Starch degradation reactions

Because a degraded starch chain is the product of the degradation reaction, the synthetic phosphorylase catalyzed reaction would require a starch chain or a maltodextrin chain to be a substrate in the synthetic reaction, as shown above. This was the origin for the primer required reaction for starch bios)uithesis from the non-reducing-end of the primer, and it has pretty much been retained for 60 years [119,120,121]. 

Where S, G, X, E and Enz are respectively the starch, glucose, cells, ethanol and enzyme concentrations inside the reactor, Si is the starch concentration on the feed, F is the feed flow rate, V is the volume of hquid in the fermentor and (pi, (p2, (ps represent the reaction rates for starch degradation, cells growth and ethanol production, respectively. The unstructured model presented in (Ochoa et al., 2007) is used here as the real plant. The ki (for i=l to 4) kinetic parameters of the model for control were identified by an optimization procedure given in Mazouni et al. (2004), using as error index the mean square error between the state variables of the unstructured model and the model for control. 

Glycosyltransferases transfer glycosyl groups from a starch chain to an acceptor. The acceptor may be another starch molecule, phosphoric acid or nucleotides. Most enzymes in this class catalyze reversible reactions some enzymes are involved in the starch biosynthesis. The only glycosyltransferase responsible for starch degradation is the cyclodextrin glycosyltransferase. 

While little more is known in detail about the mechanism of the enzymatic degradation reactions of cellulose and starch, it is likely that, in many cases, once an enzyme molecule associates with a polymer chain and causes the first chain cleavage, either by an exo- or an endo-cleavage, it remains associated with the fragmented chain, and it can catalyze the hydrolysis of several more units before dissociating. At present, however, little quantitative information is available about this type of process. 

In order to describe the kinetics of enzymatic starch depolymerization, information on reaction rate, reaction extent, and product distribution profiles are required. Traditional end-group analysis can be used to a limited extent in the first two areas, but will not provide information about the last important subject. Hence, SEC profiles can provide sufficient insight into the mechanism of starch degradation. 

The catalytic effect of the hydroxide ion would normally be represented as part of the reaction rate constant (k ) for each temperature, because catalyst concentration normally remains constant. However, in the case of alkaline degradation of starch or cellulose, organic acids are produced which are converted to their salts by the alkali present, thus reducing the hydroxide ion concentration. Therefore, it seemed that this degradation reaction could be represented by second-order kinetics, with the hydroxide ion concentration determined by the stoichiometry of conversion of starch or cellulose to organic acids. 

Similarly, the choice of enzymes in pathway design is of importance. In the case of the conversion of cellulose to starch, cellobiose phosphorylase and a-glucan phosphorylase are responsible for reversibly eonverting from eello-biose to amylose or vice versa. It was found that a-gluean phosphorylase from potato is a key enzyme to drive the reaction toward starch synthesis. In contrast, the same enzyme from Clostridium thermocellum eannot generate amylase from cellobiose because it prefers the starch degradation direetion. ... 

A dynamic mediatory role between starch synthesis and degradation has been ascribed to starch phosphorylase (3). Actually, plant starch phosphorylase has been rather believed to be involved in the degradation reactions. The various biochemical properties have been reviewed. However, there are different varieties of starch phosphorylase. The importance of plastidial and cytoplasmic starch phosphorylase have been discussed as well as various biotechnological aspects. 

Hydrolysis Hydrolysis is a degradation reaction resulting from reaction with water. For example, starch can be hydrolysed to smaller molecules by extended storage in water. 

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