Download biochemistry-n-6-protein-metabolism

Protein and amino acid metabolism
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Protein Digestion and Amino Acid Absorption
I. Protein Digestion
A. Digestion in the Stomach
B. Digestion by Enzymes From the Pancreas
C. Digestion by Enzymes From Intestinal Cells
Synthesis and Degradation of Amino Acids
IV. Amino Acid Degradation
I1. Fate Of Amino Acid Nitrogen
A. Transamination Reactions
B. Removal of Amino Acid Nitrogen as Ammonia
C. Role of Glutamate in the Metabolism of Amino Acid Nitrogen
D. Ammonia Toxicity
Reference :Marks' Medical Biochemistry: A Clinical Approach.
• Protein Digestion and Amino Acid
Absorption
• I. Protein Digestion
• Digestion of proteins begins in the stomach
and is completed in the intestine (Fig. 30.1).
• A. Digestion in the Stomach
• 1-Pepsinogen is secreted by the chief cells of
the stomach.
• 2-The gastric cells secrete hydrochloric acid
(HCl). The acid in the stomach lumen alters
the conformation of pepsinogen so
producing the active protease pepsin.
3-Dietary proteins are denatured by the acid in
the stomach. . However, at the low pH of the
stomach, pepsin is not denatured; it acts as
an endopeptidase, cleaving peptide bonds at
various points within the protein chain.
Smaller peptides and some free amino acids
are produced.
Anatomy of an amino acid
Chemistry of peptide bond formation
• B. Digestion by Enzymes From the Pancreas
• 1-As the gastric contents empty into the intestine,
they exposed to the secretions from the exocrine
pancreas.
• 2- One of these secretions is bicarbonate, which, in
addition to neutralizing the stomach acid, raises the
pH.
• 3- pancreatic proteases, which are also present in
pancreatic secretions, can be active. cleavage of
trypsinogen to the active enzyme trypsin,
•
which then cleaves the other pancreatic zymogens,
producing their active forms (see Fig. 30.2).
4-The smaller peptides formed by the action of trypsin,
chymotrypsin, and elastase ,and they are cleave one
amino acid at a time from the end of the chain.
• C. Digestion by Enzymes From Intestinal Cells
• 1- Exopeptidases produced by intestinal epithelial
cells act within the brush border and also within the
cell.
• 2- Aminopeptidases, located on the brush border,
cleave one amino acid at a time from the amino end
of peptides. Intracellular peptidases act on small
peptides that are absorbed by the cells.Finally a.a
absorbed from small intestine.
Protein half life
Body proteins have life times. Life time of a protein is
expressed in terms of half-life.
It is defined as time required for initial amount of
protein to be reduced to half.
Half life of proteins ranges from minutes to years.
Protein turn over
In all forms of life, proteins once formed may not
remain forever, proteins are synthesized and
degraded. Hence, body protein is in dynamic state.
Continuous synthesis and degradation of protein is
called as protein turnover.
The rates of protein synthesis and degradation vary
according to physiological needs.
The rate of protein synthesis is high during growth
lactation and post operative recovery.
In starvation, cancer, fever and during rate of
degradation of protein is more.
• Source of plasma amino acids
Amino acids in the plasma are mainly derived from
endogenous protein breakdown and dietary protein
breakdown. Intracellular synthesis may contribute to
plasma amino acid pool to some level.
Synthesis and Degradation of Amino Acids
1-Humans can synthesize only 11 of the 20 amino acids
required for protein synthesis; the other 9 are
considered essential amino acids for the diet.
2- The nonessential amino acids can be synthesized
from glycolytic intermediates (serine, glycine,
cysteine and alanin), tricarboxylic acid cycle
intermediates (aspartate, asparagine, glutamate,
glutamine, proline, arginine, and ornithine), or from
existing amino acids (tyrosine from phenylalanine).
3- When amino acids are degraded, the nitrogen is
converted to urea, and the carbon skeletons are
classified as either glucogenic (a precursor of
glucose) or ketogenic (a precursor of ketone bodies).
• Amino acid catabolism
Amino acid catabolism occurs in two main stages.
• First stage is the removal of amino group of amino
acids as ammonia. Ammonia is converted to urea
and excreted in urine.
• In the second stage carbon skeletons of amino
acids are converted into intermediates of TCA cycle
and acetyl-CoA. Then they are either oxidized in TCA
cycle for generation of energy or used for glucose
synthesis or ketone body formation.
Deamination of amino acids
Since the removal of α-amino group is the first stage of
amino acid degradation we shall see now how it
occurs. There are several ways for the removal of
α-amino group of amino acids.
They are
(1) Transamination followed by oxidative deamination
(2) (2) Oxidative deamination
(3) Non-oxidative deamination.
(1)A/ Transamination
Removal of amino group of amino acids by
transamination is the first step in the catabolism
of most of the amino acids. The enzymes involved in
this process are known as transaminases.
They transfer α-amino group to an acceptor mostly to
α-keto glutarate.
(B) Oxidative deamination
Amino groups that are collected by glutamate are
removed as ammonia by enzyme glutamate
dehydrogenase in presence of NAD or NADP+.
Glutamate dehydrogenase catalyzes oxidative
deamination of glutamate to yield α-ketoglutarate and
ammonia (Fig. before).
Thus, the combined action of transaminases
and glutamate dehydrogenase results in the
net removal of amino groups of most amino
acids as ammonia.
(2) Oxidative deamination by a.a oxidase
Amino acids undergo NAD+ independent oxidative
deamination also. Amino acid oxidases are
the enzymes which catalyzes this type of oxidative
deamination. They are of two types and
dependent on FMN or FAD. They are D-amino acid
oxidase and L-amino acid oxidase. Since
the activity of L-amino acid oxidase is low its
contribution to ammonia production is less.
D-amino acid oxidase
It is present in liver and kidney. It cannot
deaminate many amino acids. It uses FAD as
cofactor. It can deaminate glycine.
L-amino acid oxidase It is also present in liver
and kidney. It cannot deaminate glycine and
dicarboxylic acids. It is dependent on FMN.
The reaction mechanism of these oxidases involves
1- first oxidation of amino acid to iminoacid. FMN (FAD)
are reduced.
2-Next imino acid undergo hydrolytic loss of ammonia
to produce α-keto acid (Fig. bellow).
3-Non-oxidative deamination
Non-oxidative deamination of amino acids is catalyzed
by specific enzymes. Some of them are given below
(Fig. below).
(Transport of ammonia)
Since free ammonia is toxic even in trace it is
transported in the form of glutamine and alanine.
• Transport of ammonia as glutamine
From the brain and other peripheral organs except
muscle ammonia released from amino acids is
transported to liver and kidney as amide nitrogen of
glutamine.
• Formation of glutamine
A widely distributed glutamine synthetase catalyzes
the formation of glutamine from glutamate
and ammonia in ATP dependent reaction (Fig. bellow).
Glutamine so formed enters circulation.
(Ammonia toxicity)
Since ammonia is toxic to central nervous
system cells, blood ammonia level must be
within normal range.
If blood ammonia level raises due to any
reason , symptoms of ammonia intoxication
appears.
They are slurred speech, blurred vision and
tremors.
Coma and death can occur in severe cases.
Mechanism of ammonia toxicity
Ammonia can cause brain toxicity by three ways.
1. The entry of ammonia into brain leads to formation of
glutamate by the reversal of glutamate dehydrogenase
reaction.
This depletes available α-keto glutarate in the brain. As a result
citric acid cycle operation is impaired and ATP production
diminishes. This leads to brain cell dysfunction.
2. Since the brain is rich in glutamine synthetase the ammonia
which enters brain is used for glutamine synthesis.
This leads to depletion of cellular ATP and cell dysfunction.
3. Since glutamate is considered as neurotransmitter the toxic
effect of ammonia may be due to over stimulation of nerve cells
by glutamate formed from ammonia and α-keto glutarate by the
action of glutamate dehydrogenase.
Causes for ammonia toxicity
1. If hepatic function is impaired plasma ammonia rises
to toxic level. Liver function can impair in cases of
poisoning due to carbon tetra chloride, heavy metals
and viral infections.
2. Liver cirrhosis
3. Consumption of protein rich diet after
gastrointestinal haemorrhage can cause ammonia
toxicity.
Metabolic fates of carbon skeletons of Amino acids
Initial deamination of aminoacids produces carbon
skeletons of amino acids. The carbon skeletons of
twenty aminoacids are converted to seven
compounds. These seven compounds are ultimately
used for the formation of carbohydrates or fat like
substances (Fig. 12.8).
Depending on the cell needs they may be used for
energy production. Hence, amino acids
are classified based on the metabolic fate of their
carbon skeletons also. They are
1. Glucogenic amino acids
2. Ketogenic amino acids
3. Glucogenic and ketogenic amino acids
1. Glucogenic amino acids
The carbon skeletons of these aminoacids are
converted to either glucose or intermediates
of TCA cycle.
The products may be pyruvate, oxaloacetate,
α-ketoglutarate, succinate and fumarate.
Glycine, alanine, valine, serine, threonine,
cysteine, methionine, aspartate
(aspargine), glutamate (glutamine), histidine,
arginine and proline are glucogenic amino
acids. Note that all non-essential amino acids
are glucogenic.
2. Ketogenic amino acids
The carbon skeletons of these aminoacids
gives rise to fat like substances or
intermediates of fatty acid catabolism. The
products may be aceto acetyl-CoA and
acetyl-CoA.
Leucine is the only ketogenic aminoacid.
Isoleucine, phenylalanine, tyrosine,
tryptophan and lysine are also ketogenic
amino acids.
3. Glucogenic and ketogenic amino acids
The carbon skeletons of these amino
acids are converted to glucose or
intermediates of TCA cycle and fat like
substance.
The products may be pyruvate,succinate,
fumarate and acetyl- CoA.
Isoleucine, phenylalanine, tyrosine,
tryptophan and lysine are glucogenic
and ketogenic amino acids.