This document discusses the different levels of protein structure, including: 1) Primary structure, which is the amino acid sequence that makes up the protein chain. 2) Secondary structure, which involves local folding into structures like alpha helices and beta sheets. 3) Tertiary structure, which describes the overall 3D shape of the protein formed by interactions between different parts of the amino acid chain. 4) Quaternary structure for proteins made of multiple subunits, describing how those subunits are arranged relative to each other.

2. Content
• Introduction to protein
• Levels of Protein structure
• Folds
• Motifs
• Domain

3. Proteins
• Proteins are family of biological macromolecules that provide a variety of 3D structure exquisitely shaped for
their many different individual functions. They are made up of long chain of amino acids. They sequence and
arrangement of amino acids determine the unique structure and function of each protein.
• They includes
• Structural Proteins
• Enzymes
• Antibodies
• Regulatory bodies
• Sensors
• Transporters and pumps

4. Levels Of Protein Structure
• The building blocks of proteins are amino acids, which are small organic molecules that consist of an
alpha (central) carbon atom linked to an amino group, a carboxyl group, a hydrogen atom, and
a variable component called a side chain.
• Danish protein chemist KU Linderstorm- Lang described
Three levels of protein structure:
 Primary
 Secondary
 Tertiary.
• For proteins composed of more than one subunit, J.D.
Bernal added the level
 Quaternary Structure.

5. • Primary Protein Structure (amino acid sequence)
The Amino acid sequence of a protein, plus intra- and interchain covalent cross-links if any, defines the primary
structure.

6. • Secondary Protein Structure (helix formation and local folding)
The secondary protein structure depends on the local interactions between parts of a protein chain, which can
affect the folding and three-dimensional shape of the protein. There are two main things that can alter the
secondary structure:
•α-helix: It is a rigid, rod like structure that form when a polypeptide chain twists into a helical conformation. The
screw sense of α- helix can be right- handed (Clockwise){energetically more favorable} or left handed
(counterclockwise).
•β-pleated sheet: It form when two or more polypeptide chain segments line up side by side. Each individual
segment refer to beta- strand. Adjacent strands can be parallel or anti- parallel. Anti-parallel sheet are more
stable than parallel sheet.

7. FOLDS in Protein Structure
Amino acids interact with each other to produce a well-
defined three-dimensional structure, the folded protein
(the right hand side of the figure), known as the native
state. All the information for the native fold appears
therefore to be contained within the primary structure
(Anfinsen received the Nobel Prize for this), and proteins
are self-folding (although in vivo, polypeptide folding is
often assisted additional molecules known as molecular
chaperones).
Protein folding is the physical process by which a linear polypeptide folds into its characteristic and functional
three-dimensional structure. A protein Fold is defined by the way the secondary structure element of the
structure are arranged relative to each other in space.

8. MOTIFs in Protein Structure (Supersecoondary Structures)
Many Globular protein contain combination of α-helix and β-pleated secondary structures. Specific geometric
arrangements of α-helix and β-pleated connected through loops are called supersecondary structures (also called as
motifs).

9. • Tertiary Protein Structure
The tertiary structure of proteins refers to the overall three-dimensional shape, after the secondary
interactions. All information needed to fold the protein into its native tertiary structure is contained
within the primary structure of the peptide chain itself.
• Hydrophobic interaction
• Electrostatic
• Hydrogen bonds
• Van der waals force of interaction
• Covalent bonds.
 DOMAINS: The fundamental unit of tertiary structure is Domain. It is
a polypeptide chain or a part of a polypeptide chain that can fold
idependently into a stable 3D tertiary structure. Domain is basically
independently folded region of the protein.
 Domains are classified into three classes :
• α-Domain
• β- Domain
• αβ- Domain

10. Quaternary structure of Protein (Protein-protein interaction or multichain association)
Some globular proteins consist of 2 or more
interacting peptide chains. Each peptide chain in
such a protein is called Subunit.These proteins are
called multimeric proteins(homomultimeric or
heteromultimeric). These chain may be identical or
different in their primary structure. The quaternary
structure refers to the spatial arrangement of
subunits and the nature of their contact. The same
force is involved in this formation.