Agriculture Reference
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involves its irreversible transformation into acetate by the enzyme pyruvate dehydro-
genase, with removal of one carbon in the form of CO 2 and association to a cofactor
known as coenzyme A (CoA). Pyruvate dehydrogenase is a multisubunit enzyme
complex susceptible to allosteric regulation by a multiplicity of metabolites and thus
is the main control point for the citric acid pathway; however, the most relevant
regulators are the relative concentrations of adenosine diphosphate (ADP)/ATP and
NAD/NADH plus H + : ADP and NAD activate it, while the other two decrease its
activity. Although in plant species the production of new glucose molecules (glu-
coneogenesis) is possible from acetate groups, in most mammals this step is com-
pletely irreversible; therefore, gluconoegenesis occurs through completely different
pathways that depend on the transformation of amino acids into pyruvate. This is
the main reason for the incapacity of mammals to produce glucose from the acetate
groups generated from the β-oxidation of lipids. The acetate resulting from the pyru-
vate decarboxylation is then linked to a molecule of oxaloacetate with production of
citric acid, from which the pathway takes its name. In the citric acid pathway, the
two-carbon acetate molecule is broken down into CO 2 with the production of large
quantities of the reduced intermediaries NADH plus H + and FADH 2 as well as some
guanosine triphosphate (GTP) molecules. NADH plus H + and FADH 2 are then used
as proton donors to generate a transmembrane electrochemical gradient that drives
the production of ATP by the mitochondrial respiratory chain, which is the end prod-
uct of glucose oxidative pathways.
In the storage pathways, when glucose-6P accumulates in the cytoplasm of liver and
muscle cells due to low concentrations of AMP and low activity of 6-phosphofructoi-
nase, glucose-6P can be isomerized to glucose-1P and then transformed into uridine
diphosphate (UDP)-glucose to follow the pathway of glycogenesis, with production of
glycogen as a form of energy storage in animal tissues. In addition, in adipose tissue
the absorption of glucose causes the diversion of the glucolysis toward the production
of glyceraldehyde-3P, which is used in the synthesis of fatty acids and triacylglycer-
ides, without production of pyruvate, lactate, or energy (Mathews et al. 1999).
f r u C t o s e m e t a b o L i s m
Fructose can be metabolized directly by the glycolysis pathway through produc-
tion of fructose-1P catalyzed by the enzyme fructokinase. By effects of the enzyme
aldolase of the glycolysis pathway, fructose-1P is cleaved into dihydroxyacetone-P
and glyceraldehyde; the last is further phosphorylated to produce glyceraldehyde-3P.
These two products are identical to the two intermediate three-carbon molecules
produced during the glycolysis from glucose and afterward follow the same meta-
bolic processing. This metabolic route involves a shortcut from the classical glycoly-
sis followed by glucose and may be the cause for the observed faster rate of fructose
metabolism (Mathews et al. 1999).
g a L a C t o s e m e t a b o L i s m
For the catabolism of galactose, its conversion into glucose is indispensable by a
complex pathway. First, galactose is phosphorylated by liver galactokinase; then,
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