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
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