1. Proteins are made of amino acid building blocks joined by peptide bonds. They fold into complex 3D shapes defined by their primary, secondary, tertiary, and quaternary structures to achieve their biologically active forms. 2. The stomach breaks down proteins through the actions of pepsinogen, hydrochloric acid, and other gastric secretions. Parietal cells secrete acid and intrinsic factor through a proton pump, while chief cells secrete the inactive pepsinogen enzyme. 3. Proteins are classified based on their shape, functions, and solubility properties. Fibrous proteins like collagen are long and thin, while globular proteins like hemoglobin are compact spheres. Enzy

1. Protein structure

2. • Proteins catalyze metabolic reactions,
power cellular motion, and forms
structural integrity to hair, bones, tendons
and teeth
• Human proteins therefore reflects the
sophistication and diversity of their biologic
roles

3. •
•
The 20 amino acids commonly found in
proteins are joined together by peptide
bonds.
PEPTIDES: Two AA covalently joined
through a substituted amide linkage –
peptide bond by removal of H2O
OH- Carboxyl group of one AA and H+ from
amino group of another AA

4. •
•
•
•
• Two AA reacts to form
dipeptides.
Three AA can be joined by
two peptide bonds to form
a tripeptide and so on.
Oligopeptide: When a few
AA are joined by various
peptide linkage •
When many amino acids
are joined, the product is
called a polypeptide.
Proteins may have
thousands of amino acid
residues

5. •
•
The linear sequence of the linked amino acids
contains the information necessary to generate a
protein molecule with a unique three-
dimensional shape.
Therefore maturation of a newly synthesized
polypeptide into a biologically functional protein
– Requires folding into a specific three-
dimensional arrangement, or conformation

6. •
1.
2.
3.
4.
The complexity of protein structure is best
analyzed by considering the molecule in terms of
four organizational levels:
Primary: linking amino acid residues in a
polypeptide chain
Secondary: stable arrangements of amino acid
residues giving rise to recurring structural
patterns into geometrically ordered units;
twisting resulting in α-helix or pleated
tertiary: the three-dimensional assembly of
secondary structural units to form larger
functional units
Quaternary: : It’s the arrangement in space of
protein having two or more polypeptide subunits

8. •
•
An examination of these hierarchies of
increasing complexity has revealed that certain
structural elements are repeated in a wide
variety of proteins, suggesting that there are
general “rules” regarding the ways in which
proteins achieve their native, functional form.
These repeated structural elements range from
simple combinations of α-helices and β- sheets
forming small motifs, to the complex folding of
polypeptide domains of multifunctional
proteins.

9. CLASSIFICATION OF PROTEINS
•
I.
Proteins are classified:
On the basis of shape and size
II. On the basis of functional properties
III. On the basis of solubility and physical
properties.

10. •
•
I. On the basis of shape and size
•Fibrous proteins: When the axial ratio of length: width
of a protein molecule is more than 10, it is called a
fibrous protein
. fiber like in shape, insoluble in water
and resistant to digestion
Examples : α-keratin from hair, collagen.
•Globular protein: When the axial ratio of length: width
of a protein molecule is less than 10, it is called as
globular protein.
Examples: Myoglobin, haemoglobin, ribonuclease,
etc.

11. •
•
•
•
•
•
•
•
•
II. On the basis of functional properties:
The second way of classifying proteins makes use of
their functional properties, such as:
Enzymes: Proteins catalyzing the biological
reactions.
Hormones: Proteins regulating physiological
responses.
Defence proteins: Immunoglobulins involved in
defence mechanisms.
Contractile proteins: Proteins of skeletal muscle
involved in muscle contraction and relaxation.
Respiratory proteins: Involved in the function of
respiration, like haemoglobin, myoglobin,
cytochromes.
Structural proteins: Proteins of skin, cartilage, nail.

12. •
III. On the basis of solubility and physical properties:
However, both the above classification schemes have many
overlapping features. Therefore a third most acceptable scheme
of classification of proteins is adopted. According to this scheme
proteins are classified on the
basis of their solubility and physical properties and are divided
in three different classes.
A. Simple proteins: These are proteins which on complete
hydrolysis yield only amino acids. Albumin, Histone
B. Conjugated proteins: These are proteins which in addition to
amino acids
contain a non-protein group called prosthetic group
in their structure. Lipoproteins, Glycoproteins, Nucleoprotein
C. Derived proteins: These are the proteins formed from native
protein by the action of heat, physical forces or chemical factors.
Fibrin

13. Digestion in stomach
•
•
•
•
Food particles that enter the stomach are
thoroughly mixed with gastric secretions to
form the solution defined earlier as chyme.
Chyme contains molecular fragments of
proteins and polysaccharides; droplets of
fat; and ions, water, and various other
molecules ingested in the food.
Virtually none of these ions or molecules,
except water, can cross the epithelium of
the gastric wall, and thus little absorption of
nutrients occurs in the stomach.
In addition to playing a role in the
breakdown of proteins, the stomach
functions as a storage vessel that
periodically empties some chyme into the
small intestine at a rate that favors the
complete digestion and absorption of a
meal.

14. •
•
•
•
•
•
The three major exocrine secretions of the
stomach—mucus, acid, and pepsinogen—is
secreted by a different cell type
The cells at the opening of the gastric
glands secrete a protective coating of
mucus and HCO 3−.
Lining the walls of the glands are parietal
cells, which secrete acid and intrinsic factor.
Intrinsic factor is a protein that binds and
allows the absorption of vitamin B12.
chief cells, which secrete pepsinogen
The gastric glands also
containenteroendocrine cells called G cells,
which secrete gastrin.
In addition, enterochromaffin-like (ECL) cells,
which release the paracrine substance
histamine

15. HCl Production and Secretion
•
•
The stomach secretes
about 2 L of hydrochloric
acid per day.
the parietal cell
contains the enzyme
carbonic anhydrase
that catalyzes the
reaction between CO2
with water to produce
carbonic acid, which
dissociates to H+ and
HCO 3−

16. •
•
•
•
H+/K+- ATPase pumps the
parietal cells pump hydrogen
ions into the lumen of the
stomacH.
This primary active transporter
also pumps K+ into the cell,
which then leaks back into the
lumen through K+ channels.
As H+ is secreted into the
lumen, HCO 3− is moved
across the membrane and into
the capillaries in exchange for
Cl−, which maintains
electroneutrality.
In this way, production and
secretion of H+ are coupled.