Digestion of proteins
Cellular Biochemistry and
metabolism 1
CLS 331
Dr. Samah Kotb
Lecturer of Biochemistry
2015
Proteins
Structure of proteins
C-terminus
COOH end
N-terminus
NH2 end
Dehydration synthesis
Amino Acid + Amino Acid --> Dipeptide
Amino Acid + Dipeptide --> Tripeptide
A.A. + A.A. + …..+ Tripeptide --> Polypeptide
A protein consists of one or more
polypeptide chains.
Dr Samah Kotb
Lecturer of
Biochemistry
proteins can be fibrous or globular
Fibrous
Dr Samah Kotb
Lecturer of
Biochemistry
Globular
Functions of proteins
Generally speaking, proteins do everything in the living
cells.
1) Catalysis: Almost all chemical reactions in a living cell are
catalyzed by protein enzymes.
2) Transport: Some proteins transports various substances
(Hemoglobin).
3) Regulatory: Many hormones are proteins (insulin).
Dr Samah Kotb
Lecturer of
Biochemistry
4) Contraction: Some have role in contractile
mechanism ( actin & myosin) (muscle contraction).
5)Proteins of immune defense: Antibodies.
6) Structural Functions: giving structure and form
to the human organism. Example: keratin, collagen.
Dr Samah Kotb
8
DIGESTION
&
ABSORPTION
OF
PROTEINS
Dr Samah Kotb
9
Dr Samah Kotb
10
Dr Samah Kotb
11
Dr Samah Kotb
12
Digestion & Absorption of proteins
Enzymes involved in the Digestion of Proteins
 Proteases are proteolytic digestive enzymes and
are
of
two
types:
Endopeptidases
and
Exopeptidases.
Dr Samah Kotb
Lecturer of
Biochemistry
Digestion & Absorption of proteins
 Endopeptidases
hydrolyze peptide bonds
between specific amino acids throughout the
molecule. They are the first enzymes to act,
yielding a larger number of smaller fragments, e.g,
pepsin in the gastric juice and trypsin,
chymotrypsin, and elastase secreted into the small
intestine by the pancreas.
Digestion & Absorption of
proteins
 Exopeptidases catalyze the hydrolysis of
peptide bonds, one at a time, from the ends of
polypeptides.
 Exopeptidase is further classified as an
 A. Carboxypeptidase or
 B. Aminopeptidase or
 C. Dipeptidases
Dr Samah Kotb
16
Carboxypeptidases, secreted in the pancreatic
juice, release amino acids from the free carboxyl
terminal.
Dr Samah Kotb
17
Aminopeptidases, secreted by the intestinal
mucosal cells, release amino acids from the amino
terminal.
Dipeptidases, secreted by the brush border of
intestinal mucosal cells, hydrolyze dipeptides into
their two component amino acids.
Dr Samah Kotb
18
ACTIVATION OF PROTEASES
Dr Samah Kotb
19
Digestion & Absorption of proteins
The
proteases
are
secreted
as
inactive
zymogens, the active site of the enzyme is
masked by a small region of its peptide chain,
which is removed by hydrolysis of a specific
peptide bond.
Digestion & Absorption of proteins
For example:
 Pepsinogen is activated to pepsin by gastric
acid and by activated pepsin.
 In
the
precursor
small
of
intestine,
trypsin,
is
trypsinogen,
the
activated
by
enteropeptidase, which is secreted by the
duodenal epithelial cells.
Digestion & Absorption of proteins
 Trypsin can then activate chymotrypsinogen to
chymotrypsin,
proelastase
procarboxypeptidase
to
to
elastase,
carboxypeptidase,
proaminopeptidase to aminopeptidase.
and
gastric acid
Pepsinogen
pepsin
activated pepsin
Enteropeptidase
Trypsin
Trypsinogen
Chymotrypsinogen
Proelastase
Trypsin
Trypsin
Chymotrypsin
Elastase
Trypsin
Procarboxypeptidase
Carboxypeptidase
Trypsin
Proaminopeptidase
Aminopeptidase
Dr Samah Kotb
23
Mechanisms of Amino
Acids Absorption
Dr Samah Kotb
24
Free amino acids & small peptides
are absorbed
by different mechanisms
 The
end
product
of
the
action
of
endopeptidases and exopeptidases is a mixture
of free amino acids, di- and tripeptides, and
oligopeptides, all of which are absorbed.
Free amino acids & small
peptides are absorbed
by different mechanisms
 Free
amino acids are absorbed across the
intestinal mucosa by sodium-dependent active
transport.
There are several different amino acid
transporters, with specificity for the nature of the
amino acid side chain (large or small; neutral,
acidic, or basic).
Absorption of protein
Absorption of protein
The various amino acids carried by any one
transporter compete with each other for absorption
and tissue uptake.
Absorption of protein
 Dipeptides and tripeptides enter the brush border
of the intestinal mucosal cells, where they are
hydrolyzed to free amino acids, which are then
transported into the hepatic portal vein.
Dr Samah Kotb
30