Agriculture Reference
In-Depth Information
gal species may have constituted significant components of their diet. Each molecule
of trehalose hydrolyzed renders two molecules of free glucose.
PRImARy Routes of cARbohydRAte
metAbolIsm In mAmmAls
Glucose, galactose, and fructose are the three main carbohydrates that can be metabo-
lized by the human body. Most other carbohydrates, if absorbed, have to be transformed
into glucose before being metabolized; otherwise, they are excreted into the urine.
Depending on energetic requirements, glucose can follow different metabolic
pathways: With high-energy expenditure, glucose follows predominantly a pathway
leading to its oxidative transformation into CO 2 , H 2 O, and energy in the form of
adenosine triphosphate (ATP). With low-energy expenditure, glucose follows path-
ways of storage in liver and muscle that consists of the formation of glycogen (glyco-
genesis). Storage in adipose tissue consists of glucose transformation into fatty acid
precursors of lipids (Mathews et al. 1999).
g L u C o s e m e t a b o L i is m
Liver is the main organ controlling the metabolic route followed by glucose after
absorption. The hepatic dominance in glucose metabolism is primarily defined by the
enterohepatic circulation that carries most absorbed components of diet into the liver
via portal circulation. Insulin and glucagon are in turn the main hormonal regulators
for the rate of absorption of glucose into hepatic as well as other body tissues. Brain
and muscle are the two extrahepatic tissues with the highest oxidative metabolism
of glucose. The incorporation and “fixation” of glucose in the tissues depends on the
glucose transporter GLUT2 and the enzyme hexokinase, which catalyzes the phos-
phorylation of glucose into glucose-6P. Hexokinase is activated by insulin, causing
the fast absorption and retention of glucose by the body cells, while glucagon has
the opposite effects on the enzyme. Once in the cytoplasm, glucose can follow either
the oxidative pathways leading to the production of energy and involving glycolysis,
citric acid pathway, and mitochondrial oxidative phosphorylation/respiratory chain
or the storage pathways leading to glycogen and fat.
For the glycolytic pathway, glucose-6P is isomerized to fructose-6P and further
phosphorylated to fructose-1,6-biphosphate. This step is the main regulator for the
entrance of glucose into the glycolytic pathway with adenosine monophosphate
(AMP), a product from consumption of ATP, as an allosteric activator of the enzyme
6-phosphofructokinase that catalyzes this phosphorylation reaction. Depending on
the intracellular redox state, the glycolytic pathway can produce pyruvate or lactate
as the main final products: A high concentration of H + in the form of NADH or
NADPH plus H + , products of the citric acid pathway, causes accumulation of lactate
and a decreased pyruvate concentration, which limits its availability for the citric
acid pathway; low concentrations of NADH or NADPH plus H + increase pyruvate
concentration, which is the main substrate of the citric acid pathway. The entrance
of pyruvate into the citric acid pathway, also known as the tricarboxylic acid cycle,
Search WWH ::




Custom Search