Starch, hydrolysis reactions

The biocatalysts obtained were evaluated with respect to the composition, morphology, activity and stability of the immobilised enzyme in the starch hydrolysis reaction. In general, two alternative methods can be used, considering the bioartificial matrix as a substrate for the enzyme (this method is used for example to drive drug release into erosion control devices), or alternatively, as in the case of this work, after blending the enzyme with a polymer, and investigating its activity against an external substrate. The apparent kinetic parameters of the reaction catalyzed by the immobilised and native enzymes were determined and compared. 

We thus elucidated that three of the four cellulase components are endo- or random-type and the other is exo-type. However, it is difficult to distinguish between the components of least or lowest random-type and those of exo-type. It is rather easy to identify an endo-type cellulase component. In contrast, it is very difficult to determine a cellulase to be exo-type because if the enzyme has a glycosyl-transferring activity the hydrolysis product is not a single sort, which is one of the necessary conditions to be an exo-type. Based on our experiments, measurement of the time course of CMC using a sample of medium substitution degree seems to be the best method of diagnosis to determine a cellulase component to be endo- or exo-type. With some enzymes, direction of mutarotation of reaction products is useful to resolve this problem, as is illustrated by the classic example of the starch hydrolysis by a- and /3-amylases. If this is true for our cellulases, the mutarotation of reaction products would be a... 

Starch can be broken down in two ways, both of which take place in the presence of water. Hence the reactions are known as hydrolysis reactions. Hydrolysis of starch is the key reaction that enables us to use this energy source. If starch is boiled for about one hour with dilute hydrochloric acid, it is broken down into its monomers, glucose molecules. 

If starch is mixed with saliva and left to stand for a few minutes, it will break down to maltose, a disaccharide (that is two joined monosaccharides). The enzyme present in the saliva, called amylase, catalyses this hydrolysis reaction. 

Enzymes are very efficient natural catalysts present in plants and animals. They do not require high temperatures to break down the starch to maltose. In humans, a salivary amylase breaks down the starch in our food. If you chew on a piece of bread for several minutes, you will notice a sweet taste in your mouth. The above hydrolysis reactions are summarised in Figure 15.21. 

Various refinements of starch content analysis have been reported. The methods are based on starch hydrolysis, followed by polarimetry,305 high-pressure liquid chromatography306 or reaction with glucose oxidase/peroxidase.307,308 An iodine reaction can be used to determine the botanical origin of starch.309 The molecular weight distribution is determined by size-exclusion chromatography.310,311... 

In addition to the application of ABRE in wood chemistry, it has been widely applied to other biomass conversion fields, particularly enzyme catalysed hydrolysis reactions such as the conversion of biopolymers (including cellulose and starch) to monosaccharides and oligosaccharides. In this area, PEG ABS systems offer a benign non-denaturing environment in contrast to organic solvent reaction media. 

Since it contains many -OH groups in its structure, starch absorbs moisture well. When starch is heated with a dilute acid, it undergoes a hydrolysis reaction resulting in glucose as the final product. 

In these expressions, t is the reaction time. These differences may be interpreted in terms of the data in Table I. However, some changes of the rate constant for starch hydrolysis depended on the concentration of the same hydrochloric acid.214... 

When the QCM technique was employed for the starch hydrolysis, all kinetic parameters both of the enzyme binding process (kon. koff and JCj) and the hydrolysis process (kcat) could be obtained simultaneously on the same device, as shown in Table 3. In the conventional enzyme reactions in the bulk solution, Michaelis-Menten kinetics have been applied to obtain both the Michaelis constant (Km) and the hydrolysis rate constant (kcat) according to Eq. 16. If koff > kcat, the Km value is thought to be the apparent dissociation constant (K = koff/kon) ... 

An efficient single-step catalytic process was recently developed for the conversion of glucan-type polysaccharides, especially starch, to sorbitol [15]. This process is characterized by the simultaneous hydrolysis of the polysaccharide and hydrogenation of the liberated monosaccharide. The catalyst used is Ru-loaded H-USY zeolite (3 % wlw Ru) in which the zeolitic material fulfils the role of metal carrier (Ru) and solid-acid catalyst. The zeolite provides the Brpnsted acidity required for the hydrolysis reaction either because of its outer surface or by introducing some homogeneous acidity, and the Ru catalyzes the hydrogenation of D-glucose to sorbitol (Scheme 2). 

As shown above, in starch the glucopyranoside ring is present in the a-form, while in cellulose the repeating units exist in the P-form. Because of this difference in structures, the enzymes that catalyze the acetal hydrolysis reactions in the biodegradation reactions of each of these two polysaccharides are different for each and are not interchangeable. 

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