(1) Protein digestion involves proteolytic enzymes that break proteins into smaller peptides and amino acids in the stomach and small intestine. (2) Amino acids are absorbed into the bloodstream and transported to tissues via three main mechanisms. (3) Within cells, amino acids undergo various reactions including transamination, oxidative and non-oxidative deamination, and transmethylation and trans-sulfuration. (4) Ammonia is a byproduct of amino acid metabolism and is shuttled to the liver where it is converted to urea to be excreted.

1. DIGESTION OF PROTEIN ,
ABSORPTION OF AMINO ACIDS &
GENERAL REACTIONS OF AMINO
ACIDS
Department of Biochemistry.

2. DIGESTION OF PROTEIN
 Digestion: It is the process to convert complex
compounds from our diet into simpler compounds
which can be easily taken up by our GIT cells.
 Zymogens: Proteolytic enzymes are secreted in the
form of inactive enzymes which are converted to
their active form in our GIT lumen.
 This prevents autodigestion of secretory glands.

3. TYPES OF PROTEOLYTIC ENZYMES
 Endopeptidases:
 Act on peptide bonds inside protein
molecule to convert it into smaller
fragments.
 E.g. Pepsin, trypsin, chymotrypsin,
elastase.
 Exopeptidases:
 Acts on ends of peptide chain, not
inside.
 Carboxypeptidase: act on C terminal.
 Aminopeptidase: act on N terminal.
 Dipeptidase
 tripeptidase

4. DIGESTION IN STOMACH
 Hydrochloric acid (HCl):
 Denatures protein
 Activates Pepsinogen  Pepsin.
 Make pH optimum for action of pepsin. (pH~2)
 Rennin:
 Active in infants & absent in adults
 Causes curdling of milk
 Casein  paracasein. Which is further digested by
pepsin & other enzymes.

5.  Pepsin:
 Secreted by chief cell as inactive pepsinogen.
 HCl removes N terminal 44 AAs & convert
pepsinogen (inactive)  Pepsin (Active)
 It hydrolyze the bond formed by carboxy groups
of Phe, Tyr, Trp & Met (aromatic AA) & convert
protein  Proteoses & peptones.

6. DIGESTION BY PANCREATIC ENZYMES
 Optimum pH of pancreatic enzyme is 8 which is
provided by alkaline bile & pancreatic juice.
 Cholecystokinin & Pancreozymin hormones
stimulates secretion of pancreatic juice.
 Enzymes are secreted as zymogen (inactive form) to
protect gland from autodigestion.

7.  Trypsin: (SERINE PROTEASE)
 Trypsinogen  Trypsin by enterokinase present
of intestinal microvillus membranes.
 Activation needs removal of hexapeptide from N
terminal end.
 Trypsin can autoactivate other trypsinogen & can
activate other zymogens.
 Trypsin hydrolyses bonds formed by carboxyl
groups of Arg & Lys (basic AA).
 Acute pancreatitis:

8.  Chymotrypsinogen  Chymotrypsin by Trypsin. (SERINE
PROTEASE)
 Proelastase  Elastase by Trypsin. (SERINE PROTEASE)
 Procarboxypeptidase  Carboxypeptidase by Trypsin.
 Required metal ions : Zn
 Intestinal digestion
 Enzymes present on intestinal juice (Succus entericus) leads
to complete digestion of small peptides into amino acids.
 Dipeptidases and tripeptidases
 aminopeptidase

9. ABSORPTION OF AMINO ACIDS.
 Site: Upper small intestine (Duodenum & Proximal
Jejunum)
 3 different mechanisms:
1. Sodium dependent secondary active transport
2. Sodium independent facilitated transport
3. Meister cycle
 After absorption they are transported via portal
blood to liver.

10. SODIUM DEPENDENT SECONDARY ACTIVE
TRANSPORT OF AMINO ACIDS:

11. SODIUM DEPENDENT SECONDARY ACTIVE
TRANSPORT:
 5 different transporter system for different groups
of amino acids:
1. Neutral AAs: Ala, Val, Leu, Met, Phe, Tyr, Ile.
 Trp & Ala show competitive inhibition.
2. Basic AAs: COAL system  Cys, Ornithine, Arg,
Lys.
3. Acidic AAs: Glu, Asp
4. Imino acids and glycine: Proline, Hydroxyproline,
Glycine.
5. Beta AAs: Beta alanine.

12. SODIUM INDEPENDENT FACILITATED
TRANSPORT SYSTEM
 Neutral & hydrophobic AAs: Phe, Leu
 Basic AAs: Lys, Arg

13. MEISTER CYCLE

14. DISORDERS OF DIGESTION & ABSORPTION
OF PROTEIN
 Disease of pancreas: Acute or chronic pancreatitis,
cystic fibrosis.
 Small intestinal diseases:
 Genetic disorders:
1. Hartnup disease: Defective transport of neutral
AAs (Trp) at PCT of kidney & jejunum  symptoms
of pellagra (Diarrhoea, Dementia, Dermatitis)
2. Glycinuria: Defect in Pro/Gly transporter at PCT &
intestine  loss of Gly/Pro in urine.
3. Cystinuria: Defect in COAL system  cystine renal
stones.

15. OVERVIEW OF AMINOACID METABOLISM

16. AMINO ACIDS
 Amino derivative of carboxylic acid
 Central α carbon with four groups attached to it:
I. Carboxyl group
II. Amino group
III. Hydrogen atom
IV. Unique side chain.
 Total 20 amino acids:
 But 21st & 22nd amino acids
are also found.

17. REACTIONS DUE TO CARBOXYL GROUP
 Decarboxylation: removal of CO2 &
formation of corresponding
amine.
 Histidine  Histamine
 Tyrosine  Tyramine
 Tryptophan  Tryptamine
 Lysine  Cadaverine
 Glutamic acid  GABA

18. REACTION DUE TO AMINO GROUP
 Transamination: α amino
group transferred to α keto
acid resulting in formation of
corresponding new amino
acid & α keto acid.
 Occur in cytosol
 α keto glutarate/Glutamate
generally act as amino
group acceptor/donor.

19. PURPOSE OF
TRANSAMINATION
 Removal & detoxification of
ammonia
 Gluconeogenesis from amino
acids
 Biosynthesis of non essential
amino acids.

20.  Oxidative deamination: α amino group is removed &
corresponding α keto acid & ammonia is formed.
 For metabolism of amino acid & production of
ammonia for urea conversion.

21. L- amino acid oxidase
L-amino acid + FMN α keto acid + NH3
+ FMNH2
Significance:
 Oxidative deamination is involved in metabolism of
amino acid & production of ammonia.

22.  Non oxidative deamination: Ammonia is released from
AA without oxidation.
Cysteine  Pyruvate + NH3 + H2S (Cys desulfhydrase)
Serine  Pyruvate + NH3 (Serine dehydratase)
Threonine  α ketobutyrate + NH3 (Thr dehydratase)
Histidine  Urocanate + NH3 (Histidase)
Significance: Metabolism of amino acid & Production of
ammonia

23. TRANSMETHYLATION
 Transfer of methyl group from S-Adenosyl
Methionine to various compounds.

24. CH3 acceptor  Methylated product
 Norepinephrine  Epinephrine
 Guanidinoacetate  Creatine
 Ethanolamine  Choline
 N acetyl serononin  Melatonin
 Carnosine  Anserine

25. TRANS-SULFURATION
1. Cystathionine
synthase
2. Cystathioninase
PLP – Pyridoxal
phosphate

26. Significance:
 Cysteine is formed from Methionine  therefore
cysteine is non-esseatial amino acid
 Inherited defect of enzymes of trans-sulfuration
leads to Homocystinuria and/or cystathionuria.

27. SOURCES OF AMMONIA
 Transamination followed by oxidative deamination:
Ala/Asp + α KG  Pyruvate/Oxaloacetate + Glu
Glu + NAD+  α KG + NH3 + NADH + H+
 Direct oxidative deamination:
L-Amino acid + FMN  α-Keto acid + NH3 + FMNH2
 Non oxidative deamination:
Ser/Thr dehydratase, Cys desulfhydrase, Histidase
 Non amino acid sources:
Degradation of urea by bacteria in intestinal lumen,
degradation of purine/pyrimidine nucleotides &
biogenic amines.

28. TRANSPORT OF AMMONIA
 3 forms: Glutamine, Glutamate, Ammonia,alanine
 Glutamine (Gln): from brain & intestine to liver
Glu + NH3 + ATP  Gln + ADP + Pi (Gln synthetase)
 Intracellular ammonia is immediately converted
to Gln to reduce toxicity & transport to liver for
final disposal
 Glutamate (Glu): Through transamination, NH3 is
donated to α KG to form Glu. Glu is transported to
liver for final disposal.
 Free ammonia: Normal level - 15-60 μmol/l. Excess
of free ammonia is toxic to cells especially neurons.

29.  Muscle : mainly release ammonia as alanine to liver
 Brain : mainly release ammonia as glutamine to liver
 Intestine : mainly release ammonia as alanine to
liver
 Kidney : mainly release ammonia as alanine to liver
and NH4+ as in urine for buffer

30. FATE OF AMMONIA
 Major fate of ammonia is  conversion to UREA.
 Other fate:
 Formation of Glutamine/Asparagine
 Formation of Glycine
 Formation of nitrogenous compound like purine ,
pyrimidine

31. SUMMARY
 Digestion of protein
 Absorption of amino acids
 Disorders related to digestion & absorption
 Reactions of amino acids:
 Transamination, Oxidative deamination, non
oxidative deamination
 Transmethylation, Trans-sulfuration.
 Ammonia:
 Sources, Transport & Fate of ammonia

32. IMPORTANT QUESTIONS
 Digestion of protein & absorption of amino acids (2
or 3 marks, viva)
 Meister cycle (2 marks, viva)
 Transamination (2 or 3 marks, viva)
 Trans-deamination (2 or 3 marks, viva)
 Transmethylation, Trans-sulfuration (2 marks each,
viva)
 Sources & transport of ammonia (viva)
 Next class on Urea cycle.