The proteins metabolism

Biochemistry for nurses: Unit 3

THE PROTEIN METABOLISM


• 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)


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)


• 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)


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


• Proteins are digested by proteases and peptidases. • Protein digestion starts in the stomach.• PEPSIN is an endoprotease which degrades food proteins in the stomach.



• 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.


Protein digestion is completed in the small intestine by brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase.



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 dipeptidesEXOPEPTIDASE:• 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 ?



Trypsin



ChymotrypsinTrypsin



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




H2N-Gly-Phe-COOH

H2N-Phe-COOH

H2N-Tyr-COOH


H2N-Ala-COOH

Tripeptidase




H2N-Gly-Phe-COOH

H2N-Phe-COOH

H2N-Tyr-COOH


H2N-Ala-COOH

Tripeptidase + Aminopeptidase




H2N-Gly-Phe-COOH

H2N-Phe-COOH

H2N-Tyr-COOH


H2N-Ala-COOH

Tripeptidase + Aminopeptidase + Carboxypeptidase




H2N-Gly-Phe-COOH

H2N-Phe-COOH

H2N-Tyr-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


Summary:


• 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)


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 »


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!


BIOSYNTHESIS of UREA

Biosynthesis of urea is composed by 4 stages:

1. Transamination2. Oxidative deamination of

Glutamate3. Ammonia transport4. 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


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)


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)


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)


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.


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


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 I2. Ornithine Transcarbamoylase3. Argininosuccinic Acid Synthase4. Argininosuccinase5. Arginase



Summary of the Urea Cycle

2 NH3 + CO2 + 3 ATP

UREA + 2 ADP + Pi

+ AMP + Pi

Liver


Summary of the ammonia elimination

Amino acids

degradation

Amino group

Keto acids« carbon skeletton »

Synthesis of glucosesand lipids


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.


Clinical correlation


Metabolic disorders of urea synthesis

• Extremely rare: Dysfunction of enzymes

• Disorders in urea synthesis





[NH3] in blood increases






NH3 intoxicationIntoxication 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)







Clinical Treatments:• Low protein diet ingested• Frequent small meals to

avoid sudden increase in blood of the NH3 levels.








Clinical Treatments:• Low protein diet ingested• Frequent small meals to

avoid sudden increase in blood of the NH3 levels.

Clinical improvement andminimization of Brain damage.


Education

The proteins metabolism