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The Nature of Proteins
The significance of proteins cannot be over-emphasised, since
they are intimately connected with all phases of the chemical
and physical activities of the living cell
Proteins function as enzymes, hormones and oxygen transporters
and they form the bulk of skin, hair, feathers, nails and cartilage
Human
hair
Feather
Proteins are huge three-dimensional molecules whose building
blocks or monomers are the variety of different amino acids
found in nature
Amino Acid Structure
Variable group
amino
group
C
C
carboxylic
acid group
Formation of a Dipeptide
C
TWO
AMINO
ACIDS
C
C
C
C
O
N
D
E
N
S
A
T
I
O
N
-H
O
2
C
C
C
p
e
p
t
i
d
e
b
o
n
d
C
D
I
P
E
P
T
I
D
E
The Variety of Amino Acids
Individual amino acids display a
tetrahedral shape due to the angles
of the bonds between the atoms
There are twenty different naturally
occurring amino acids that differ from
one another by virtue of the R group
The simplest of the amino acids
possesses a hydrogen atom for its R
group. This amino acid is called
GLYCINE
Proteins - Levels of Structure
Amino Acid Sequence
Peptide bond
Polypeptide chains form when amino acids bond together
in a particular sequence. THE PRIMARY STRUCTURE
of a protein is the number, type and sequence of amino acids
that make up this linear chain together with the peptide bonds
that hold them together
Different proteins have different primary structures. Different
proteins are made up of different types, numbers and sequences
of amino acids making up the primary chain
Proteins - Levels of Structure
Secondary Structure
The secondary structure of proteins is the arrangement in
space of the atoms that form the backbone or linear chain
of the protein
The amino acid chain can coil into a helix shape or form
a shape called the beta pleated sheet
The helix and beta pleated sheet shapes are secondary
structures of protein molecules
Alpha helix
Beta pleated sheet
Tertiary Structure of Proteins
HAEM GROUP
MYOGLOBIN MOLECULE
All globular proteins
display tertiary structure.
Once the secondary
structures have formed,
the molecule bends and
folds into a 3-D globular
shape
Myoglobin is a globular
protein found in muscle
cells. This tertiary shape
is the highest level of
structure for this protein
and a variety of bonds
help to stabilise its
structure
Quaternary Structure of Proteins
beta chain
beta chain
Quaternary structure is a
level of structure displayed
by proteins that consist of
more than one polypeptide
chain
Haemoglobin is a protein
displaying quaternary
structure
Haemoglobin consists of four
polypeptide chains that are
held together by weak van
der Waals forces
alpha chain
alpha chain
iron-containing
haem group
The Haemoglobin Molecule
Each polypeptide chain contains
an iron containing HAEM group
that binds to molecules of oxygen
BONDS THAT STABILISE SECONDARY & TERTIARY
STRUCTURE
As the chains of amino acids bend
& fold to form secondary & tertiary
structures, various atoms are brought
into close proximity and form bonds
Hydrogen and oxygen atoms from both the
main chain and the R groups may form
hydrogen bonds
The R groups of two amino acids contain
sulphur atoms. When these atoms are in
close proximity they form DISULPHIDE
BRIDGES
Many of the carboxylic acid and amino groups
form charged groups in solution. Oppositely
charged groups form IONIC BONDS
Many hydrophobic R groups tend to cluster
towards the interior of the protein molecule
forming Hydrophobic Interactions
SUMMARY
 The building blocks of proteins are monomers
called amino acids
 Every amino acid possesses an amino end and a
carboxylic acid end
 There are twenty different naturally occurring
amino acids
 Amino acids differ by virtue of the nature of
their R groups
 Amino acids bond together forming peptide
bonds
 When two amino acids bond during a
condensation reaction, the resulting molecule is
a dipeptide
 When many amino acids bond together, the
resulting molecule is referred to as a
polypeptide
 Chains of amino acids numbering greater than
100 are generally referred to as proteins
 Individual amino acids may be neutral, basic or
acidic
 Two amino acids, namely cysteine and
methionine, possess sulphur atoms in their R
groups
 The type, number and sequence of amino acids
forming the original linear chain of a protein is
termed the PRIMARY STRUCTURE OF A
PROTEIN
 Different proteins have different primary
structures
 The primary structure determines the final
shape of the protein molecule
 The linear chain of amino acids making up the
primary structure of the protein bends and folds
in various ways to form the SECONDARY
STRUCTURE OF THE PROTEIN
 Two main types of secondary structure are found
in proteins - the beta pleated sheet and the alpha
helix
 The alpha helix forms when the linear chain coils
into a right handed helix
 The beta pleated sheet forms when the linear
chain folds back on itself many times
 Hydrogen bonds play a major part in stabilising
the secondary structure of proteins
 Many proteins bend and fold further to form
globular TERTIARY STRUCTURES
 Myoglobin is a globular protein displaying the
tertiary level of structure
 Myoglobin is a protein found in muscle cells
 Proteins consisting of more than one polypeptide
chain display quaternary structure
 Haemoglobin is a protein consisting of more than
one polypeptide chain
 Haemoglobin consists of four separate polypeptide
chains held together by weak van der Waals forces
 Each polypeptide chain in haemoglobin contains a
haem group that binds to molecular oxygen
 The role of haemoglobin is to transport oxygen
molecules from the lungs to the body tissues
 A variety of different bonds stabilise the
secondary and tertiary structures of proteins
 Hydrogen bonds form between oxygen and
hydrogen atoms within the main amino acid
chain and between the R groups
 Disulphide bridges form between sulphur
atoms in the R groups of amino acids such as
cytseine
 Ionic bonds form between charged amino
groups and charged carboxylic acid groups
 Hydrophobic interactions occur between R
groups that have clustered towards the centre
of protein molecule due to their hydrophobic
nature