Download Metabolism II BCH 440

Metabolism II
BCH 440
Dr. Amina R ELGezeery
Biochemistry Dept
King Saud University
Continuous Assessment Tests (CAT)
• Two Tests -------------------------- 50 Marks
• Final----------------------------------50 Marks
• Dates for CAT:
– 1st CAT:Wed.
14-11- 1432
– 2nd CAT:Wed.
4-1- 1433
Time: 11-12
Lecture Room: B 8, R 696
Ref. Books
• Lehninger: Pronciples of Biochemistry by DL.
Nelson and MI. Cox .
Course Outline
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Lipoproteins properties and their metabolism.
Prostaglandins metabolism.
Sterol metabolism.
Digestion and absorption of amino acids.
Amino acids catabolism.
Amino acids biosynthesis.
Conversion of amino acids to specialized
products.
• Biochemistry of prophyrins.
• Integration of metabolism
Digestion and Apsorbtion of
proteins
Introduction
• Proteins are the most abundant biological macromolecules .
• Occurring in all cells and all parts of cell.
• Proteins occur in great variety; thousands of different kinds,
ranging in size from relatively small peptides to huge polymers
with molecular weights in the millions.
• Proteins have enormous diversity of biological function.
• Amino acids are the building blocks of proteins.
• There are 20 different amino acids that make up human
proteins.
• Dietary proteins are a vital source of amino acids.
• Discarded cellular proteins are another source of amino
acids.
Proteins functions
• Have several diverse functions:
- Catalytic functions [enzymes]
- Receptor [insulin receptor]
- Structural function [collagen]
- Transport [haemoglobin, myoglobin]
- Protective functions [immunoglobulins]
- Hemostasis [clotting factors]
- Hormonal functions [insulin, glucagon, GH]
- Control of gene expression [transcription factors]
- DNA packing [histones]
- Act as buffers
Digestion of Dietary Proteins
• Digestion involves hydrolyzing food molecules into
smaller molecules for absorption through the
gastrointestinal epithelium.
• Proteins are generally too large to be absorbed by the
intestine. [ An example of an exception to this rule is
that newborns can take up maternal antibodies in
breast milk.]
• They shouled be hydrolyzed to yield their constituent
amino acids, which can be absorbed.
• Proteolytic enzymes responsible for degrading
proteins are produced by three different organs: the
stomach, the pancreas, and the small intestine.
Digestive Enzymes
• Several Groups of Enzymes Catalyze the Digestion of
Proteins:
There are two main classes of proteolytic digestive enzymes
(proteases), with different specificities for the amino acids which
form the peptide bond to be hydrolyzed.
Endopeptidases hydrolyze peptide bonds between specific amino
acids throughout the molecule.
Exopeptidases catalyze the hydrolysis of peptide bonds, one at a
time, from the ends of peptides.
These enzymes have different specificities with no single enzyme
can completely digest a protein. However by acting simultaneously,
they can digest dietary proteins to amino acids.
Enzymatic Degradation of Dietary Protein
to Amino Acids
• In humans, the degradation of ingested proteins to their constituent amino
acids occurs in the gastrointestinal tract.
• Entry of dietary protein into the stomach stimulates the gastric mucosa to
secrete the hormone gastrin.
• Gastrin stimulates the secretion of hydrochloric acid by the parietal cells and
pepsinogen by the chief cells of the gastric glands.
• The acidic gastric juice (pH 1.0 to 2.5) is both an antiseptic, killing most
bacteria and other foreign cells, and a denaturing agent, unfolding globular
proteins and rendering their internal peptide bonds more accessible to
enzymatic hydrolysis.
• Pepsinogen , an inactive precursor, or zymogen is converted to active pepsin
either by HCl, or by other pepsin molecules that have already been activated.
• In the stomach, pepsin hydrolyzes ingested proteins at peptide bonds on the
amino-terminal side of the aromatic amino acid residues Phe, Trp, and Tyr ,
cleaving long polypeptide chains into a mixture of smaller peptides.
Protein Digestion in Stomach
– HCl from parietal cells
• Stomach pH 1.6 to 3.2
• Denatures 40, 30, and 20 structures
– Pepsinogen from chief cells
HCl
Pepsinogen
Pepsin
• Cleaves at phenylalanine, tyrosine, tryptophan
Aromatic amino acids
• Protein leaves stomach as mix of insoluble protein,
soluble protein, peptides and amino acids
Enzymatic Degradation of Dietary Protein
to Amino Acids ….cont
• As the acidic stomach contents pass into the small intestine, the low
pH triggers secretion of the hormone secretin into the blood.
• Secretin stimulates the pancreas to secrete bicarbonate into the
small intestine to neutralize the gastric HCl, increasing the pH to
about 7. (All pancreatic secretions pass into the small intestine
through the pancreatic duct.)
• Arrival of amino acids in the upper part of the intestine (duodenum)
causes release into the blood of the hormone cholecystokinin,
which stimulates secretion of several pancreatic enzymes with
activity optima at pH 7 to 8.
• Cholecystokinin and secretin are two polypeptide hormones of
the digestive tract.
Protein Digestion in Small Intestine
• Pancreatic enzymes: are synthesized and
secreted by the exocrine cells of the pancreas.
– Trypsinogen
– Chymotrypsinogen
– Procarboxypeptidase A&B
– Proelastase
Zymogens
Protein Digestion in Small
Intestine…cont
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Trypsinogen is converted to its active form, trypsin, by
enteropeptidase (formerly called enterokinase), a proteolytic
enzyme secreted by intestinal cells.
• Trypsin can then activate:
(1) chymotrypsinogen to chymotrypsin,
(2) proelastase to elastase,
(3) procarboxypeptidase to carboxypeptidase.
• Synthesis of the enzymes as inactive precursors protects the
exocrine cells from destructive proteolytic attack.
• The pancreas further protects itself against self-digestion by making
a specific inhibitor, a protein called pancreatic trypsin inhibitor that
effectively prevents premature production of active proteolytic
enzymes within the pancreatic cells.
Digestion in Small Intestine
• Zymogens must be converted to active form
– Trypsinogen
Enteropeptidase/Trypsin Trypsin
• Endopeptidase
– Cleaves on carbonyl side of Lys & Arg
– Chymotrypsinogen
Trypsin
Chymotrypsin
• Endopeptidase
– Cleaves carboxy terminal Phe,Tyr,Trp,Met and Leu.
– Elastase
– Cleaves at carboxy terminal of ala, gly and ser.
– Procarboxypeptidase
Trypsin
• Exopeptidase
– Removes carboxy terminal residues
Carboxypeptidase
• Trypsin and chymotrypsin further hydrolyze the peptides
that were produced by pepsin in the stomach.
• This stage of protein digestion is accomplished very
efficiently, because pepsin, trypsin, and chymotrypsin
have different amino acid specificities.
• Degradation of the short peptides in the small intestine is
then completed by other intestinal peptidases.
• These include carboxypeptidases A and B (both of which
are zinc-containing enzymes), which remove successive
carboxyl-terminal residues from peptides, and an
aminopeptidase that hydrolyzes successive amino-terminal
residues from short peptides.
Protein Digestion
• Small intestine (brush border)
– Aminopeptidases
• Cleave at N-terminal AA
– Dipeptidases
• Cleave dipeptides
Summary of Proteolytic enzymes
• Exo- or endopeptidases
• Inactive proenzymes
• Stomach
– pepsinogen --> pepsin
• Endopeptidase
• Activated by acidic pH
• Preferentially splits Phe, Tyr, Glu and Asp bonds
• Pancreas (duodenum)
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trypsinogen --> trypsin
chymotrypsinogen --> chymotrypsin
Carboxypeptidase A
Carboxypeptidase B
• Enterocytes
– various aminopeptidases and dipeptidases
• The resulting mixture of free amino acids is transported into
the epithelial cells lining the small intestine through which the
amino acids enter the blood capillaries in the villi and travel to
the liver.
• In humans, most globular proteins from animal sources are
almost completely hydrolyzed to amino acids in the
gastrointestinal tract, but some fibrous proteins, such as
keratin, are only partly digested.
• In addition, the protein content of some plant foods is
protected against breakdown by indigestible cellulose husks.
Enzymatic Degradation
of Dietary Proteins
• (a) gastrin -> secretion of HCl by
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parietal cells and pepsin by chief
cells
(b) exocrine cells synthesize
zymogens
– zymogen granules fuse with
plasma membrane
– zymogens released into the
lumen of the collecting duct
– collecting ducts -> pancreatic
duct -> small intestine.
(c) Amino acids -> villi ->
capillaries -> portal vein -> liver
Summary of Protein Digestion
• Luminal digestion yields
• 40% free amino acids and
• 60% peptides with two to six amino acids
Abnormalities in protein digestion
• In individuals with a deficiency in pancreatic
secretion (for example, due to chronic
pancreatitis, cystic fibrosis, or surgical removal
of the pancreas), the digestion and absorption
of fat and protein is incomplete. This results in
the abnormal appearance of lipids
(steatorrhea) and undigested protein in the
feces .
Acute pancreatitis
• Acute pancreatitis is a disease caused by
obstruction of the normal pathway by which
pancreatic secretions enter the intestine.
• The zymogens of the proteolytic enzymes are
converted to their catalytically active forms
prematurely, inside the pancreatic cells, and
attack the pancreatic tissue itself.
• This causes excruciating pain and damage to
the organ that can prove fatal.
Trypsin Inhibitors
• Small proteins or peptides
• Present in plants
– Soybeans, peas, beans, wheat
• Block digestion of proteins
• Inactivated by heat
Absorption of amino acids and dipeptides
• The end product of the action of endopeptidases and
exopeptidases is a mixture of free amino acids, di- and
tripeptides, and oligopeptides, all of which are absorbed.
• Free amino acids are absorbed across the intestinal mucosa
into enterocytes by sodium-dependent active transport.
• There are several different amino acid transporters, with
specificity for the nature of the amino acid side chain (large or
small; neutral, acidic, or basic).
• The various amino acids carried by any one transporter
compete with each other for absorption and tissue uptake.
Free Amino Acid Absorption
• Free amino acids
– Carrier systems
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Neutral AA
Basic AA
Acidic AA
Imino acids
– Entrance of some AA is
via active transport
• Requires energy
Na+
Na+
Amino Acid Transporters – Brush
Border Membrane
Transport
system
L
B
IMINO
y+
Bo,+
bo,+
Energy
required
Substrates carried
No
Yes
Yes
No
Yes
No
Leu, other neutral
Phe, Tyr, Trp, Ile, Leu, Val
Pro, Gly
Basic amino acids
Most neutral and basic
Most neutral and basic
Absorption of amino acids and dipeptides….cont
• Di- and tripeptides are absorbed by a H+-linked transport system
where they are hydrolyzed to free amino acids, and are then
transported into the hepatic portal vein.
• Thus, only free amino acids are found in the portal vein after a meal
containing protein.
• These amino acids are either metabolized by the liver or released
into the general circulation. [ Branched-chain amino acids are
important examples of amino acids that are not metabolized by the
liver, but instead are sent from the liver into the blood.]
• Relatively large peptides may be absorbed intact, either by uptake
into mucosal epithelial cells (transcellular) or by passing between
epithelial cells (paracellular).
• Many such peptides are large enough to stimulate antibody
formation—this is the basis of allergic reactions to foods.
Peptide Absorption
• Form in which the majority of
protein is absorbed
• More rapid than absorption of
free amino acids
• Active transport
– Energy required
• Metabolized into free amino
acids in enterocyte
• Only free amino acids absorbed
into blood
Protein Transport in the Blood
• Amino acids diffuse across the basolateral
membrane
– Enterocytes → portal blood → liver → tissues
– Transported mostly as free amino acids
• Liver
– Breakdown of amino acids
– Synthesis of non-essential amino acids
Basolateral Membrane
• Transport of
free amino acids
only
– Peptides are
hydrolyzed
within the
enterocyte
• Transport mainly
by diffusion and
Na-independent
carriers
Groff & Gropper, 2000
Transport of Amino Acids into Cells
• The concentration of free amino acids in the extracellular fluids is
significantly lower than that within the cells of the body. This
concentration gradient is maintained because active transport systems,
driven by the hydrolysis of ATP, are required for movement of amino acids
from the extracellular space into cells.
• At least seven different transport systems are known that have
overlapping specificities for different amino acids.
• The small intestine and the proximal tubule of the kidney have common
transport systems for amino acid uptake; therefore, a defect in any one of
these systems results in an inability to absorb particular amino acids into
the gut and into the kidney tubules.
• For example, one system is responsible for the uptake of cystine and the
dibasic amino acids, ornithine, arginine, and lysine (represented as
“COAL”). In the inherited disorder cystinuria, this carrier system is
defective, and all four amino acids appear in the urine.
Cystinuria
• Cystinuria occurs at a frequency of 1 in 7,000
individuals, making it one of the most common
inherited diseases, and the most common genetic error
of amino acid transport. The disease expresses itself
clinically by the precipitation of cystine to form kidney
stones (calculi), which can block the urinary tract. Oral
hydration is an important part of treatment for this
disorder.
• Defects in the transport of tryptophan (and other
neutral amino acids) can result in Hartnup disorder and
pellagra-like dermatologic and neurologic symptoms.