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Faculty of nursing
CHEM 203
Biochemistry
UNIT VII
Amino acids , Protein chemistry and
metabolism
Part
Dr. Ola Fouad Talkhan
The process of digestion is defined as the
‘process by which macromolecules in food are
broken down into their component smallmolecule subunits’.
The macromolecules are the proteins or
polypeptides themselves, and the subunits are
the amino acids.
The bonds holding the subunits together are
peptide bonds
the degradation of proteins by cellular enzymes
enzymes in a process called hydrolysis.

Protein digestion takes place in two different phases:
◦ In the stomach
◦ In the small intestine

Both of these phases of digestion are based on several
types of enzymes that are called proteinases and
proteases.
Gastrin
-stimulates Parietal cells to secrete HCL; Chief cells of the
gastric glands to secrete pepsinogen
Hydrochloric acid
-Denatures protein structure
-Activates pepsinogen (zymogen) to pepsin
Pepsin
-hydrolyzes proteins to smaller polypeptides and some free
amino acids.
Proteins
Pepsin , H+
pH 1-2
proteoses
+ peptones
The remainder of protein digestion occurs in the small intestine as
the result of the action of enzymes such as
1- trypsin , chemotrypsin and carboxypeptidase (secreted by the
pancreas)
proteoses
peptones
Trypsin, chemotrypsin
HCO3 , pH 8.0
polypeptides + A.A.
aminopeptidase
polypeptides
di, and tripeptidase + A.A.
carboxypeptidase
2- peptidases ( aminopeptidase ,dipeptidase and tripeptidase )
(located in the cells that line the small intestine).
di, and tripeptidase
dipeptidase
tripeptidase
A.A.

Proteins are broken down to
◦ Tripeptides
◦ Dipeptides
◦ Free amino acids
Free amino acid  small intestine(villi)Liverblood
circulation
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


Amino Acid Degradation and Synthesis
The catabolism of the amino acids involves the removal
of α-amino groups (NH3), followed by the breakdown of
the resulting carbon skeletons.
These pathways converge to form seven intermediate
products:oxaloacetate, pyruvate, α-ketoglutarate,
fumarate, succinyl coenzyme A (CoA), acetyl CoA, and
acetoacetate.
these will enter in the synthesis of glucose or lipid or in
the production of energy through their oxidation to CO2
by the citric acid cycle
Nonessential amino acids can be
synthesized from the intermediates
of metabolism or, as in the case of
cysteine and tyrosine, from essential
amino acids
Fate of NH3 produced in deamination
Ammonia produced from deamination of A.A. are very
toxic to the brain and nervous tissues so, it is rapidly
removed by :
1. Urea : Formation of urea in the liver is
quantitatively the most important disposal route
for ammonia.
Urea travels in the blood from the liver to the kidneys,
where it passes into the glomerular filtrate.
2. Glutamine synthesis in brain and liver
 Circulating glutamine is removed
by the liver and the kidneys and
deaminated by glutaminase.
 In the liver, the NH3 produced is
detoxified through conversion to urea
, and in the kidney it can be used
in the excretion of protons.
Hyperammonemia
The capacity of the hepatic urea cycle exceeds the normal rates
of ammonia generation, and the levels of serum ammonia are
normally low (5–35 μmol/L).
However, when liver function is compromised, due either to
genetic defects of the urea cycle or liver disease, blood levels
can rise above 1,000 μmol/L.
Such hy per ammon emia is a medical emergency, because
ammonia has a direct neurotoxic effect on the CNS
the symptoms of ammonia intoxication,
1-tremors, slurring of speech, somnolence, vomiting, cerebral
edema, and blurring of vision.
2-At high concentrations, ammonia can cause coma and death.
The two major types of hyperammonemia are:
1. Acquired hyperammonemia:
Liver disease is a common cause of hyperammonemia in adults,
and may be due, for example, to viral hepatitis or to hepatotoxins
such as alcohol
2. Congenital hyperammonemia:
Genetic deficiencies of each of the five enzymes of the urea cycle
have been described
METABOLIC DEFECTS IN AMINO ACID
METABOLISM
1- Phenylketonuria (PKU).
Phenylketonuria (PKU), caused by a
deficiency of phenylalanine hydroxylase
A deficiency in phenylalanine
hydroxylase results in the disease
phenylketonuria (PKU).
Characteristics of classic PKU:
1-Elevated phenylalanine:Phenylalanine is
present in elevated concentrations in tissues,
plasma, and urine.
2-CNS symptoms: Mental retardation,
failure to walk or talk,
seizures, hyperactivity, tremor, microcephaly,
and failure to grow are characteristic findings in
PKU.
3-Hypopigmentation: Patients with
phenylketonuria often show a deficiency of
pigmentation (fair hair, light skin color, and
blue eyes).
2- Maple syrup urine disease
(MSUD) is a rare (1:185,000), autosomal recessive disorder
in which there is a partial or complete deficiency in
branched-chain α-keto acid dehydrogenase, an enzyme
complex that decarboxylates leucine, isoleucine, and valine
Accumulation of these A.A. in the blood, causing a toxic
effect that interferes with brain functions.
Characteristics of Maple syrup urine disease
 feeding problems, vomiting, dehydration, severe
metabolic acidosis, and a characteristic
maple syrup odor to the urine.
 If untreated, the disease leads to mental retardation,
physical disabilities, and even death.
3- Homocystinuria
The homocystinurias are a group of disorders
involving defects in the metabolism of
homocysteine.
The diseases are inherited as autosomal
recessive illnesses,due to a defect in
the enzyme cystathionine β-synthase,
Characteristics of Homocystinuria
high plasma and urinary levels of homocysteine
and methionine and low levels of cysteine.
ectopia lentis (displacement of the lens of the
eye), skeletal abnormalities, a tendency to
form thrombi (blood clots), osteoporosis, and
neurological deficits.
Patients can be responsive or nonresponsive to oral
administration of pyridoxine (vitamin B6)