Km=0.36 mM, the highest kcat=12.86 s(-1), and a kcat/Km value of 35.72 s(-1) mM(-1), producing maltotriose (G3) and maltohexaose (G6) as the major product pair. Maltooctaose (G8) was hydrolyzed into two pairs of products: G3 and maltopentaose (G5), and G2 and G6 with cleavage frequencies of 0.45 and 0.30, respectively. Therefore, we propose a model with nine subsites: six in the terminal non-reducing end-binding site and three at the reducing end-binding site in the binding region of BLA.

Subsite profile of the active center of porcine pancreatic alpha-amylase. Kinetic studies using maltooligosaccharides as substrates

The hydrolysis of several maltooligosaccharides catalysed by porcine pancreatic alpha-amylase was performed in order to determine their kinetic parameters. Maltose behaves as a substrate. Molecular activity (Ko) increases with chain length up to maltopentaose, remaining practically unchanged from maltopentaose to maltoheptaose. Maltose shows the highest Km value while the one for maltotriose is the lowest. Only maltose and maltotriose were directly cleaved to glucose. From Km and Ko values, the binding energy of the total complexes and of the productive ones respectively were calculated. The binding energy at each subsite was determined assuming that each substrate forms a single productive complex and that maltose and maltotriose differ in their binding mode from higher oligosaccharides. The model was checked by calculating theorical Km and Ko. Theorical values agree reasonably well with experimental ones.

Amylases: enzymatic mechanisms

Many types of amylases are found throughout the animal, vegetable and microbial kingdoms. They have evolved along different pathways to enable the organism to convert insoluble starch (or glycogen) into low molecular weight, water soluble dextrins and sugars. Alpha amylases are dextrinogenic and can attack the interior of starch molecules. The products retain the alpha anomeric configuration. Beta amylases act only at the non-reducing chain ends and liberate only beta maltose. Both alpha and beta amylases exhibit multiple (repetitive) attack, that is, after the initial catalytic cleavage, the enzyme may remain attached to the substrate and lead to several more cleavages before dissociation of the enzyme-substrate complex. Amylases have extended substrate binding sites, in the range 4-9 glucose units. This enables the enzyme to stress the substrate and lower the activation energy for hydrolysis. Similarly the enzyme exerts a torsion on the glucose unit at the catalytic site, inducing a transition state conformation (oxycarbonium ion). Alpha and beta amylases differ in the stereospecific hydration of the oxycarbonium ion, in the sequence of liberation of the right-hand vs the left-hand product, and the direction of motion of the retained substrate to give multiple attack.