Agriculture Reference
In-Depth Information
highly conserved gene in all genomes. In contrast, the α-lactalbumin gene is lim-
ited to mammalian genomes but has an ancestral root as a lysosomal enzyme gene.
N
-Acetyllactosamine synthase is expressed in seven isoforms and plays a crucial role
in protein
N
-glycosylations. As lactose synthase complex, these two genes are cen-
tral to human and bovine lactation, for which lactose production drives the volume
of milk produced.
Small intestinal mucosal lactase is the hydrolase that digests lactose to the mono-
saccharide units. The lactase protein is an internally duplicated enzyme that belongs
to the glycosyl hydrolases family GH 1 (Benson et al., 2008). The enzyme is bound
at the C-terminal to enterocyte lumenal plasma membrane and has both lactase and
phlorizin hydrolase activity. The second activity also hydrolyzes β-glucosides of lip-
ids and micronutrients such as pyridoxine-5′-β-D-glucoside and other glycosylated
phytochemicals. The enzyme has a glutamic acid proton donor and glutamic acid
nucleophile and a (β/α)
8
barrel structure. In the human, the gene for lactase activity is
downregulated at about 4 years of age, resulting in symptomatic lactose intolerance. A
mutation in regulation of this gene permits lactase persistence in adults, and onset of
the mutation is correlated with the development of dairy cattle about 10,000 years ago
(see Chapter 1). Symptoms of lactose intolerance are treated by elimination of lactose
in the diet or by oral lactase enzyme supplementation (Robayo-Torres et al., 2006).
table sugar
Sucrose is a disaccharide composed of glucose and fructose: α-D-glucopyranosyl
β-D-fructofuranoside (Robayo-Torres et al., 2006). The sweetness, for which it is
favored in candy and pastries, arises from the fructose unit. It is hydrolyzed to mono-
saccharides by small intestinal membrane-bound sucrase-isomaltase (SI) activity.
The two subunits are both glucohydrolase family 31 α-glucosidase activities that play
a prominent role in the digestion of starch. Given that sucrose only became avail-
able as a cultivated crop about 4000 years ago, the role in starch digestion is likely
its primitive function. SI is bound to the membrane by the isomaltase containing
N-terminal. The human SI gene is located on chromosome 2 in a context conserved
among rodents. The family GH 31 genome extends throughout the archaea, bacte-
ria, and plant kingdoms (Benson et al., 2008). In rodents, SI intestinal activities are
low until the time of weaning to a starch-based diet. A human disorder, congenital
sucrase-isomaltase deficiency (CSID) has been discovered with clinical symptoms
similar to lactose intolerance. These patients are relieved if sucrose is removed from
the diet or an oral supplement of sucrase enzyme is fed with the sugar. Some CSID
patients also have symptoms when fed starch (Robayo-Torres et al., 2006).
starch digestion
The plant kingdom converts radiant to chemical negative energy by fixation of atmo-
spheric CO
2
through photosynthesis by leaf chloroplasts (Quezada-Calvillo et al.,
2007a, 2007b). The immediate product of carbon fixation by chloroplasts is starch,
which is synthesized in light and degraded in dark cycles. The disaccharide sucrose is
produced in leaf cytosol from starch-derived adenosine diphosphate (ADP)-glucose.
Sucrose is then transported from leaves to amyloplasts in reproductive tissues, where
it is converted to storage molecules composed of thousands of polymeric glucose
