1. The document discusses amino acid metabolism, including the digestion and absorption of proteins, nitrogen balance in the body, and pathways of amino acid catabolism. 2. Key aspects of protein digestion are outlined, from breakdown in the stomach by pepsin to further digestion by pancreatic enzymes like trypsin in the small intestine. 3. Amino acids are then absorbed across the intestinal mucosa using secondary active transport and transported to tissues via the bloodstream.

1. GENERAL PATHWAYS OF AMINO
ACIDS METABOLISM
Digestion and absorbtion of
proteins in the
gastrointestinal tract.
Nitrogenous balance.

2. Proteins function in the organism.
 All enzymes are proteins.
 Storing amino acids as nutrients and as building
blocks for the growing organism.
 Transport function (proteins transport fatty acids,
bilirubin, ions, hormones, some drugs etc.).
 Proteins are essential elements in contractile and
motile systems (actin, myosin).
 Protective or defensive function (fibrinogen,
antibodies).
 Some hormones are proteins (insulin, somatotropin).
 Structural function (collagen, elastin).

3. GENERAL PATHWAYS OF AMINO ACIDS METABOLISM
Proteins of food
Metabolites of
Amino acids glycolysis and
Krebs cycle
Anabolic ways Catabolic ways
Synthesis of Synthesis of Trans- Deami- Decar-
cell and peptide ami- nation boxila-
extracell physiologi- nation tion
proteins cally active
substances Amines
Proteins and peptides Urea, CO2, H2O
of the organism

4. Nitrogen Balance (NB):
 Nitrogen balance is a comparison between
Nitrogen intake (in the form of dietary
protein) and Nitrogen loss (as undigested
protein in feces,NPN as urea, ammonia,
creatinine & uric acid in urine,sweat &
saliva & losses by hair, nail, skin).
 NB is important in defining
1.overall protein metabolism of an individual
2.nutritional nitrogen requirement.

5. Nitrogenous balance
It may be positive, negative and neutral (zero).
Positive nitrogenous balance – the amount of nitrogen entered the
organism is more than amount of nitrogen removed from the
organism. It occurs in young growing organism, during
recovering after severe diseases, at the using of anabolic
medicines pregnancy, lactation and convulascence
Negative nitrogenous balance – the amount of nitrogen removed
from the organism is more than amount of nitrogen entered the
organism. It occurs in senile age, destroying of malignant tumor,
vast combustions, poisoning by some toxins. High loss of tissue
proteins in wasting diseases like burns, hemorrhage & kidney
diseases with albuminurea (High breakdown of tissue proteins )
in hyperthyroidism, fever, infection
Zero nitrogenous balance – the amount of nitrogen removed from the
organism is equal to the amount of nitrogen entered the organism. It
occurs in healthy adult people
Normal adult: will be in nitrogen equilibrium, Losses = Intake

6. A deficiency of
even one amino
acid results in a
negative nitrogen
balance.
In this state, more
protein is
degraded than
synthesized.

7. Protein Requirement for humans
in Healthy and Disease Conditions
The normal daily requirement of protein for
adults is 0.8 g/Kg body wt. day-1.
• That requirement is increased in healthy
conditions:
during the periods of rapid growth, pregnancy,
lactation and adolescence.
• Protein requirement is increased in disease
states:
illness, major trauma and surgery.
• RDA for protein should be reduced in:
hepatic failure and renal failure

8. Biological Value for Protein (BV)
BV is : a measure for the ability of dietary
protein to provide the essential amino acids
required for tissue protein maintenance.
•Proteins of animal sources (meat, milk, eggs)
have high BV because they contain all the
essential amino acids.
•Proteins from plant sources (wheat, corn,
beans) have low BV thus combination of more
than one plant protein is required (a
vegetarian diet) to increase its BV.

9. Protein digestion
Chemical composition of digestive juices.
Gastric juice contains water, enzymes, hydrochloric acid,
mineral salts and other compounds. About 2,5 l of
gastric juice is secreted per day.
The role of hydrochloric acid in digestion.
 Denaturate proteins (denaturated proteins easier
undergo digestion by pepsin than native proteins).
 Stimulates the activity of pepsin.
 Hydrochloric acid has bactericidial properties.
 Stimulates the peristalsis.
 Regulate the enzymatic function of pancreas.

10. Digestion in Stomach
Stimulated by food acetylcholine, histamine and gastrin are
released onto the cells of the stomach
The combination of acetylcholine, histamine and gastrin cause
the liberation of the gastric juice.
Mucin - is always secreted in the stomach
HCl - pH 0.8-2.5 (secreted by parietal cells)
Pepsinogen (a zymogen, secreted by the chief cells)

11. Proteolytic enzymes and their activation.
Three enzymes are in gastric juice: pepsin, gastricsin and rennin. All these enzymes cleave
proteins or peptides.
Pepsinogen (MW=40,000) is activated by the enzyme pepsin, which is already present in the
stomach and by hydrochloric acid.
Pepsinogen cleaved off to become the enzyme pepsin (MW=33,000) and a peptide fragment to
be degraded.
Pepsin partially digests proteins by cleaving the peptide bond formed by aromatic amino
acids: Phe, Tyr, Trp

12. Optimal pH for gastricsin is 2,0-3,0. The ratio between
gastricsin and pepsin in gastric juice is 1:5,5. This ratio can be
changed in some pathological states.
Rennin also possesses a proteolytic activity and causes
a rapid coagulation of ingested casein. But this enzyme
plays important role only in children because the optimal pH
for it is 5-6.

13. Digestion in the Duodenum
Stimulated by food secretin and cholecystokinin regulate the
secretion of bicarbonate and zymogens trypsinogen,
chymotrypsinogen, proelastase and procarboxypeptidase by
pancreas into the duodenum
Bicarbonate changes the pH to about 7
The intestinal cells
secrete an enzyme
called enteropeptidase
that acts on trypsinogen
cleaving it into trypsin

14. Enteropeptidase secreted by the mucosa of duodenum initiates
the activation of the pancreatic proenzymes
14

15. Proteolytic enzymes exhibit the preference for particular
types of peptide bonds
Proteinases preferentially attacks the bond after:
Pepsin aromatic (Phe, Tyr) and acidic AA (Glu, Asp)
Trypsin basic AA (Arg, Lys)
Chymotrypsin hydrophobic (Phe, Tyr, Trp, Leu) and acidic AA (Glu,
Asp)
Elastase AA with a small side chain (Gly, Ala, Ser)
Peptidases:
Carboxypeptidase A nearly all AA (not Arg and Lys)
Carboxypeptidase B basic AA (Arg, Lys)
aminopeptidase nearly all AA
Prolidase proline
15
Dipeptidase only dipeptides

16. The splitting of elastin in an intestine is catalyzed by elastase
and collagen is decomposed by collagenase.
Digestion of protein takes place not only in the intestinal cavity
but also on the surface of mucosa cells.

17. Mechanism of amino acid absorbtion.
This explanation is called the sodium cotransport
theory for amino acid transport; it is also called secondary
active transport of amino acid.
Absorption of amino acids through the intestine mucosa can occur
far more rapidly than protein can be digested in the lumen of the
intestine.
Since most protein digestion occurs in the upper small intestine
most protein absorption occurs in the duodenum and jejunum.

18. Most proteins are completely digested to free amino acids
Amino acids and sometimes short oligopeptides are absorbed by the
secondary active transport
Amino acids are transported via the blood to the cells of the body.

19. The sources of amino acids:
1) absorption in the intestine;
2) formation during the protein decomposition;
3) synthesis from the carbohydrates and lipids.
Using of amino acids:
1) for protein synthesis;
2) for synthesis of nitrogen containing compounds (creatine, purines,
choline, pyrimidine);
3) as the source of energy (oxidation – deamination, transamination,
decarboxilation);
4) for the gluconeogenesis;
5) for the formation of biologically active compounds.

20. Overview of Amino Acid Catabolism:
Interorgan Relationships

21. Overview of Amino Acid Catabolism:
Interorgan Relationships
• Liver
– Synthesis of liver and plasma proteins
– Catabolism of amino acids
• Gluconeogenesis
• Ketogenesis
• Branched chain amino acids (BCAA) not
catabolized
• Urea synthesis
– Amino acids released into general
circulation
• Enriched in BCAA (2-3X)

22. Overview of Amino Acid Catabolism:
Interorgan Relationships
• Skeletal Muscle
– Muscle protein synthesis
– Catabolism of BCAA
• Amino groups transported away as alanine and
glutamine (50% of AA released)
– Alanine to liver for gluconeogenesis
– Glutamine to kidneys
• Kidney
– Glutamine metabolized to a-KG + NH4
• a-KG for gluconeogenesis
• NH4 excreted or used for urea cycle (arginine
synthesis)
– Important buffer from acidosis

23. PROTEIN TURNOVER
Protein turnover — the degradation and resynthesis
of proteins
Half-lives of proteins – from several minutes to many years
Structural proteins – usually stable (lens protein crystallin lives
during the whole life of the organism)
Regulatory proteins - short lived (altering the amounts of these
proteins can rapidly change the rate of metabolic processes)
How can a cell distinguish proteins that are meant
for degradation?

24. Ubiquitin - is the tag that marks
proteins for destruction ("black
spot" - the signal for death)
Ubiquitin - a small (8.5-kd) protein
present in all eukaryotic cells
Structure:
 extended carboxyl terminus
(glycine) that is linked to other
proteins;
 lysine residues for linking
additional ubiquitin molecules

25. Proteasomes degrade regulatory proteins (short half-life)
and abnormal or misfolded proteins
- hollow cylindric supramolecule,
28 polypeptides Protein-Ub
- four cyclic heptamers (4 × 7 =
28) regulation of
- the caps on the ends regulate
the entry of proteins into cell cycle,
destruction chamber, upon ATP
apoptosis,
hydrolysis
- inside the barrel, differently angiogenesis
specific proteases hydrolyze
target protein into short (8 AA)
peptides cytosolic
- Ub is not degraded, it is peptidases
Ub + short peptides AA
released intact
25

26. GENERAL WAYS OF AMINO
ACIDS METABOLISM
The fates of amino acids:
1) for protein synthesis;
2) for synthesis of other nitrogen containing compounds
(creatine, purines, choline, pyrimidine);
3) as the source of energy;
4) for the gluconeogenesis.

27. The general ways of amino acids degradation:
 Deamination
 Transamination
 Decarboxilation
The major site of amino acid degradation - the liver.
Deamination of amino acids
Deamination - elimination of amino group from amino acid with
ammonia formation.
Four types of deamination:
- oxidative (the most important for higher animals),
- reduction,
- hydrolytic, and
- intramolecular

28. Reduction deamination:
R-CH(NH2)-COOH + 2H+ → R-CH2-COOH + NH3
amino acid fatty acid
Hydrolytic deamination:
R-CH(NH2)-COOH + H2O → R-CH(OH)-COOH +
NH3
amino acid hydroxyacid
Intramolecular deamination:
R-CH(NH2)-COOH → R-CH-CH-COOH + NH3
amino acid unsaturated fatty acid

29. General scheme of oxydative
transamination
R CH COOH + HOOC C CH2CH2COOH
NH2 O
aminokyselina
amino acid 2-oxoglutarate
2-oxoglutarát
aminotransferase
aminotransferasa
pyridoxalfosfát
pyridoxal phosphate
R C COOH + HOOC CH CH2CH2COOH
O NH2
2-oxokyselina
2-oxo acid glutamát
glutamate 29

30. Glutamate dehydrogenase (GMD, GD, GDH)
• requires pyridine cofactor NAD(P)+
• GMD reaction is reversible: dehydrogenation with NAD+,
hydrogenation with NADPH+H+
• two steps:
• dehydrogenation of CH-NH2 to imino group C=NH
• hydrolysis of imino group to oxo group and ammonia
30

31. In transaminations, nitrogen of most
!
AA is concentrated in glutamate
Glutamate then undergoes
dehydrogenation + deamination
and releases free ammonia NH3
31

32. Oxidative deamination
L-Glutamate dehydrogenase plays a central role in amino acid
deamination
In most organisms glutamate is the only amino acid that has
active dehydrogenase
Present in both the cytosol and mitochondria of the liver

33. Transamination of amino acids
Transamination - transfer of an amino group from an α -
amino acid to an α -keto acid (usually to α -ketoglutarate)
Enzymes: aminotransferases (transaminases).
α -keto acid
α -amino acid α -keto acid α -amino acid

34. There are different transaminases
The most common:
alanine aminotransferase alanine + α-ketoglutarate ⇔ pyruvate +
glutamate
aspartate aminotransferase
aspartate + α-ketoglutarate ⇔ oxaloacetate + glutamate
Aminotransferases funnel α -amino groups from a variety of
amino acids to α-ketoglutarate with glutamate formation
Glutamate can be deaminated with NH4+ release

35. Mechanism of transamination
All aminotransferases require the
prosthetic group pyridoxal
phosphate (PLP), which is derived
from pyridoxine (vitamin B6).
Ping-pong kinetic mechanism
First step: the amino group of
amino acid is transferred to
pyridoxal phosphate, forming
pyridoxamine phosphate and
releasing ketoacid.
Second step: α-ketoglutarate
reacts with pyridoxamine
phosphate forming glutamate

36. Ping-pong kinetic mechanism of aspartate transaminase
aspartate + α-ketoglutarate ⇔ oxaloacetate + glutamate

37. Decarboxylation of amino acids
Decarboxylation – removal of carbon dioxide from amino
acid with formation of amines.
amine
Usually amines have high physiological activity
(hormones, neurotransmitters etc).
Enzyme: decarboxylases
Coenzyme – pyrydoxalphosphate

38. DECARBOXYLATION OF AMINO
ACIDS
α-decarboxilation
ω-decarboxilation
Decarboxilation with transamination
Decarboxilation with conjugation of two molecules

39. Significance of amino acid decarboxylation
1. Formation of physiologically active compounds
glutamate gamma-aminobutyric acid
(GABA)
histidine histamine

40. 1) A lot of histamine is formed in inflamatory place;
It has vasodilator action;
Mediator of inflamation, mediator of pain;
Responsible for the allergy development;
Stimulate HCI secretion in stomach. -CO2
2) Tryptophan → Serotonin
Vasokonstrictor
Takes part in regulation of arterial pressure, body
temperature, respiration, kidney filtration, mediator of
nervous system
3) Tyrosine → Dopamine
It is precursor of epinephrine and norepinephrine.
mediator of central nervous system
4) Glutamate → γ -aminobutyrate (GABA)
Is is ingibitory mediator of central nervous system. In
medicine we use with anticonvulsion purpose (action).

41. 2. Catabolism of amino acids during the decomposition
of proteins
Enzymes of microorganisms (in colon; dead organisms)
decarboxylate amino acids with the formation of diamines.
ornithine putrescine
lysine cadaverine