1. Amino Acids Metabolism:
Disposal of Nitrogen

2. No Storage of Amino Acids in the body
So, amino acids must be obtained from
Diet
De novo synthesis (of non-essential aa)
Degradation of protein (normal turnover)
Unlike fats and carbohydrates, amino acids are not stored by the body, that is, no protein exists whose sole function is to maintain a supply of amino acids for future use.

3. Overall Nitrogen Metabolism
De novo
synthesis
Diet
Degradation
Amino Acids Pool
Protein
synthesis
Glucose & Fas ..etc
Overall Nitrogen Metabolism
Amino acid catabolism is part of the larger process of the metabolism of nitrogen-containing molecules.
Nitrogen enters the body in a variety of compounds present in food, the most important being amino acids contained in dietary protein.
Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism.
The role of body proteins in these transformations involves two important concepts: the amino acid pool and protein turnover.
Other Nitogen-
containing comp.

5. Protein turnover
Protein turnover
For many proteins, regulation of synthesis determines the concentration of protein in the cell, with protein degradation assuming a minor role.
For other proteins, the rate of synthesis is constitutive, that is, relatively constant, and cellular levels of the protein are controlled by selective degradation.
Most proteins in the body are constantly being synthesized & then degraded, permitting the removal of abnormal or unneeded proteins

6. By Two Major Enzyme Systems
Protein Degradation
By Two Major Enzyme Systems
1- Ubiquitin-proteasome mechanism
Energy-dependent
Mainly for endogenous proteins
(proteins synthesized within the cell)
2- Lysosomes
Non-energy-dependent
Primarily for extracellular proteins as:
- plasma proteins that are taken into cells by endocytosis
- cell surface membrane proteins: for receptor-mediated endocytosis

7. Amino Acids Catabolism

8. Amino Acids Catabolism - Overview
Unlike glucose and fatty acids, amino acids are not stored by the body
Amino acids in excess of biosynthetic needs are degraded.
Degradation of amino acids involves:
First Stage:
Removal of α-amino group Ammonia (NH3)
Second Stage:
Remaining carbon skeleton Energy metabolism

9. 1st phase of catabolism of amino acids: Removal of the α-amino groups
With production of
Free Ammonia
A portion of the free ammonia is excreted in urine but most is used in the synthesis of urea
In Liver
Small amount excreted in urine
Urea

10. Ammonia is produced by all tissues from the catabolism of amino acids
Ammonia is produced by all tissues from the catabolism of amino acids. Ammonia is mainly disposed is via formation of urea in liver
Blood level of ammonia must be kept very low, otherwise, hyperammonemia & CNS toxicity will occur
To solve this problem, ammonia is transported from peripheral tissues to liver via formation of:
1- Glutamine (most tissues)
2- Alanine (muscle)

11. 2nd phase of A. A. catabolism
Carbon skeletons of the α-ketoacids are converted to common intermediates of energy producing, metabolic pathways
ATP, CO2 & H2O (by Citric acid cycle)
Glucose (by gluconeogenesis)
Fatty Acids (from acetyl CoA)
Ketone Bodies (from acetyl CoA)
Any amino acids in excess of the biosynthetic needs of the cell are rapidly degraded.
The first phase of catabolism involves the removal of the α-amino groups (usually by transamination and subsequent oxidative deamination), forming ammonia and the corresponding α-keto acid—the “carbon skeletons” of amino acids.
A portion of the free ammonia is excreted in the urine, but most is used in the synthesis of urea (Figure 19.1), which is quantitatively the most important route for disposing of nitrogen from the body.

13. Amino Acids Metabolism Removal of Nitrogen from Amino Acids
Removing the a-amino group
Essential for producing energy from any amino acid
An obligatory step for the catabolism of all amino acids
Deamination Pathways
Amino group (nitrogen) is removed from an amino acid by either:
1- Transamination : by transaminases
2- Oxidative Deamination: by glutamate dehydrogenase
3- Trans deamination ?

14. 1- Transamination ALL Amino Acids (except lysine & threonine)
a-ketoglutarate accepts the
amino group from amino acids to
become glutamate by:
Transaminases (aminotransferases)
Glutamate:
Glutamate dehydrogenase
Ammonia
Transaminase
Energy, glucose, FAs or KB

15. Glutamate (from transamination steps)
2- Oxidative deamination by Glutamate Dehydrogenase
Glutamate (from transamination steps)
by enzyme Glutamate Dehydrogenase
Ammonia a-ketoglutarate
Urea
Cycle
Urea used for transamination
of further amino acids

16. Diagnostic Value of Plasma Aminotransferases
Aminotransferases are normally intracellular enzymes
Plasma contains low levels of aminotransferases
representing release of cellular contents during normal cell turnover
Elevated plasma levels of aminotransferases
indicate damage to cells rich in these enzymes
(as physical trauma or disease to tissue)
Plasma AST & ALT are of particular diagnostic value

17. 1- liver disease:
Plasma ALT & AST are elevated in nearly all liver diseases
but, particularly high in conditions that cause cell necrosis as:
viral hepatitis
toxic injury
prolonged circulatory collapse
ALT is more specific for liver disease than AST
AST is more sensitive (as liver contains a large amount of AST)
2- Nonhepatic disease:
as: Myocardial infarction
Skeletal muscle disorders
These disorders can be distinguished clinically from liver disease

18. Glutamate transaminase
Transdeamination
► It is combination of transamination & oxidative deamination.
It includes the transamination of most a.as with α– keto glutarate to form glutamate then the glutamate is oxidatively deaminated reforming α– keto glutarate and giving ammonia. This provides a pathway by which the amino group of most a.as is released in the form of ammonia.
α- a.a.
α- keto acid
α- keto glutarate
Glutamic acid ◄
Glutamate transaminase ►
H2O
NH3
NAD(P)
NAD(P)H+H
L-GLUTAMATE
DEHYDROGENASE ENZYME ►

19. Metabolism of Ammonia So, There must be a mechanism by which
Ammonia is produced by all tissues during metabolism
of a variety of compounds
Ammonia is disposed of primarily by formation of urea in the liver
The level of ammonia in blood must be kept very low
Slightly elevated concentrations (hyperammonemia) are toxic to CNS
So,
There must be a mechanism by which
Ammonia is moved from peripheral tissues to the liver for disposal as urea
While at the same time
Ammonia must be maintained at low levels in blood

20. Disposal of Ammonia 1- Urea in the liver
is quantitatively the most important disposal route for ammonia
Urea is formed in the liver from ammonia (urea cycle)
Urea travels in the blood from the liver to the kidneys where it is filtered to appear in urine

21. 2- Glutamine
in most peripheral tissues especially brain, sk.ms. & liver
In most peripheral tissues, glutamate binds with ammonia by action of glutamine synthase
in the brain, it is the major mechanism of removal of ammonia from the brain
This structure provides a nontoxic storage & transport form of ammonia
Glutamine is transported to blood to other organs esp. liver & kidneys
In the liver & Kidney, glutamine is converted to ammonia & glutamate by the enzyme glutaminase.

22. in skeletal muscles 3- Alanine
Ammonia Pyruvate form alanine in skeletal muscles
Alanine is transported in blood to liver
In liver, alanine is converted to pyruvate & ammonia
Pyruvate can be converted to glucose (by gluconeogenesis)
Glucose can enter the blood to be used by skeletal muscles (GLUCOSE - ALANINE PATHWAY)

23. Disposal of Ammonia cont.
Glutamine
in Most Tissues
Esp. brain & Kidneys
Urea
in Liver
Alanine
in Skeletal Muscles

26. Urea Cycle Urea is composed of: Two nitrogen atoms
Urea is produced in the liver
From the liver, it is transported in the blood to the kidneys for excretion in urine
Urea is composed of:
Two nitrogen atoms
First nitrogen atom is from free ammonia
Second nitrogen atom is from aspartate
Carbon & oxygen atoms are from CO2

27. Reactions of the Urea Cycle
First two reactions occur in the mitochondria
Remaining reactions occur in the cytosol
Ammonia + Aspartate + CO2 + 3 ATP
UREA + Fumarate + 2 ADP + AMP + 2 Pi + PPi + 3 H20
Synthesis of urea is irreversible
4 high-energy phosphates are consumed for synthesis of one molecule of
urea

28. Overview of Urea Cycle

29. In stool Reabsorbed in blood
Fate of Urea
Urea
(synthesized in the liver)
Blood
Kidney intestine
Urine cleaved by bacterial urease
Ammonia CO2
In stool Reabsorbed in blood

30. Hyperammonemia = Increase of ammonia level of blood
Blood Ammonia
Normal level of blood ammonia is 5-50 mmol/L
Hyperammonemia
A medical emergency as ammonia has a direct neurotoxic effect on CNS
Ammonia intoxication:
It is defined as toxicity of the brain due to increase in ammonia level in the systemic blood.
This increased ammonia will be directed to α ketoglutarate to form glutamic acid then glutamine leading to interference with citric acid cycle  so decrease ATP production in the brain cells.
Clinical manifestations:
Tremors, slurring of speech, somnolence, vomiting, cerebral edema & blurring of vision. At high concentrations, ammonia can cause coma & death

31. Types of Hyperammonemia
1- Acquired Hyperammonemia
1- Liver diseases: are common causes in adults
1- Acute causes:
viral hepatitis, ischemia, hepatotoxins
2- Chronic causes:
liver cirrhosis due to alcoholism, hepatitis, biliary obstruction…etc may
result in the formation of collateral circulation around the liver
So, portal blood is shunted directly into systemic circulation & detoxication
of ammonia to urea is markedly impaired
2- Gatrointestinal Bleeding
By action of bacteria of GIT on blood urea with production of big amounts of ammonia that is absorbed to blood.

32. Types of Hyperammonemia cont.
2- Hereditary Hyperammonemia
Genetic deficiencies can occur for each of the five enzymes of the urea cycle (overall prevalence 1:300,000 live births)
Ornithine transcarbamoylase (OTC) deficiency
X-linked
Most common deficiency among all 5 enzymes
Males are predominantly affected
Females carriers are clinically affected
All other urea cycle disorders are autosomal recessive
In each case, failure to synthesize urea leads to hyperammonemia during the first weeks following birth
All inherited disorders of the urea cycle enzymes result in mental retardation

33. Treatment of Hyperammonemia
Limiting protein in diet
Administration of compounds that bind covalently to amino acids
To produce nitrogen-containing molecules that are excreted in the
urine for example:
Phenylbutyrate given orally converted to phenylacetate that
condenses with glutamine to form phenylacetylglutamine which is
excreted in urine

34. Hyperammonemia in Renal Failure
Rena Failure
blood urea levels are elevated
Transfer of urea to intestine is increased
Much amounts of Ammonia is formed by bacterial urease
Absorbed to blood
Hyperammonemia
To reduce hyperammonemia:
Oral neomycin reduces the amount of intestinal bacteria
responsible for ammonia production