Metabolism of Protein and Amino Acids following the curriculum of medical biochemistry of MBBS course

1. Metabolism of
Protein & Amino Acid
DR. FARHANA ATIA
ASSOCIATE PROFESSOR, BIOCHEMISTRY
NILPHAMARI MEDICAL COLLEGE, NILPHAMARI

2. Structure of Amino Acid & Peptide bond

4. Protein Turnover
The total amount of protein in the body remain
constant, because rate of protein synthesis is just
sufficient to replace the protein that degraded. This
process is called protein turnover.
It is 1-2% (300-400gm/day) of total body protein(12-14
Kg), principally muscle protein

5. Protein Turnover
The continuous degradation & synthesis of cellular
proteins occur in all forms of life
High rates of protein degradation occur in tissues that
are undergoing structural rearrangement such as
uterine tissue during pregnancy, skeletal muscle in
starvation

6. Amino Acid Pool
Amino acid released by hydrolysis of dietary or
tissue protein or synthesized de novo mix with other
free amino acid distributed throughout the body
collectively they constitute the amino acid pool.
It contains 100 gm of amino acid.

7. Dietary protein
30-50 gm/d
Body protein
400 gm/d
Synthesis of NEAA
(acc. to requirement)
Body protein
400 gm/d
Amino acid pool Synthesis of NPN
30 gm/d
Catabolism
(Transamination & Deamination)
Carbon skeleton of amino acid NH₃ →Urea synthesis →Excrete
Ketogenic
AA
Glucogenic AA Reamination
Acetyl coA Gluconeogenesis CO₂+ H₂O
Fatty acid
Steroid
Cholesterol
Ketone body
Glucose

8. NPN
(Non-Protein Nitrogenous Substance)
• Porphyrin
• Creatinine
• Neurotransmitter
• Purine
• Pyrimidine
• Other Nitrogen containing compounds

9. Amino Acids
•Glucogenic : Amino acids whose catabolism
yields pyruvate or one of the intermediates of
TCA cycle are termed glucogenic.
•Ketogenic : Amino acids whose catabolism
yields either acetoacetate or one of its
precursors are termed ketogenic.

10. Glucogenic
&
Ketogenic
amino acids

11. Amino Acids
Glucogenic Ketogenic
Alanine
Arginine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
Histidine
Methionine
Threonine
Valine
Isoleucine
Phenyl alanine
Tryptophan
Tyrosine
Leucine
Lysine
Isoleucine
Phenyl alanine
Tryptophan
Tyrosine

12. Metabolic loss of protein
• 16 gm/day
• Route of nitrogen loss
1. Urine: 12- 15 gm/d
(99% of total N loss)
2. Feces: 2 gm/d
3. Sweat
Urinary nitrogen
 Urea 11 gm/d (80% of
total urinary nitrogen)
 Uric acid
 NH₄⁺ ion
 Creatinine
 Undetermined nitrogen

13. Nitrogen Balance
The difference between intake and output of nitrogenous
compounds.
NB = Total N intake (protein) - Total N loss (urinary)
 3 conditions
 In equilibrium
 Positive nitrogen balance
 Negative nitrogen balance

14. Nitrogen Balance
 In equilibrium
• If the difference is zero
• In normal healthy person
 Positive nitrogen balance
• N consumption is greater than N excreted
• In growth, pg, convalescence phase

15. Negative nitrogen balance
• N excretion is greater
than N consumption
• Occurs in
• Prolong starvation
• Uncontrolled DM
• Trauma (severe)
• Malignancy
• Extensive surgery
• Burn
• Any debilitating disease
• PEM- marasmus

16. Overview of amino acid metabolism

17. Transamination
• Transfer of amino group from an α-amino acid to an α-
keto acid with simultaneous production of a α-keto acid &
α-amino acid respectively

18. Transamination
• Substrate : α amino acid, α ketoacid
• Product : Ketoacid, Amino acid
• Site : Liver, Kidney, Skeletal muscle, Heart
• Compartment : Cytoplasm (mainly)
Mitochondria (partly)
• Nature : Amphibolic

19. Transamination
• Reversible reaction
• Catalyzed by a family of
enzymes called
Aminotransferase or
transaminase with coenzyme
Pyridoxal Phosphate (vitamin
B6)

20. Transamination
• Mainly 3 keto acids
participate
–α-ketoglutarate
–Oxaloacetate
–Pyruvate
• All amino acids participate
except
–Lysine
–Threonine
–Proline

21. Importance of Transamination
• Redistribution of amino group among amino acids
• Help in synthesis of non-essential amino acids as
per cellular need
• Divert excess amino acid to energy production

22. Importance of Transamination
• Provide link between Carbohydrate, protein & fat
metabolism as ketoacids form compounds
common to their metabolic cycle
• Funneling of amino group to α-ketoglutarate to
form glutamate (ultimately to urea cycle)

23. Deamination
• Removal of amino group
from an amino acid as
ammonia with
simultaneous production
of corresponding
ketoacid

24. Salient Feature
• Substrate : Amino acid (mainly glutamate)
• Product : Ammonia,
α-ketoglutarate
• Site : Liver, Kidney, other tissues (some
extent)
• Compartment : Mitochondria

25. Types of Deamination
1. Oxidative deamination
Glutamate is unique for this process
NAD⁺ NADH NH₃
Glutamate
Glutamate
dehydrogenase
α-
ketoglutarate

26. 2. Non-oxidative
deamination
Serine
Threonine
Cystine
Histidine
Dehydratase
Pyruvate NH₃

27. Importance of Deamination
•Supply NH₃ to the urea cycle
•Support gluconeogenesis
•Produce ATP (NADH)
•Peak amino group from an amino acid by
α-ketoglutarate

28. Metabolism of Ammonia
Ammonia is highly toxic
It arises primarily from α-amino nitrogen of amino acids
Tissues then convert ammonia to nontoxic amino acid
glutamine
Subsequent deamination of glutamine in the liver
releases ammonia, which is converted to urea

29. Sources of Ammonia
1. Amino acid by transamination & deamination
2. Purine & pyrimidine catabolism
3. Catabolism of dietary amine & endogenous monoamine
4. Bacterial urease activity on urea in intestinal lumen
5. Glutamine by the action of renal & intestinal
glutaminase in kidney & intestine respectively

30. Disposal of Ammonia
• Formation of UREA & excrete
• Formation of GLUTAMINE in mitochondria of
Brain Liver
Muscle Kidney
• Formation of GLUTAMATE in liver
• Excretion of NH₄⁺ through urine

31. Functions of NH₃
• NH₃ is not just a waste product of nitrogen
metabolism. It is involved in the synthesis of many
compounds in the body (directly or via glutamine)
• NH₄⁺ is very important to maintain acid base
balance of the body.

32. Compounds synthesized from NH₃
NEAA
Purine
Pyrimidine
Amino sugar
Asparagine

33. Hyperammonemia
• Normal level in serum: 10-20 µg/L
• Ammonia has directly neurotoxic effect on CNS
• Elevated concentration of ammonia in blood
causes symptoms of ammonia intoxication.

34. Hyperammonemia
Symptoms of ammonia intoxication-
Tremor Vomiting
Slurring of speech Cerebral edema
Blurring of vision Coma
Somnolence Death

35. Acquired Hyperammonemia
 Acute liver disease
 Viral hepatitis (severe type)
 Cirrhosis due to alcoholism
 Other hepatitis
 Biliary obstruction
 Ischemia
 Hepatotoxins

36. Congenital Hyperammonemia
•Congenital hyperammonemia occurs in
genetic deficiency of enzymes of urea cycle

37. UREA CYCLE
• Urea is synthesized in liver and transported to kidney for
excretion
• Small amount of urea is converted to CO₂ & NH₃ by
urease in intestine
• Synthesis of 1 mol of urea requires 3 mol of ATP, 1 mol
of NH₄⁺ and 1 mol of aspartate, and employ 5 enzymes
• Six amino acids participates in this cycle

38. UREA CYCLE
• Substrate : NH₃
• Product : Urea
• Site : Liver
• Compartment : Mitochondria & cytosol
• Two amino group of urea comes from ammonia &
aspartate; C comes from CO₂

39. Steps of Urea cycle
1. Condensation of NH₄⁺ with CO₂ to form
carbamoyl phosphate; consumes 2 ATP.
[enzyme- Carbamoyl phosphate synthase-1]
2. Citrulline is synthesized from carbamoyl
phosphate & ornithine by ornithine carbamoyl
transferase

40. Steps of Urea cycle
3. Argininosuccinate synthase condenses citrulline with
aspartate to produce argininosuccinate; requires ATP
4. Argininosuccinate lyase cleaves argininosuccinate to
arginine & fumarate
5. Arginase cleaves arginine to yield UREA & ornithine

41. Harper’s illustrated review, 31st edn, page-275

42. Importance of Urea cycle
Major disposal form of amino group. Provide about
90% of Nitrogen containing compound of urine.
Toxic NH₃ is converted to non-toxic urea. Any defect
in this cycle leads to hyperammonemia
Arginine produced in this cycle is used for protein
synthesis
Fumarate takes part in TCA cycle & gluconeogenesis