Protein metabolism
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Why to study protein
metabolism?
• Most serious nutritional problem in
world. 63% Indians malnourished?
• Affected groups
• Pregnant and lactating women
• New born and growing children
• Poor and aged persons
• Person habitually avoiding protein rich
food

Dietary proteins
• Contains simple and conjugated proteins
• Primary source of nitrogen to the body
• 16 gm protein nitrogen is excreted/day in health
• About 70 to 100 gm of proteins are required per
day by adult to replace loss.
• Regular supply necessary for cell integrity,
growth and cellular functions. (Nitrogen
balance)

Name major functions performed
by dietary proteins.
• Performs four basic functions
1. Synthesis of body proteins 300 to 400gm/ day
2. Oxidised to supply energy 30 to 50 gm/day
3. Required for nitrogenous compound synthesis
4. Source of non- nitrogenous substances (Glu,
FA)

What is peptidase? Or proteolytic enzymes?
• Hydrolase group of enzymes that hydrolyse
(digest) proteins are called peptidase or
proteolytic enzymes or protease .
• Two Type 1. Endopeptidase 2. Exopeptidase

Why digestion of proteins? How?
• Proteins can not be absorbed as such.
• Hence need digestion by proteolytic enzymes.
• Cooking denatures proteins and make them more
digestible, palatable and tasteful.
• Complex molecules of proteins are digested to simple
amino acids before absorbed.

Name proteins that resist
digestion?
• Group of proteins called scleroproteins
resist digestion.
• Hair, Nail, Hoof & Bone proteins can not
be digested

Name the types of proteolytic
enzymes.
Proteolytic enzymes are of two types
Endopeptidase Exopeptidase
Act in the interior of
proteins
Act on the periphery of
the protein molecule
Cleaves internal peptide
bonds
Cleaves terminal peptide
bonds
End product – Small
peptides,
End product – free amino
acids and dipeptides
Eg. Pepsin, Rennin, Trypsin,
Chymotrypsin, Elastase
Eg. Carboxypeptidase,
Aminopeptidase

Give examples of Endopeptidase & their
specificity.
Enzyme Origin Specificity
Pepsin Chief cells /
Stomach
Aromatic amino
acids and acidic
amino acids
Trypsin Pancreatic juice Basic amino acids
Lysine and
Arginine
Chymotrypsin Pancreatic juice Aromatic amino
acids and Leu,
Arg, Met, His
Elastase Pancreatic juice Glycine, Serine,
Alanine

Digestion of protein
No digestion in mouth
Due to lack of
proteolytic enzymes
in saliva

In stomach -
HCl
• Vegal stimulus and Gastrin --- secrete HCl
• 2000 – 3000 ml / day
• Act on native and cooked protein
• End product  Metaprotein, Coagulated
protein
Digestion of protein

Digestion of
protein
• Secreted as pepsinogen (inactive form)
• Activated by HCl and Pepsin itself (Autocatalysis)
• Optimum pH 2.0
• Act on peptide bonds formed by carbonyl group of
aromatic and acidic amino acids in proteins
• End product  Proteoses and peptones
In stomach -
by Pepsin

Pepsinogen Pepsin
[H
+
]
Pro enzyme
Chief cells
ActiveAutocatalysis
Act on aromatic amino acid P T T and
acidic amino acids A G
In stomach - by Pepsin

Digestion of protein by pepsin
how much essential?
• It is important but not essential
• Gastroctomy person can maintain
proper nitrogen balance.

Digestion of
protein
• Secreted only in infants and child age,
Absent in adult.
• Optimum pH 4.0 – 4.5
• Act on milk protein casein
• End product  Para casein which is further
acted upon by pepsin for digestion
In stomach - by
Rennin

Milk Casein Para casein
[Rennin , Ca2+]
Acted up on
by pepsin
Curdling
Help to keep milk in stomach for long time
Ensures complete digestion
In stomach - by Rennin

Digestion of
protein In duodenum
Acidic chyme stimulates secretion of Secretin and
pancreozymin Which in turn stimulates pouring of
pancreatic juice
Pancreatic juice contains
• Trypsinogen
• Chymotrypsinogen
• Pro-Elastase
• Pro carboxy peptidase
• Por-Collagenase

Activation of pancreatic peptidases
Trypsinogen Trypsin
[Enterokinase]
Pro enzyme
ActiveAutocatalysis
Chymotrypsinogen Chymotrypsin
[Enterokinase, Trypsin ]
Pro Carboxy peptidase Carboxy peptidase

Action of pancreatic enzymes on protein
Enzyme Specificity
Trypsin Basic amino acids Lysine and
Arginine
Chymotrypsin Aromatic amino acids and Leu,
Arg, Met, His
Elastase Glycine, Serine, Alanine
Carboxy peptidase Act on C terminal peptide bonds
End product Dipeptides, Tripeptides and free amino

Digestion of protein
In small intestine
Intestinal mucosal cell produces Amino
peptidase , dipeptidases and tripeptidases.
Amino peptidase act on N terminal peptide
bonds of protein And dipeptidases act on
Dipeptides
End product is
Free amino acids

Dietary Protein
Denatured Protein
Metaprotein
Proteose and Peptones
Small peptides Di and tri peptides
Amino acids
Cooking
HCl
Trypsin
Chymotrypsin
Elastase
Pepsin
Carboxy peptidase
Amino peptidase
Dipeptidases
Tripeptidases
Digestion of protein

Absorption of amino acids
• Only amino acids are absorbed
• Intestinal epithelial cell & renal
tubular cell
• Carrier system exists or
• By Secondary active transport
• Genetic defect -- improper absorption
of particular amino acid (Hartnup’s
disease, Glycinuria)

Absorption of amino acids
Three mechanism exists
1. Sodium independent facilitated
transport system
2. Sodium dependent secondary
active transport system
3. γ- Glutamyl cycle (meister cycle)

Mechanism of amino acid
absorption
Sodium independent facilitated transport
Purely on concentration
gradient

ATP ADP
Active transport
Secondary active
Mechanism of amino acid absorption
Sodium dependent Secondary active transport

γ- Glutamyl cycle (meister cycle)
Operates in small intestine
Requires glutathione (γ - glutamyl
cysteinyl glycine )
Energy expenditure is large

γ- Glutamyl cycle (meister cycle)
Amino acids
Glutathione
Glutamyl
transferase
Cystenyl
Glycine
Glutamyl
AA
Gutamate
γ -Glutamyl
cysteine
Glycine
ATP
ATP
A D P
+ Pi
A D P + Pi
1
2
3
4
5
ATP
A D P
+ Pi
Amino
acids
6
7

Some Exceptions
• In case of infants
direct absorption of
IgA from the
mothers milk
(Colostrum)
• This provides
passive immunity to
the
• newborn
1. Some plant proteins
are endocytosed in
partially digested
form
• Basis of allergens
and anaphylactic
reactions
Carrier proteins
defect
2. Hartnup disorder
3. Amino acidurias

Thank you
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