Amylase, action on starch

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)...

Now as the limit dextrins are stable toward the amylases, it seems sound to assume that they are built according to another scheme than are those parts of the starch which 3ueld maltose. In order to explain the incomplete saccharification and the formation of the limit dextrins, the author has assumed - that, although the starch molecules are built substantially according to the maltose scheme, there are at certain intervals anomalies of one kind or other which constitute hindrances to the enzyme action. Since in most cases 70 to 80% of maltose is formed from starch, it must be concluded that the normal action of the amylases is the opening of 1,4-a-D-glucosidic links. When the constitution of the chain differs in some way from that of maltose, the enzyme cannot attach itself to the anomalous part of the chain and no hydrolysis occurs. The author assumed that when, for instance, /5-amylase acts on starch, the... 

Malt amylase is a mixture of a- and /8-amylase and has the following action on starch. 

We will discuss first whether there is an absolutely definite limit of action for all amylases. In the case of the action of /5-amylase on starch and on a-dextrins this question seems to be settled, but in the case of the malt a-amylase the answer is less certain. But certainly the action of the malt amylase practically stops at a certain limit. There is, however, almost always a very slow further action. It is possible that this slow saccharification of the limit dextrins is due not to the amylases but to other carbohydrases which have no action on starch but which are capable of attacking products with short chains. Under all circumstances it must be kept in mind that when in an experiment the saccharification for practical purposes has stopped and the limit dextrins have been isolated, this does not necessarily mean that the limit dextrins will not be further attacked by the enzyme used. But the velocity of this action is certainly very small compared with the velocity of the action on starch. Thus, it must be admitted that experiments involving the isolation of the limit dextrins after the action of a certain amylase on starch are in most cases not strictly reproducible. TJie total yield and chain length distribution of limit dextrins may vary, but their general character is not affected. If a limit dextrin produced by a certain amylase is treated with the same enzyme for a very long time, it is very often transformed to another limit dextrin of lower molecular weight with concomi-... 

Maltose is a disaccharide that is present free in small quantities in barley grains and other plants. However, it is more commonly produced by the action of the amylase enzyme on starch or glycogen. Its melting point is 102-103 °C. 

Evidence that the n-glucosidic interchain linkages in starch are, in fact, (l->6)-a-n in nature came originally from the isolation of small proportions of 2,3-di-O-methyl-n-glucose after methylation and hydrolysis. This evidence was, however, hardly definitive, as the small amounts of this dimethyl sugar obtained could equally well have arisen as a result of undermethylation or demethylation. The isolation of isomaltose after the action of crude, bacterial alpha-amylase on the polysaccharides is equally equivocal. No pure alpha-amylase is known that gives isomaltose as a product of its action on starch, and it is likely that this product was formed by a contaminating enzyme in the crude preparation of olpho-amylase used. 

These water-soluble molecules are cyclic oligomers of a-D-glucose formed by the action of certain bacterial amylases on starches (Bender and Komiyama, 1978 Saenger, 1980 Szejtli, 1982). a-Cyclodextrin (cyclohexa-amylose) has six glucose units joined a(l, 4) in a torus [1], whereas /3-cyclodextrin (cycloheptaamylose) and y-cyclodextrin (cyclooctaamylose) have seven and eight units, respectively. 

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. 

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