This document discusses the structure of proteins at multiple levels. Proteins are composed of amino acids linked together by peptide bonds. There are four levels of protein structure: primary, secondary, tertiary, and quaternary. The primary structure is the amino acid sequence. Secondary structure includes alpha helices and beta sheets formed by hydrogen bonding between amino acids. Tertiary structure is the three-dimensional folding of the polypeptide chain, stabilized by interactions between amino acid side chains. Quaternary structure refers to the arrangement of multiple polypeptide subunits in a protein. Protein structure determines its function in the cell.

3.  The term "protein" originates from the Greek word 'Proteios'
meaning holding the first place, as proposed by Berzelius, a
Swedish chemist.
 Proteins are the predominant organic molecules in living
organisms.
 They are present in all parts of the cell and make up
approximately 50% of the cellular dry weight.
 play a crucial role in the structure and function of life.
 About 3/4th of the total dry body weight is comprised of
proteins.
INTRODUCTION

4.  Proteins are intricate organic molecules with molecular weights
varying from 6000 to several million.
 They consist of carbon, hydrogen, oxygen, nitrogen, and
occasionally phosphorus and sulfur.
 Amino acids are the building blocks of proteins, linked together
by peptide bonds.
 The properties and actions of proteins are determined by the
specific amino acids they contain.
Composition

5.  Proteins contain carbon, hydrogen, oxygen, nitrogen, and small
amounts of sulfur.
 Composed of amino acids linked together by peptide bonds.
 Act as catalysts, enzymes that speed up chemical reactions.
 Provide structural support for cells.
 Transport substances across cell membranes.
 Act as a defense mechanism against pathogens (antibodies).
 Respond to chemical stimuli.
 Secrete hormones.
Physical Properties

6.  Proteins are essential biological macromolecules composed of
amino acid polymers.
 Biochemists have identified
distinct levels of structural
organization for proteins.
 The levels of protein
structure include:
 1. Primary structure
 2. Secondary structure
 3. Tertiary structure
 4. Quaternary structure
STRUCTURE OF PROTIEN

7.  The primary structure of a protein is determined by the specific
sequence of amino acids in the polypeptide chain.
 Amino acids are connected through peptide bonds to form the
polypeptide chain.
 Each individual amino acid in a polypeptide chain is referred to
as a "residue" or "moiety".
 Conventionally, the primary structure of a protein is described
starting from the amino-terminal (N) end and ending at the
carboxyl-terminal (C) end.
PRIMARY STRUCTURE

8. IMPORTANCE OF PRIMARY STRUCTURE
• To predict 20 and 30 structures from sequence
homologies with related proteins. (Structure
prediction)
• Many genetic diseases result from abnormal amino acid
sequences.
• To understand the molecular mechanism of action of proteins.
• To trace evolutionary paths.
METHODS OFAMINOACID SEQUENCE DETERMINATION
• End group analysis – Edman degradation.
• Gene sequencing method.
IMPORTANCE OF PRIMARY
STRUCTURE

9. SECONDARY STRUCTURE
• Localized arrangement of adjacent amino acids formed as the polypeptide
chain folds.
• It consists of
• Linus Pauling proposed some essential features of peptide units and
polypeptide backbone. They are:
– The amide group is rigid and planar as a result of resonance. So rotation
about C-N bond is not feasible.
– Rotation can take place only about N- Cα and Cα – C bonds.
– Trans configuration is more stable than cis for R grps at Cα
• From these conclusions Pauling postulated 2 ordered structures α helix and
β sheet
α-helix
β-pleated sheet
β-bends
Non repetitive structures
Super secondary structures
SECONDARY STRUCTURE

10. Spiral structure
Tightly packed, coiled
polypeptide backbone core.
Side chain extend outwards
Stabilized by H bonding
b/w carbonyl oxygen and
amide hydrogen.
Amino acids per turn – 3.6
Pitch is 5.4A
Alpha helical segments are found
in many globular proteins like
myoglobins, troponin- C etc.
ALPHA HELIX
H bonding

11. BETA PLEATED SHEET
 Formed when 2 or more polypeptides
line up side by side.
 Individual polypeptide - β strand
 Each β strand is fully extended.
 They are stabilized by H bond b/w
N-H
and carbonyl grps of adjacent chains.
 2 types
Parallel Anti -Parallel
N
N
C
C
C
N
N
C

12. TERTIARY STRUCTURE
Tertiary structure is the three-
dimensional conformation of a
polypeptide.
The common features of protein
tertiary structure reveal much about
the biological functions of the proteins
and their evolutionary origins.
The function of a protein depends on
its tertiary structure. If this is
disrupted, it loses its activity.

13. INTERACTIONS STABILIZING 30
STRUCTURE
• This final shape is
determined by a variety of
bonding interactions
between the "side chains"
on the amino acids.
• Hydrogen bonds
• Ionic Bonds
• Disulphide Bridges
• Hydrophobic Interactions:

14. • The biological function of some
molecules is determined by
multiple
polypeptide chains –
multimeric proteins.
• Arrangement of polypeptide sub
unit is called quaternary
structure.
• Sub units are held together by
non covalent interactions.
• Eg: Hemoglobin has the
subunit composition a2b2
QUATERNARY STRUCTURE
Quaternary structure of hemoglobin.

15. CONCLUSION
• Proteins are extraordinarily complex molecules. Of all the
molecules encountered in living organisms, proteins have the
most diverse functions.
• So a basic understanding of the structure of proteins is
necessary to comprehend its role in organisms.
• Further researches will provide more insight into the structure
of several other proteins in the coming year.

16. REFERENCE
• Voet, Donald; Voet Judith. Biochemistry, 3rd edition, John
Wiley and sons.
• Champe, Pamela.C, Harvey, RichardA, Ferrier Denise R
(2005). Lippincott’s Illustrated Reviews: Biochemistry, 3rd
edition. Lippincott William and Wilkins.
• McKee Trudy, McKee James R (2003), Biochemistry: The
molecular basis of life, 3rd edition, McGraw Hill.
• http://esciencenews.com/articles/2011/06/01/new.antibiotics.a.
step.closer.with.discovery.bacterial.protein.structure
• http://www.eurekalert.org/pub_releases/2010-04/sri-
srs042610.php
• http://www.physorg.com/news/2011-10-cell-survival-protein-
reveals.html