Agriculture Reference
In-Depth Information
units. Starches with mostly linear α-1,4 glucose linkages are known as amyloses,
and those with a mixture of α-1,4 and α-1,6 branched linkages are amylopectins. In
the plant, these are stored as chemical energy within semicrystalline granules whose
glucose units become available during reproduction. Plant cell walls are composed
of β-linked glucoside (cellulose) and nonglucose polymers that are poorly digested
by the human small intestine; these carbohydrates are classed as dietary fibers.
These polymers are not primary products of DNA transcription but are products of
regulated multienzyme metabolic pathways; the product is synthesis of a family of
starch and fiber polymers varying in size.
Starches are a gift of the vegetable kingdom to animal diets and a major food
source of negative entropy. However, animal digestion is made complicated by
the hundreds of botanical varieties of food starches. In contrast to the digestion of
sucrose by a single mucosal enzyme, starch digestion requires a committee of six
enzymes. The multiplicity of animal starch-digesting enzymes mirrors the multi-
plicity of the starch synthetic enzymes of plants (Quezada-Calvillo et al., 2007a,
2007b). Amylase was the first enzyme ever identified by biochemists. The activity is
increased by the process of malting by sprouting grain. The same enzyme activity
is found in animal salivary and pancreatic secretions. The product maltose was first
discovered in malted grains, and mucosal maltase activity was found by brewers.
Four different membrane-bound maltase enzyme activities of the small intestine can
be identified. Two are associated with SI activities and two lack any other identify-
ing activities (maltase-glucoamylase, MGAM). Because the four mucosal maltase
enzymes hydrolyze the nonreducing end of all α-1,4 glucose oligomers to free glu-
cose, here they are referred to as α-glucosidase activities (Table 2.2).
Amylase solubilization of starch
The Amylase gene (AMY) is highly represented in 615 species of bacteria and ver-
tebrates. It is α-endoglucosidase and a member of family GH 13 (Stam et al., 2006;
Benson et al., 2008). The gene is found in some archaea, suggesting differentiation
before 2.5 bya. The human gene is on chromosome 2, where six paralogs are found
(Quezada-Calvillo et al., 2007a). The grouping with neighboring genes is conserved
in rodents. The secreted enzyme has a glutamic acid proton donor and aspartic acid
nucleophile and a (β/α)
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barrel structure. It requires bound calcium and chloride
ions for activity. The catalytic action is on internal α-1,4 linkages of starch granules,
and the hydrolyzed products are a mixture of soluble maltodextrins. When there is
no remaining hydrolysis possible, the produced oligomers are referred to as α-limit
dextrins. The amylase activities produce very little free glucose during hydrolysis
of starch granules (Quezada-Calvillo et al.). The redundant nature of the root gene
expression of this secreted protein suggests that it coded digestion by the most primi-
tive cells (de Duve, 2007).
α
-glucosidase digestion of starch
The most active mucosal α-glucosidase is the C-terminal of MGAM, which is
located on human chromosome 7. This gene is in family GH 31 along with the
complimentary gene SI (as discussed in the starch digestion section). The genes
for GH 31 are expressed in 491 species, including archaea and bacteria (Sim et al.,
