2. 3-2
Protein Structure
Present in all types of cells and in all parts of the
cell.
Proteins are chain-like polymers of small repeating
subunits called amino acids
– Proteins have 20 different amino acids with:
• An amino group
• A hydroxyl group
• A hydrogen atom
• A specific side chain
When small molecules/units/monomers are combined to form
large molecule, this is called polymer.
A small molecule is called monomer (amino acid)
3. What is an amino acids?
• Amino acids are building block of protein. Amino acids is
actually a molecule in which a carbon atom is present to
which 2 functional groups are attached.
• R may be a hydrogen atom as in glycine, or CH3 as in
alanine, or any other group. So amino acids mainly differ
due to the type or nature of R group.
3-3
4. 3-4
Polypeptides
• Amino acids are joined together via peptide bonds
• Chains of amino acids are called polypeptides
• Proteins are composed of 1 or more polypeptides
– Free amino group at one end is the amino- or N-terminus
– Free hydroxyl group at the other end is the carboxyl- or
C-terminus
5. 3-5
Types of Protein Structure
• Linear order of amino acids is a protein’s
primary structure
• Interaction of the amino acids’ amino and
carboxyl groups gives rise to the secondary
structure of a protein
– Secondary structure is the result of amino acid and
carboxyl group hydrogen bonding among near
neighbors
– Common types of secondary structure:
a-helix
b-sheet
6. 3-6
Helical Secondary Structure
• In a-helix secondary
structure, polypeptide
backbone groups
forms H bond with
each other
• Dashed lines in figure
indicate hydrogen
bonds between
nearby amino acids
7. 3-7
Sheet Secondary Structure
• The b-sheet pattern of also
occurs when polypeptide
backbone groups form H
bonds
• In the sheet configuration,
extended polypeptide
chains are packed side by
side
• This side-by-side packing
creates a sheet appearance
8. 3-8
Tertiary Structure
• Total three-dimensional
shape of a polypeptide
is its tertiary structure
• A prominent aspect of
this structure is
interaction of the amino
acid side chains
• The globular form of a
polypeptide is a roughly
spherical structure
10. Protein Function
• They build many structures of the cell.
• All enzymes are proteins and in this way they control the
whole metabolism of the cell.
• As hormones, proteins regulate metabolic processes.
• Some proteins (e.g. hemoglobin) work as carriers and
transport specific substances such as oxygen, lipids, metal
ions, etc.
• Some proteins called antibodies, defend the body against
pathogens, Defensive protein
• Blood clotting proteins prevent the loss of blood from the
body after an injury.
• Movement of organs and organisms, and movement of
chromosomes during anaphase of cell division, are caused by
proteins.
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11. Classification of Proteins
1. Fibrous proteins:
They consist of molecules having one or more polypeptide
chains in the form of fibrils.
Secondary structure is most important in them.
They are insoluble in aqueous media.
They are non-crystalline and are elastic in nature.
They perform structural roles in cells and organisms.
• Examples are myosin (in muscle cells), fibrin (of blood
• clot), and keratin (of nails and hair).
3-11
12. 2. Globular proteins:
• These are spherical or ellipsoidal due to multiple folding
of polypeptide chains.
• Tertiary structure is most important in them.
• They are soluble in aqueous media such as salt
solution, solution of acids or bases, or aqueous alcohol.
They can be crystallized.
• They disorganize with changes in the physical and
physiological environment.
• Examples are enzymes, antibodies, hormones and
hemoglobin.
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