Agriculture Reference
In-Depth Information
structure and composition of starches, their intermediary products of degradation, or
the different catalytic properties of each enzyme molecule interacting with specific
substrate molecules. In this model, two luminal α-1,4 endoglucosidases, namely sali-
vary and pancreatic amylases, respectively, hydrolyze linear unbranched segments
with five or more glucose residues present in amylose and amylopectin molecules,
releasing oligomers that have from two (maltose:
O
-α-d-glucopyrinosyl-(1,4)-α-d-
glucopyranoside; the simplest glucose oligomer) to ten glucose residues but minimal
production of free glucose (Gray 1975; Brayer et al. 2000; Horvathova et al. 2001).
The segments containing α-1,6 branches are resistant to these amylase activities.
To attain the effective release of free glucose, linear glucose oligomers resulting
from the amylase digestion have to be further hydrolyzed by four α-1,4 exoglucosidic
activities present in the mucosal epithelial cells of the small intestine. These activities
are constituted by the enzymes sucrase-isomaltase (SI) and maltase-glucoamylase
(MGAM), each composed of two subunits containing catalytic sites that act on the
nonreducing ends of linear glucose oligomers with substantial release of free glucose
monomers (Gray 1975; Corring 1980; Jones et al. 1983; Fujita and Fuwa 1984; Gray
1992). In addition, the isomaltase subunit of SI also shows α-1,6 glucosidic activity
that cleaves the linkages present in the branching points of intermediate branched
oligomers and isomaltose, releasing free glucose and the corresponding unbranched
glucose oligomer.
More recent studies of the details of the catalytic mechanisms for starch diges-
tion have demonstrated the oversimplification of that model (Quezada-Calvillo et
al. 2007, 2008). For instance, amylase digestion of starches derived from different
botanical sources or subject to different processing yields end products with charac-
teristic composition of glucose oligomers that depended primarily on the nature of
the initial substrate. The α-glucogenic abilities of MGAM and SI are not homoge-
neous for all glucose oligomers since differences of up to two orders of magnitude
can be observed in α-glucogenesis from different glucose oligomers, which suggests
that the efficiency of α-glucogenesis from starch-derived oligomers depends substan-
tially on their relative abundance. Together, the α-glucogenic activities of luminal
pancreatic amylase and of the two mucosal enzymes MGAM and SI are higher than
the sum of each of the individual activities, indicating that they have a synergistic
interaction enhancing the efficiency of the starch digestion. Despite the high specific
activity displayed by MGAM in the hydrolysis of glucose oligomers, in humans this
enzyme contributes only 20 to 30% of the total small intestinal mucosa α-glucogenic
capacity due in part to the marked predominance of SI molecules in the apical mem-
brane of epithelial cells. In addition, while MGAM experiences substrate inhibition
by maltotriose and maltotetrose (three and four glucose residues, respectively), no
inhibition of SI has been reported by glucose oligomers. This suggests that MGAM
displays full activity only under low concentrations of these starch digestion prod-
ucts, while SI maintains its slow hydrolytic activity even at high concentrations of
oligomers (Quezada-Calvillo et al. 2007, 2008). The effect may constitute a built-
in physiologic mechanism to constrain the glucose overload that could occur dur-
ing the digestion of large starchy meals, with MGAM working at full α-glucogenic
capacity.
