The document discusses the four levels of protein structure: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids in the peptide chain. The secondary structure involves hydrogen bonding that causes the chain to fold into structures like alpha helices or beta sheets. Tertiary structure describes further folding and interactions that result in the protein's three-dimensional shape. Quaternary structure refers to multiple peptide chains linked together in a protein.

1. Mahpara Gondal
Pharm-D
MS Pharmaceutical Chemistry
Rashid Latif College of Pharmacy

2. Protein structure is normally described at
four levels of organisation.
1. Primary
2. Secondary
3. Tertiary
4. Quaternary

3.  Primary structure is the linear sequence of
amino acids held together by peptide bonds
in its peptide chain.
 The free -NH2 group of the terminal amino
acid is called as N-terminal end and the free -
COOH end is called as C-terminal end.
 It is a tradition to number the amino acids
from N-terminal end as No. 1 towards the C-
terminal end.

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5.  Presence of specific amino acids at a specific
number is very significant for a particular function
of a protein. Any change in the sequence is
abnormal and may affect the function and
properties of protein.
 For example: straight chain of Hb single
replacement of glutamic acid with valine in
position 6 cause a disease called sickle cell
anemia.

6. .

7. The peptide chain thus formed assumes a
three dimensional secondary structure by
way of folding or coiling consisting of a
helically-coiled, zigzag, linear or mixed
form.
The linkages or bonds involved in the
secondary structure formation are
hydrogen bonds and disulfide bonds.

8.  Disulphide bonds: These are formed
between two cysteine residues. They are
strong, high energy covalent bonds.
 Hydrogen bonds are formed in secondary
structure by sharing H-atoms between
oxygen of CO and nitrogen of -NH of different
peptide bonds. The hydrogen bonds in
secondary structure may form either an α-
helix or β-pleated sheet structure.

9. α-Helix:
 A peptide chain forms regular helical coils
called α-helix. These coils are stabilised by
hydrogen bonds between carbonyl O of 1st
amino and amide N of 4th amino acid
residues. Thus in α-helix intra chain hydrogen
bonding is present. The α-helices can be
either right handed or left handed. Left
handed α-helix is less stable than right
handed α-helix.

10. Each turn of helix is 5.4˚A long and contain
3-6 amino acids residues.
The lengthof each residue is 1.5 ˚A or
0.15nm.

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14. β-Pleated Sheet Structure: β-Keratins
present in spider’s web, reptilian claw,
fibres of silk form almost fully extended
chain.
β-pleated sheet structure is thus formed
when hydrogen bonds are formed between
the carbonyl oxygens and amide
hydrogens of two or more adjacent
extended polypeptide chains.

15. The structure is not absolutely planar but is
slightly pleated due to the angles of bonds.
The adjacent chains in β-pleated sheet
structure are either parallel or
antiparallel, depending on whether the
amino to carbonyl peptide linkage of the
chains runs in the same or opposite
direction.

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18.  .

19. The polypeptide chain with secondary
structure mentioned previous may be
further folded, superfolded twisted
about itself forming many sizes. Such a
structural conformation is calle tertiary
structure.
The bonds responsible for interaction
between groups of amino acids are
describe further:

20.  Hydrophobic interactions:
 Normally occur between nonpolar side chains of
amino acids such as alanine, leucine,
methionine, isoleucine and phenylalanine.
 They constitute the major stabilising forces for
tertiary structure forming a compact thre
dimensional structure.
 • Hydrogen bonds: Normally formed by the polar
side chains of the amino acids.

21.  Ionic or electrostatic interactions: These
are formed between oppositely charged polar
side chains of amino acids, such as basic and
acidic amino acids.
 • Van der Waal Forces: Occur between
nonpolar side chains.
 • Disulfide bonds: These are the S–S bonds
between -SH groups of distant cysteine
residues.

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24. when a protein consists of two or more
peptide chains held together by non-
covalent interactions or by covalent
cross-links, it is referred to as the
quaternary structure.
Example: Heamoglobin

27.  Proteins are the most versatile macromolecules
in living systems and serve crucial functions in
essentially all biological processes. They
function as catalysts, they transport and store
other molecules such as oxygen, they provide
mechanical support and immune protection, they
generate movement, they transmit nerve
impulses, and they control growth and
differentiation.

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