DEGRADATION OF STARCH
Starch is a storage polysaccharide found in many plants. It is a polymer of glucose linked by α-1,4 bonds. Starch actually consists of a mixture of linear polymers— amylose—and branched polymers—amylopectin. The branches of amylopectin are due to α-1,6 bonds, and they occur approximately every 20 glucose residues along the polymer chain. Chain lengths vary from 100 to 500,000 glucose residues for amylose and up to 40 million for amylopectin. The proportions of linear and branched polymers also vary depending on the source of the starch. Glycogen is a storage polysaccharide of animals. It is essentially starch with a high proportion of branched polymers.
Starch is used in the food and brewing industries and is mostly converted to glucose by the use of the purified enzymes α-amylase and glucoamylase, rather than by microorganisms (Fig. 13.4). The α-amylase cleaves the linear regions of the starch chains at random, whereas the glucoamylase is needed for cutting the branches. The glucose may then be converted to fructose by the enzyme glucose isomerase or to alcohol by microbial fermentation. Because of the large size of the food and brewing industries, α-amylase and glucoamylase plus glucose isomerase account for over 25% of the cost of all enzymes used industrially. Many microorganisms make these enzymes. However, industrially, α-amylase is usually obtained from the bacterium Bacillus amyloliquefaciens and glucoamylase from the fungus Aspergillus niger.
A variety of improvements to the process of starch degradation might be made by genetic engineering. Recombinant organisms could be made that produce more enzyme. Furthermore, the enzymes themselves could be engineered for better thermal stability or higher rates of reaction as discussed, Protein Engineering. Alternatively, it should be possible to construct microorganisms that carry out all or part of starch hydrolysis, as well as converting the resulting glucose to alcohol.
The gene for glucoamylase has been cloned from Aspergillus niger and inserted into a suitable yeast strain. The fungal gene was placed under control of a strong yeast promoter and carried on a yeast plasmid. The engineered yeast was able to degrade solubilized starch and ferment the glucose released to alcohol. Ultimately it may be possible to engineer yeast strains that also express (and secrete) high enough levels of α-amylase to completely convert raw starch to ethanol.
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