Action of Salivary Amylase

Glycosidases such as a-amylases are very common in nature. The numerous variants expressed by different organisms show poor amino acid sequence homology, but nevertheless share a well-conserved three-dimensional structure. Moreover, this same three-dimensional structure is shared by a-glycosyl transferases such as the glucosyl- and 

Hydrolysis of substrates by the GH13 family begins two glucose residues from a nonreducing end. Catalysis occurs in two steps the formation of a P-diglucosyl intermediate covalently attached to the enzyme, followed by hydrolysis of the intermediate to maltose with inversion of the P-configuration (Fig. 12.11a). These steps are reiterated 

Arrow-root starch (500 g.) in 8 liters of water containing 1 g. of sodium chloride was hydrolyzed at pH 6.5 with 50 ml. of saliva. After twenty-four hours, 50% of maltose had been formed. The reaction mixture was kept under toluene for thirty-two days. The reducing power then corresponded to 89% maltose. Fermentation experiments, however, showed that 63% maltose and 14% D-glucose had been formed, the latter by the secondary action of maltase. As mentioned above, the salivary amylase does not form D-glucose as a primary product but differs in this respect from malt a-amylase. The pancreatic enzyme is 

Limit Dextrina Produced by Pancreatic and Salivary Amylases 

When acted upon by large amounts of amylases, the limit dextrins mentioned above are hydrolyzed further. Taka-amylase, for example, gave after prolonged action nearly 100% D-glucose. 

It was mentioned above that salivary amylase hydrolyzes normal a-dextrins, for instance, a malto-he.xaose (and the corresponding acid) at practically the same velocity as it hydrolyzes starch. This means that the affinity of the enzyme for long and for short chains is about the same. Consequently, the curve for the hydrolysis by salivary amylase has not the peculiar form of the curve for malt a-amylase but is continuous and has no sharp break. The a-dextrins which are formed at first are saccharified very rapidly. The normal a-dextrins are saccharified completely. The anomalous a-dextrins form the limit dextrins. 

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Protein and starch digestion, on the other hand, have potent nonpancreatic compensatory mechanisms. Due to the compensatory action of salivary amylase and brush border oligosaccharidases, a substantial proportion of starch digestion can be achieved without pancreatic amylase. Similarly, protein denaturation and hydrolysis is initiated by gastric proteolytic activity (acid and pepsin) and continued by intestinal brush border peptidases, and is thus partly maintained even in the absence of pancreatic proteolytic activity. 

The Schardinger dextrins have also been reported " to lie stable to alpha-type amylases. However, in a study of the action of salivary amylase, French and coworkers " found that while the a-dextrin is essentially completely resistant, the /3-dextrin is attacked very slowly indeed and the 7-dextrin is attacked about 1 % as rapidly as is starch. Here it is clear that the ring size exerts an effect possibly the smaller rings have greater rigidity and hence cannot adapt their shape to that inquired by the enzyme. 

Ans. Although starch digestion begins in the mouth with the action of salivary amylase, only a small portion of dietary starch digestion is completed there. The bulk of the starch digestion occurs in the small intestine under the action of pancreatic amylase. 

Table VI summarizes the properties of the native, partially sulfated, and 3,6-anhydro- derivative of elsinan. The 3,6-anhydro-elsinan contained 3,6-anhydro-glucose residues, approximately a half of the original (1 4)-linked glucose residues. The anhydro-elsinan showed a low optical rotation. In i.r. spectrum the formation of a new absorption band at 895 cm" was recognized. These results suggest the changes in the conformation of a-(1 4)-linked glucose residues from C-1 to 1-C forms. The introduction of 3,6-anhydroring to elsinan gives very low viscosity, and also the resistance to the action of salivary amylase.

Other clinicians knew that acid stops the diastatic action of salivary amylase, and Carl Anton Ewald and Ismar Boas knew as well that addition of fat to a test meal of bread delays the appearance of acid in the stomach. Therefore, the presence of fat in a meal should prolong the action of salivary amylase. In 1886 they fed their human subjects starch paste with or without an admixture of fat. When they recovered their subjects gastric contents they saw that in samples containing fat, free acid was rarely present and digestion of starch had occurred. 

Products of the Action of Salivary alpha-Amylase on Maltodextrins " ... 

Whelan and Roberts have devised an alternative method involving the successive action of salivary a-amylase and R-enzyme on glycogen. -By determining the number of reducing groups produced by action of R-enzyme on the a-dextrins, the proportion of (1 6) linkages can be calcu-... 

After the action of salivary and pancreatic a-amylases on dietary starch and glycogen, the carbohydrate content of the small intestine consists of newly formed maltose ingested monosaccharides dietary disaccharides, such as lactose, sucrose, maltose, and trehalose oligosaccharides, such as dextrins and maltotriose and indigestible oligosaccharides and polysaccharides, such as cellulose, agar, and other oligosaccharide dietary fibers. 

The structure of the aldotetraouronic acid is very similar to that of the smallest oligosaccharide produced by the action of salivary a-amylase on... 

These oligosaccharides were analyzed by h.p.l.c. It must be noted that the main products by the actions of salivary, hog pancreas and bacterial saccharifying a-amylase appear to be identical to each other. It was characterized as 4-0 -a-nigerosyl-D-glucose. The mode of action with Taka amylase seems to differ from those with salivary and pancreas amylases. 

Molecular weight Action of salivary C(-amylase 3X10 -h 2 X 10 2 X 10 ... 

The investigations carried out by Professor French and his students were based on sound experimental approaches and on intuitive theoretical considerations. The latter often resulted in new experiments for testing a hypothesis. On the basis of theoretical considerations, Professor French proposed a model for the structure of the amylopectin molecule, and the distribution of the linear chains in this molecule. This model was tested by utilizing enzymes that selectively cleave the linear chains, and the results substantiated the theoretical deductions. He proposed a theory on the nature and types of reactions occurring in the formation of the enzyme - starch complex during the hydrolysis of starch by amylases. In this theory, the idea of multiple attack per single encounter of enzyme with substrate was advanced. The theory has been supported by results from several types of experiments on the hydrolysis of starch with human salivary and porcine pancreatic amylases. The rates of formation of products, and the nature of the products of the action of amylase on starch, were determined at reaction conditions of unfavorable pH, elevated temperatures, and increased viscosity. The nature of the products was found to be dramatically affected by the conditions utilized for the enzymic hydrolysis, and could be accounted for by the theory of the multiple attack per single encounter of substrate and enzyme. 

The second and third of these steps depend on a supply of appropriate carbohydrate substrates, most favorably sucrose, in the mouth. The latter can become available either directly (sugar ingested in food or drink) or be derived from dietary starch by the action of bacterial or salivary amylases, or both. Of particular relevance in this context is the trapping of carbohydrates as or on food particles remaining in the mouth for considerable periods. 

Figure 7.3 Primary products of the action of human salivary and porcine pancreatic a-amylases acting on starch. Seven of the primary products are hydrolyzed slowly at specific bonds (indicated by arrows) to give secondary products that are limit dextrins. (From Robyt and French13,19and Kainuma and French17 18)...

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