The document summarizes protein metabolism and the urea cycle. It discusses that 1-2% of body proteins are degraded and renewed daily. Ammonia produced is highly toxic, so the liver converts it to urea. Glutamine synthase fixes ammonia as non-toxic glutamine and glutaminase releases it for urea synthesis. The urea cycle uses ammonia, CO2, and aspartate to synthesize urea using 5 enzymes in the liver. Rare metabolic disorders can cause urea synthesis blockage, increasing blood ammonia and risking intoxication and brain damage. Treatments include low-protein diets and frequent small meals to avoid sudden ammonia increases.

1. Biochemistry for nurses: Unit 3
THE PROTEIN METABOLISM

2. Biochemistry for nurses: Unit 3
• Protein Turnover = Continuous degradation and synthesis of proteins.
Replacement of 1-2% of the total body protein each day
• Amino acid pool = Accumulation of free AA in the liver and the blood: 75 % of
liberated AA from tissue proteins are reutilized
• Degradation (catabolism of AA) = Excess of AA are not stored! but rapidly
degraded for the synthesis of glucose (glycolosis) and lipids. Degradation of excess AA
causes an excess of nitrogen.
• Waste = Nitrogen excess is transformed into urea (80%) and ammonium (NH4
+) in
order to be thrown away in the urine.
(Liver and Blood)

3. Biochemistry for nurses: Unit 3
NITROGEN BALANCE
• The amino acids are the main source of Nitrogen.
• Nitrogen balance (NB) is a comparaison between nitrogen
intake (Dietary proteins) and nitrogen loss (indigested
proteins in feces, waste excretion as urea (80%) and ammonia
(NH4
+) in the urine).
- For normal adult: Ingested nitrogen = excreted nitrogen
- Positive NB = Ingested nitrogen > excreted nitrogen
(children growth, pregnancy)
- Negative NB = Ingested nitrogen < excreted nitrogen
(may follow surgery, advanced cancer, marasmus)

4. Biochemistry for nurses: Unit 3
• Protein Turnover = Continuous degradation and synthesis of proteins.
Replacement of 1-2% of the total body protein each day
• Amino acid pool = Accumulation of free AA in the liver and the blood: 75 % of
liberated AA from tissue proteins are reutilized
• Degradation (catabolism of AA) = Excess of AA are not stored! but rapidly
degraded for the synthesis of glucose (glycolosis) and lipids. Degradation of excess AA
causes an excess of nitrogen.
• Waste = Nitrogen excess is transformed into urea (80%) and ammonium (NH4
+) in
order to be thrown away in the urine.
(Liver and Blood)

5. Biochemistry for nurses: Unit 3
Digestion and absorption of proteins
Digestion = degradation of the protein into AA by the digestive
system to make it absorbable by intestine.
Absorption = Transfert of the AA from the intestine to the blood

6. Biochemistry for nurses: Unit 3
• Proteins are digested by proteases and peptidases.
• Protein digestion starts in the stomach.
• PEPSIN is an endoprotease which degrades food proteins in the stomach.

7. Biochemistry for nurses: Unit 3

8. Biochemistry for nurses: Unit 3
• TRYPSIN = Endopeptidase cleaves the peptide bond at the carboxyl
side of the Lysine and Arginine.
• CHYMOTRYPSIN = Endopeptidase cleaves the peptide bond at the
carboxyl side of the Tryptophan, Tyrosine and Phenylalanine.

9. Biochemistry for nurses: Unit 3
Protein digestion is completed in the small intestine by brush border
enzymes carboxypeptidase, aminopeptidase, and dipeptidase.

10. Biochemistry for nurses: Unit 3

11. Biochemistry for nurses: Unit 3
Practise
ENDOPEPTIDASE:
• Trypsin = cleaves at the COOH side of Lysine and Arginine.
• Chymotrypsin = cleaves at the COOH side of Tryptophan, Tyrosine and
Phenylalanine.
• Tri or Dipeptidase = cleaves between AA of tri or dipeptides
EXOPEPTIDASE:
• Carboxypeptidase = removes AA from the COOH end
• Aminopeptidase = removes AA from the NH2 end
H2N-Val-Cys-Ala-Leu-Lys-Val-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala-COOH
Trypsin + Chymotrypsin
?
Tripeptidase + Aminopeptidase
+ Carboxypeptidase
Final products ?

12. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-Val-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala-COOH
Trypsin

13. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-Val-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala-COOH
ChymotrypsinTrypsin

14. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-Val-Glu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala-COOH
ChymotrypsinTrypsin
H2N-Val-Cys-Ala-Leu-Lys-COOH
H2N-Val-Glu-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-Pro-Lys-COOH
H2N-Ala-COOH

15. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-COOH
H2N-Val-Glu-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-Pro-Lys-COOH
H2N-Ala-COOH
Tripeptidase

16. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-COOH
H2N-Val-Glu-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-Pro-Lys-COOH
H2N-Ala-COOH
Tripeptidase
+ Aminopeptidase

17. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-COOH
H2N-Val-Glu-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-Pro-Lys-COOH
H2N-Ala-COOH
Tripeptidase
+ Aminopeptidase
+ Carboxypeptidase

18. Biochemistry for nurses: Unit 3
H2N-Val-Cys-Ala-Leu-Lys-COOH
H2N-Val-Glu-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-Pro-Lys-COOH
H2N-Ala-COOH
Tripeptidase
+ Aminopeptidase
+ Carboxypeptidase
H2N-Val-COOH
H2N-Cys-Ala-Leu-COOH
H2N-Lys-COOH
H2N-Val-COOH
H2N-Glu-COOH
H2N-Arg-COOH
H2N-Gly-Phe-COOH
H2N-Phe-COOH
H2N-Tyr-COOH
H2N-Thr-COOH
H2N-Pro-COOH
H2N-Lys-COOH
H2N-Ala-COOH

19. Biochemistry for nurses: Unit 3
Summary:

20. Biochemistry for nurses: Unit 3
• Protein Turnover = Continuous degradation and synthesis of proteins.
Replacement of 1-2% of the total body protein each day
• Amino acid pool = Dietary proteins and the catabolism of tissue proteins
provide free AA. 75 % of liberated AA from tissue proteins are reutilized.
• Degradation (catabolism of AA) = Excess of AA are not stored! but
rapidly degraded for the synthesis of glucose (glycolosis) and lipids.
Degradation of excess AA causes an excess of nitrogen.
• Waste = Nitrogen excess is transformed into urea (80%) and ammonium
(NH4
+) in order to be thrown away in the urine.
(Liver and Blood)

21. Biochemistry for nurses: Unit 3
Definition of the Keto Acid
• The deamination of an Amino Acid (= removing of the amino group) forms
the corresponding Keto Acid.
• The Keto acid is also called « the carbon skeleton »

22. Biochemistry for nurses: Unit 3
Definition of the Keto Acid
• The deamination of an Amino Acid (= removing of the amino group) forms
the corresponding Keto Acid.
• The Keto acid is also called « the carbon skeleton »
WASTE
REUSED!

23. Biochemistry for nurses: Unit 3
BIOSYNTHESIS of UREA
Biosynthesis of urea is composed
by 4 stages:
1. Transamination
2. Oxidative deamination of
Glutamate
3. Ammonia transport
4. Reactions of the urea cycle.
80% of the excess amino acid
nitrogen forms Urea in order to
be thrown away in the urine.
Tissues
Liver

24. Biochemistry for nurses: Unit 3
Transamination
•Transfert of the α-amino group (NH2) to the ketoglutarate to give GLUTAMATE
• The reaction is reversible.
• The reaction is catalysed by an enzyme (Aminotransferase) in presence of a
co-enzyme (PLP = Vit B6)
(TISSUES)

25. Biochemistry for nurses: Unit 3
Oxidative deamination of Glutamate
• Formation of ammonia (NH3) from the amino group (NH2) of the Glutamate
by oxidative deamination.
• Glutamate is the only Amino Acid that undergoes oxidative deamination.
• Enzyme = Glutamate Dehydrogenase (GDH); Coenzyme = NAD+
(LIVER)

26. Biochemistry for nurses: Unit 3
Amino acid oxidase reaction
• The amino acid oxidase (AAO) of liver
and kidney removes the nitrogen as
ammonium ion (NH4
+).
• Conversion of Amino Acids to an Imino
acids which are decomposed to a Keto
acid with release of NH4
+.
• Enzyme = AAO ; Coenzyme = Flavin
• The reduced Flavin is reoxidized by O2,
forming hydrogen peroxide (H2O2) which
then is split to O2 and H2O by
CATALASE.
(LIVER and KIDNEY)

27. Biochemistry for nurses: Unit 3
Ammonia (NH3) Transport
• NH3 is very toxic to the nervous system!
• The NH3 produced by tissue are rapidly removed from circulation by the
Liver and converted to UREA
• Only traces (10-20 uG/dL) of NH3 are present in blood in normal
conditions
• Liver damage and metabolic disorders are associated with elevated
concentration of NH3 in the blood.
• In case of CIRRHOSIS (hepatic disease), NH3 rises to toxic levels,
consequently: Tremor, blurred, coma and ultimately death.
• The transport of NH3 from the tissue to the liver is done by
GLUTAMATE or GLUTAMINE as nontoxic forms.

28. Biochemistry for nurses: Unit 3
Glutamine Synthase fixes NH3 as Glutamine.
• NH3 is fixed by GLUTAMATE to give
GLUTAMINE
• Enzyme = Glutamine Synthase (inside
tissue mitochondria)
• That reaction needs ENERGY to work!
(hydrolysis of ATP )
Tissues

29. Biochemistry for nurses: Unit 3
UREA CYCLE
• UREA is the major end product of Nitrogen catabolism in human body.
• Synthesis of 1 molecule of UREA requires:
1. 3 molecules of ATP (Energy!)
2. 1 molecule of NH4+
3. 1 molecule of α- amino group (NH2) of Aspartate
• 5 enzymes catalyse the Urea Cycle in the liver cells:
1. Carbamoyl Phosphate Synthase I
2. Ornithine Transcarbamoylase
3. Argininosuccinic Acid Synthase
4. Argininosuccinase
5. Arginase

30. Biochemistry for nurses: Unit 3

31. Biochemistry for nurses: Unit 3
Summary of the Urea Cycle
2 NH3 + CO2 + 3 ATP
UREA + 2 ADP + Pi
+ AMP + Pi
Liver

32. Biochemistry for nurses: Unit 3
Summary of the ammonia elimination
Amino acids
degradation
Amino group
Keto acids
« carbon skeletton »
Synthesis of glucoses
and lipids

33. Biochemistry for nurses: Unit 3
Summary of the ammonia elimination
• 1 – 2 % of the body proteins are degraded and renewed daily
• Ammonia (NH3) is highly toxic.
• Ammonia (NH3) is converted to Urea
• Glutamine synthase converts NH3 to nontoxic glutamine
• Glutaminase releases NH3 for use in urea synthesis
• NH3, CO2 and the amide nitrogen of aspartate provide the
atoms of urea
• Hepatic urea synthesis takes place in part in the mitochondrial
matrix and in part in the cytosol.

34. Biochemistry for nurses: Unit 3
Clinical correlation

35. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis

36. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis
[NH3] in blood increases

37. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis
[NH3] in blood increases
NH3 intoxication
Intoxication is more severe
when the urea synthesis is
blocked at reactions 1 or 2

38. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis
[NH3] in blood increases
NH3 intoxication
Intoxication is more severe
when the urea synthesis is
blocked at reactions 1 or 2
Clinical symptoms:
•Vomiting
• Avoidance of high protein foods
• Irritability
• Lethargy
• Mental Retardation (Brain damage)

39. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis
[NH3] in blood increases
NH3 intoxication
Intoxication is more severe
when the urea synthesis is
blocked at reactions 1 or 2
Clinical Treatments:
• Low protein diet ingested
• Frequent small meals to avoid
sudden increase in blood of the
NH3 levels.
Clinical symptoms:
•Vomiting
• Avoidance of high protein foods
• Irritability
• Lethargy
• Mental Retardation (Brain damage)

40. Biochemistry for nurses: Unit 3
Metabolic disorders of urea synthesis
• Extremely rare: Dysfunction of enzymes
• Disorders in urea synthesis
[NH3] in blood increases
NH3 intoxication
Intoxication is more severe
when the urea synthesis is
blocked at reactions 1 or 2
Clinical Treatments:
• Low protein diet ingested
• Frequent small meals to avoid
sudden increase in blood of the
NH3 levels.
Clinical
improvement and
minimization of
Brain damage.
Clinical symptoms:
•Vomiting
• Avoidance of high protein foods
• Irritability
• Lethargy
• Mental Retardation (Brain damage)