Lec:5
biochemistry
Dr. Anwar J amzaiel
sodium–potassium pump (Na-K pump)
The single most important process that takes place in the small gut,
which makes nutrient absorption possible, is the establishment of an
electrochemical and concentration gradient of sodium across the
epithelial cell boundary of the lumen. To remain viable, all cells of the
body are required to maintain a low intracellular concentration of sodium.
Low intracellular sodium concentration is maintained by a large number
of Na+K+ ATPases, so-called sodium–potassium pumps
The transport of sodium creates both an electrical and a chemical
gradient across the plasma membrane. In turn this provides:
• a cell’s resting membrane potential, the gradient of which is the basis
for excitability in nerve and muscle cells
• export of sodium from the cell, providing the driving force for several
facilitated transporters, which import glucose, amino acids and other
nutrients into the cell
• translocation of sodium from one side of an epithelium to the other,
creating an osmotic gradient that drives absorption of water.
Cells contain huge numbers of these pumps. Abnormalities in the
number or function of Na+K+-ATPases are thought to be involved in
several pathologic states, in particular heart disease and hypertension.
Absorption of lipids
The products of lipid digestion include a mixture of partially hydrolyzed
lipids (primarily, monoacylglycerols and lysophospholipids), free fatty
acids, cholesterol fat-soluble vitamins, and other lipophilic molecules
(e.g., carotenoids). The main bulk dietary fat of man is triacylglycerol
with mixed long chain fatty acids. No hydrolysis of fat in the stomach(i.e.
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Lec:5
biochemistry
Dr. Anwar J amzaiel
no lipase). The suspension must be rendered water-soluble so they can
enter the intestine cells. All the products of lipid digestion ultimately
become solubilized by bile salts to form small mixed micelles that diffuse
from the lumen of the intestine toward the duodenum and jejunum,
where the dietary lipids undergo lipolysis by pancreatic lipase and
absorbed.
Whereas absorbed glucose can readily be transported as such to the
liver and other tissues in the bloodstream, this process is not suitable for
free fatty acids, because of both their limited solubility and their
detergent properties, which could disrupt cell membranes and inhibit
enzymes. Although free fatty acids released from adipocytes are
transported
in
plasma
bound
to
serum
albumin,
the
higher
concentrations of free fatty acids present after a meal would overwhelm
this transport system.
Instead, the absorbed fatty acids are reesterified into less polar products
for transport in the form of large lipoprotein aggregates called
chylornicrons. Triacylglycerol also is packaged with cholesterol,
lipoproteins and other lipids into chylomicrons. the hydrolysed
cholesterol ester and the free
cholesterol together with most of the
biliary is absorbed after transportation in the micelles. the mixed micelles
penetrate the membrane of the cell either by dissolving in the membrane
or by a process of pincytosis
Bile acids and pancreatic lipase act in the lumen to emulsify and digest
triacylglycerols into their monoacylglycerols and free fatty acids. Bile,
stored in the gall bladder and released into the duodenum, contains the
bile salts, sodium glycocholate and sodium taurocholate. Such
amphipathic molecules have a ‘detergent effect’ on lipids, intercalating
and breaking them down to smaller aggregates, and eventually to
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Lec:5
biochemistry
Dr. Anwar J amzaiel
micelles (4–8 nm diameter), thereby enabling the action of pancreatic
lipase.
Absorption of Proteins
Protein is digested in the stomach and intestine to peptide and amino
acid, where proenzymepepsinogen is autocatalytically converted to
pepsin A. Most proteolysis takes place in the duodenum via enzymes
secreted by the pancreas, including trypsinogen, chymotrypsinogen and
pro-carboxypeptidase
These proteases are produced in the form of their respective
proenzymes; thier combined action leads to the production of amino
acids, dipeptides and tripeptides. Enterokinase, activates trypsinogen
into trypsin, which in turn converts a number of precursor pancreatic
proteases into their active forms. The mechanism by which amino acids
are absorbed is conceptually identical to that of monosaccharides.
In health about 90% of the proteins are absorbed at very high rate. Most
natural a.a is L-isomer which transport actively against concentration
gradient. D.isomer transport passively at much slower rate to the portal
blood
Hartnup disorder is an autosomal recessive impairment of neutral amino
acid transport affecting the kidney tubules and small intestine. It is
believed that the defect is in a specific system responsible for neutral
amino acid transport across the brush-border membrane of renal and
intestinal epithelium, but the defect has not yet been characterised
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