BIC 101- LEC 15-STRUCTURE OF PROTEINS-PRIMARY,SECONDARY,TERTIARY AND QUATERNARY STRUCTURE.ppt

LECTURE -15
STRUCTURE OF PROTEINS
PRIMARY ,SECONDARY
TERTIARY , QUATERNARY
STRUCTURE

• Proteins are polymers of amino acid
monomers.
• The amino acids are linked together to form a
chain called peptide chain .
• The peptide chain has a variable length .
• it depends on the number of amino acids
linked.
• The amino acids of proteins are called
residues.
3

• The peptide chain is named according to the
number of amino acids residues.
1.Dipeptide: A dipeptide contains two amino
acid.
2.Tripeptide : A tripeptide contains three amino
acid .
3.Oligopeptide : An Oligopeptide is made up of
less than ten amino acid .
4.Polypeptide : A polypeptide chain is made up
more than 10 amino acids.
4

Chemical Bonds involved in protein structure
 Proteins are the polymers of amino acid
monomers.
 Any two amino acid monomers are linked
together by a chemical bond.
 There are 5 types of bonds which occur in
proteins.
 They are
1. Peptide bond
2.Disulfide bond
6

3. Hydrogen bond
4. Non polar or hydrophobic bond
5.Ionic bond
1. peptide bond
 The peptide bond is an amide bond where
the CO group of COOH group of one amino
acids is linked with the NH group of NH2
group of the another amino acids.
The peptide bond produces the linear primary
structure of proteins.
7

2.Disulfide bond
Disulfide bond is formed between two poly
peptide chains by a cystine residue.
The disulfide bond is formed by the oxidation of thiol(-
SH) groups of two cysteine molecules.
 this result in the formation of a molecule of cystine, an
amino acid with a disulfide bond.
Oxytocin hormone of pituitary gland has a disulfide
bond.
The disulfide bond links two cysteine units of
the same polypeptide chain.
8

• Insulin is another example for disulfide bond .
 Linderstrom-Lang suggests four types of
structural organization for proteins.
• They are
1. Primary structure
2. Secondary structure
3. Tertiary structure
4. Quaternary structure
9

1. Primary structure
• primary structure refers to the linear sequence of
amino acids residues making up its polypeptide
chain.
• The protein may be formed of one or more
polypeptide chains.
• The amino acids are arranged in specific sequence
in the polypeptide chain.
• The amino acid residues are linked by peptide bond.
10

• The peptide bond formed between the
carboxyl group of one amino acid and the
amino group of adjacent amino acid.
• Sometimes adjacent polypeptide chains are
linked by disulfide bonds.
• Many structural proteins which form fibres are
linear and unfolded types.
• Eg. Silk fibroin.
• Each polypeptide chain of any length has at
one end an N- terminal amino acid containing
11

Free amino group and at the other end a c-
terminal amino acid containing a free carboxyl
group.
• The amino acids in polypeptide chain are
numbered from the N – terminal end.
• Most of the structural proteins which are in
the form of fibres exhibit primary structure.
• Eg .
Fibroin of silk.
12

2. Secondary structure
 Secondary structure refers to the helical
nature of the proteins.
The secondary structure derived from the
Primary structure by the formation of
hydrogen bond interactions between amino
acids residues fairly close to one another.
This leads to the folding of the polypeptide
chain into a helix.
15

• The intramolecular hydrogen bonds occurs
between an amide nitrogen (-NH group) and
the carbonyl oxygen (-C=O group) of the 4th
amino acid residue of the polypeptide chain.
• In addition disulfide bonds also occurs either
in the same chain or between polypeptide
chains.
• By configuration , the secondary structure is
classified into two types , namely helical
structure and pleated sheets.
C=O…H-N
16

• Helical structure
• The polypeptide chain is coiled as a rope
wound around a cylinder.
• The helical structure of protein is formed by
the hydrogen bonds b/w peptide groups
within the same polypeptide chain.
• The most common form of coiling is the right
Handed alpha helix occurs in nature.
The alpha helical structure model was
proposed by Pauling and Cori (1951).
17

• The main features of alpha helix are,
• The helix is formed by a series of amino acids
residues woven into a spiral chain.
• The alpha helix is so named because of the
mobility of alpha-carbon atoms.
• The helical chain has a screw type symmetry.
• A hydrogen bond formed between the peptide
groups(CO and NH) of every first and fourth
amino acid residues.
• The regular appearance of hydrogen bonds
b/w 1st
and 4th
peptide groups determines the
20

Regularity of turns in the polypeptide chain.
 The shape of the helix is maintained by
hydrogen bonds.
 Each complete coil contains 3.6 amino acid
residues.
Pleated sheet structure of beta –structure
 It is a secondary structure formed from
hydrogen bonding between two peptide chains.
 This bonding leads to the formation of sheets
of parallel chains in the from of pleated sheets.
21

• In pleated sheet structure, the chain may be
parallel or antiparallel .
• In a parallel chain pleated sheet structure , the
N-terminal end of the polypeptide chains
point in the same direction.
Eg. Keratin.
• In Antiparallel chain pleated sheet structure
the N-terminal end of the polypeptide chains
point in opposite direction.
• Eg . Fibroin of silk.
23

• Tertiary structure
• The tertiary structure of protein is more complex
than the secondary structure. Tertiary structure is
exhibited by proteins having only one polypeptide
chain.
• The tertiary structure is attained by globular
proteins. In such cases the secondary helical
structure is further folded to form a globular,
ellepsoidal shape.
• The folding is established by the appearance of
more disulfide bonds as well as hydrogen
bonds,ionic bonds and hydrophobic bonds. 24

• Myoglobin , Ribonuclease , chymotrypsin ,
cytochrome C , etc.exist in tertiary structure.
• Myoglobin is abundant in muscles .
• It contains a single polypeptide chain of 153
amino acids and a single iron- porphyrin [heme]
group . The tertiary structure of myoglobin has
a sausage-like outline.
• The backbone of the molecule is made up of 8
almost straight helical segments alternated by
nonhelical bends which twist to produce the
characteristic folding.
26

• The longest helical segment has 23 amino acids
and the shortest has 7 amino acids .
• All the helical segments are of alpha type and
right handed , there being no beta structure.
• Nearly 80% of amino acids remain on the helical
regions and the 20% are distributed over the
nonhelical areas.
• Ribonuclease is another globular protein with
tertiary structure.
• It is made up of a single polypeptide chain
containing 124 amino acids with 4 disulfide
bonds. 27

4.Quanternary structure
• Two or more polypeptide chains associate
together to produce a quaternary structure .
• It is exhibited by proteins containing more than
one polypeptide chains.
• Quaternary structure is formed by the
combination of primary , secondary and tertiary
structures.
• The quaternary structure presents itself as an
aggregation of two or more polypeptide chains,
with a tertiary structure organized into a single
functional protein molecule. 28

• A protein with a quaternary structure is
referred to as an oligomer and its polypeptide
chain with tertiary structure are referred to as
protomers .
• The protein insulin is made up of two
polypeptide
• Haemoglobin has 4 polypeptide chains chains..
• Apoferritin has 24 identical chains.
• Aspartate transcarbamylase has 72 chains.
• The quaternary structure is of two types
depending on the nature of polypeptide chains
29

1.Homogeneousquaternarystructure
2.Heterogeneous quaternary structure.
1.Homogeneousquaternarystructure
In this quaternary structure the polypeptide
chains are identical.
Eg. phosphorylase, lactic acid dehydrogenase ,
Isozymes H4 and M4,etc.
2.Heterogeneous quaternary structure.
In this quaternary structure the polypeptide
chains are non identical.
Eg . Haemoglobin , keratin ,etc.
30

• The quaternary structure maintained by
globular and fibrous protein.
• For example , haemoglobin is a globular
protein and keratin is a fibrous protein and
both them exhibit quaternary structure.
• The quaternary structure is maintained by
hydrogen bonds ,disulfide bonds , hydrophobic
bonds and ionic bonds , the same bonds
involved in the formation of tertiary structure
of proteins.
• Haemoglobin contains four independent
polypeptide chains , two identical alpha chains
31

• And two identical beta chains.
• Each chain is characterized by its own
secondary and tertiary structure.
• Each chain has a heme prosthetic group. The
alpha chain has 141 amino acid residues and
the beta chain has 146 amino acid residues.
The protein has thus 574 amino acid residues.
• [141x2+146x2=574]
• The alpha and beta chains have several
segments of beta helix , separated by bends.
32

33
• The alpha and beta chains are held together
as a pair by ionic and hydrogen bonds.
• The two pairs of chains are joined to each
other by additional ionic bonds , hydrogen
bonds and hydrophobic bonds.
• Thus the 4 polypeptide chains fit together to
produce the quaternary structure .
• The hemes are 2.5 nm apart from each other
and are tilted at different angles.

BIC 101- LEC 15-STRUCTURE OF PROTEINS-PRIMARY,SECONDARY,TERTIARY AND QUATERNARY STRUCTURE.ppt

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