Download 29_Metabolism of amino acids. Digestion of proteins

PROTEIN
METABOLISM:
NITROGEN CYCLE;
DIGESTION OF
PROTEINS
Red meat is an important dietary
source of protein nitrogen
The Nitrogen Cycle and Nitrogen
Fixation
• Nitrogen is needed for amino acids,
nucleotides, etc
• Atmospheric N2 is the ultimate source of
biological nitrogen
• Nitrogen fixation: biosynthetic process of
the reduction of N2 to NH3 (ammonia)
• Higher organisms are unable to fix nitrogen.
• Some bacteria and archaea can fix nitrogen.
Symbiotic Rhizobium
bacteria invade the roots of
leguminous plants and form
root nodules.
Nodules of Rhizobium
bacteria
Rhizobium bacteria fix
nitrogen supplying both
the bacteria and the
plants.
Archaea
The amount of
N2 fixed by
nitrogen-fixing
microorganisms
is about 60% of
Earth's newly
fixed nitrogen.
25% is fixed by
industrial processes
(fertilizer factories)
Lightning and ultraviolet
radiation fix 15%
Nitrogen-fixing bacteria possess nitrogenase complex
which can reduce N2 to ammonia
The nitrogenase complex consists of two proteins:
 reductase, which provides electrons
 nitrogenase, which uses these electrons to reduce N2 to NH3.
The transfer of electrons from the reductase to the
nitrogenase is coupled to the hydrolysis of ATP.
The high-potential
electrons come from
protein ferredoxin,
generated by
photosynthesis or
oxidative processes.
16 molecules of ATP are
hydrolyzed for each
molecule of N2 reduced.
• Nitrogenase reaction:
N2 + 8 H+ + 8 e- + 16 ATP 
2 NH3 + H2 + 16 ADP + 16 Pi
Reductase – dimer
containing Fe-S clusters
and ATP-binding site
The nitrogenase
component is an 22
tetramer.
Contains P cluster
(Fe-S) and FeMo
cofactor.
FeMo cofactor is the
site of nitrogen
fixation.
Ammonia in the presence of
water becomes NH4+ which can
be used by plants
NH4+ can be rapidly oxidized
by soil bacteria Nitrosomonas
and Nitrobacter to NO2- and
NO3- (nitrification)
Nitrosomonas
NO2- and NO3- are used by higher plants
Another soil bacteria can reverse the nitrification
process and convert NO2- and NO3- back to nitrogen
Nitrogen from plants and animals is recycled to soil
(excretion of nitrogen in the form of urea or uric acid;
decay of plants and animals) - nitrogen cycle
Assimilation of Ammonia
• Ammonia generated from N2 is assimilated into
amino acids such as glutamate or glutamine
A. Ammonia Is Incorporated into Glutamate
• Reductive amination of a-ketoglutarate by
glutamate dehydrogenase occurs in plants, animals
and microorganisms
This reaction establishes the stereochemistry of the
-carbon atom in glutamate. Only the L isomer of
glutamate is synthesized.
B. Glutamine Is a Nitrogen Carrier
• A second route in assimilation of ammonia is via
glutamine synthetase
All organisms have both enzymes: glutamate
dehydrogenase and glutamine synthetase.
Amino acids are used for the synthesis of proteins.
Animals and humans consume proteins.
Proteins undergo digestion in the stomach and intestine.
The nitrogen cycle
Nitrogen Balance (NB):
 Nitrogen balance is a comparison between Nitrogen
intake (in the form of dietary protein) and Nitrogen
loss (as undigested protein in feces, NPN as urea,
ammonia, creatinine & uric acid in urine, sweat &
saliva & losses by hair, nail, skin).
 NB is important in defining
1.overall protein metabolism of an individual
2.nutritional nitrogen requirement.
Nitrogenous balance
Positive nitrogenous balance – the amount of nitrogen entered the
organism is more than amount of nitrogen removed from the
organism. It occurs in young growing organism, during recovering
after severe diseases, at the using of anabolic medicines pregnancy,
lactation and convulascence
Negative nitrogenous balance – the amount of nitrogen removed
from the organism is more than amount of nitrogen entered the
organism. It occurs in senile age, destroying of malignant tumor, vast
combustions, poisoning by some toxins. High loss of tissue proteins
in wasting diseases like burns, hemorrhage & kidney diseases with
albuminurea (High breakdown of tissue proteins ) in
hyperthyroidism, fever, infection
Zero nitrogenous balance – the amount of nitrogen removed from the
organism is equal to the amount of nitrogen entered the organism. It
occurs in healthy adult people
Normal adult: will be in nitrogen equilibrium, Losses = Intake
A deficiency of
even one amino
acid results in a
negative nitrogen
balance.
In this state, more
protein is
degraded than
synthesized.
Protein Requirement for humans
in Healthy and Disease Conditions
The normal daily requirement of protein for adults is
0.8 g/Kg body wt. day-1.
• That requirement is increased in healthy conditions:
during the periods of rapid growth, pregnancy, lactation
and adolescence.
• Protein requirement is increased in disease states:
illness, major trauma and surgery.
• RDA for protein should be reduced in:
hepatic failure and renal failure
The Fate of Dietary Protein
The intake of dietary protein is in the
range of 50-100g/day.
 Digestion and absorption .
 Maintenance of body protein stores.
 Net protein synthesis.
 Synthesis of non-protein compounds
 Oxidative deamination
Biological Value for Protein (BV)
BV is : a measure for the ability of dietary protein to
provide the essential amino acids required for tissue
protein maintenance.
• Proteins of animal sources (meat, milk, eggs) have high
BV because they contain all the essential amino acids.
• Proteins from plant sources (wheat, corn, beans) have
low BV thus combination of more than one plant
protein is required (a vegetarian diet) to increase its BV.
PROTEINS in the BODY
• Amino Acid Pool – amino acids that are available
throughout the body (tissues and fluids) for use
when needed.
• Protein Turnover – of the ~ 300 grams of protein
synthesized by the body each day, 200 grams are
made from recycled amino acids.
Protein digestion
Chemical composition of digestive juices.
Gastric juice contains water, enzymes, hydrochloric acid,
mineral salts and other compounds. About 2,5 l of gastric
juice is secreted per day.
The role of hydrochloric acid in digestion.
 Denaturate proteins (denaturated proteins easier undergo
digestion by pepsin than native proteins).
 Stimulates the activity of pepsin.
 Hydrochloric acid has bactericidial properties.
 Stimulates the peristalsis.
 Regulate the enzymatic function of pancreas.
Protein digestion
Digestion in Stomach
Stimulated by food acetylcholine, histamine and gastrin
are released onto the cells of the stomach
The combination of acetylcholine, histamine and gastrin
cause the release of the gastric juice.
Mucin - is always secreted in the stomach
HCl - pH 0.8-2.5 (secreted by parietal cells)
Pepsinogen (a zymogen, secreted by the chief cells)
Hydrochloric acid:
 Creates optimal pH for
pepsin
 Denaturates proteins
 Kills most bacteria and
other foreign cells
Proteolytic enzymes and their activation.
Three enzymes are in gastric juice: pepsin, gastricsin and rennin. All
these enzymes cleave proteins or peptides.
Pepsinogen (MW=40,000) is activated by the enzyme pepsin present
already in the stomach and the stomach acid.
Pepsinogen cleaved off to become the enzyme pepsin (MW=33,000) and a
peptide fragment to be degraded.
Pepsin partially digests proteins by cleaving the peptide bond formed by
aromatic amino acids: Phe, Tyr, Trp
Optimal pH for gastricsin is 2,0-3,0. The
ratio between gastricsin and pepsin in
gastric juice is 1:5,5. This ratio can be
changed in some pathological states.
Rennin also possesses a proteolytic
activity and causes a rapid coagulation of
ingested casein. But this enzyme plays
important role only in children because
the optimal pH for it is 5-6.
Digestion in the Duodenum
Stimulated by food secretin and cholecystokinin regulate
the secretion of bicarbonate and zymogens trypsinogen,
chymotrypsinogen, proelastase and procarboxypeptidase
by pancreas into the duodenum
Bicarbonate changes the pH to about 7
The intestinal cells
secrete an enzyme
called
enteropeptidase
that acts on
trypsinogen cleaving
it into trypsin
Trypsin converts chymotrypsinogen into chymotrypsin,
procarboxypeptidase into carboxypeptidase and
proelastase into elastase, and trypsinogen into more
trypsin.
Trypsin which cleaves peptide bonds between basic
amino acids Lys and Arg
Chymotrypsin cleaves the bonds between aromatic amino
acids Phe, Tyr and Trp
Carboxypeptidase which cleaves one amino acid at a
time from the carboxyl side
Aminopeptidase is secreted by the small intestine and
cleaves off the N-terminal amino acids one at a time
Enteropeptidase secreted by the mucosa of duodenum initiates
the activation of the pancreatic proenzymes
27
Proteolytic enzymes exhibit the preference for particular types of peptide bonds
Proteinases preferentially attacks the bond after:
Pepsin
aromatic (Phe, Tyr) and acidic AA (Glu, Asp)
Trypsin
basic AA (Arg, Lys)
Chymotrypsin
hydrophobic (Phe, Tyr, Trp, Leu) and acidic AA (Glu, Asp)
Elastase
AA with a small side chain (Gly, Ala, Ser)
Peptidases:
Carboxypeptidase A
Carboxypeptidase B
nearly all AA (not Arg and Lys)
basic AA (Arg, Lys)
aminopeptidase
nearly all AA
Prolidase
proline
Dipeptidase
only dipeptides
The splitting of elastin in an intestine is catalyzed by elastase and collagen is
decomposed by collagenase.
Digestion of protein takes place not only in the intestinal cavity but also on the
surface of mucosa cells.
Mechanism of amino acid absorbtion.
This explanation is called the sodium cotransport
theory for amino acid transport; it is also called secondary
active transport of amino acid.
Absorption of amino acids through the intestine mucosa can occur
far more rapidly than protein can be digested in the lumen of the
intestine.
Since most protein digestion occurs in the upper small intestine
most protein absorption occurs in the duodenum and jejunum.
Most proteins are completely digested to free amino acids
Amino acids and sometimes short oligopeptides are
absorbed by the secondary active transport
Amino acids are transported via the blood to the cells of
the body.
The ways of entry and using of amino
acids in tissue
The sources of amino acids:
1) absorption in the intestine;
2) protein decomposition;
3) synthesis from the carbohydrates and lipids.
Using of amino acids:
1) for protein synthesis;
2) for synthesis of other nitrogen containing
compounds (creatine, purines, choline, pyrimidine);
3) as the source of energy;
4) for the gluconeogenesis.