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1. LEVELS OF PROTEIN
STRUCTURE

3.  A key concept in understanding how proteins work is that
their functions are derived from three-dimensional structure,
and three-dimensional structure is specified by the amino acid
sequence.
 Every protein has a unique 3-D structure –native conformation.

4.  THE PRIMARY STRUCTURE:
 The primary structure consists of a specific sequence of amino
acids linked together by peptide bonds and includes any
disulfide bonds (covalent bonds).
 Linear polypeptide sequence
 Each protein has a distinctive number, type and sequence of
amino acid residues.
 It is the sequence and not the composition that determines
the primary structure.
 Such sequence variation is the most imp element of protein
diversity.
 Possible sequence= 20n , where n= no.of aa residues.

5. INSULIN

7. SECONDARY STRUCTURE
 Hydrogen bonding between amino groups and carboxyl groups in
neighboring regions of the protein chain sometimes causes certain
patterns of folding to occur. As a result, the polypeptide chain is
twisted or bent and is stabilized .
 These stable folding patterns make up the secondary structure of a
protein.
 Also, it is important to note that the hydrogen bonds between the
binding atoms are weak, but the summation of all the hydrogen
bonds allows the structure to maintain its shape.

9. Figure: Myoglobin

10.  The α-helix is a right-handed helical coil that is held
together by hydrogen bonding between every fourth
amino acid .
 i.e. In the α-helix chain, the hydrogen bond forms
between the oxygen atom in the polypeptide
backbone carbonyl group (C=O) in one amino acid
(n) and the hydrogen atom in the polypeptide
backbone amino group (N-H) of another amino acid
that is four amino acids farther along the chain
(n+4).
ALPHA HELIX:

11.  Proline -“helix-breaker”, disrupts the structure
because of the presence of imino group, which
isn’t compatible with helix formation.
 R-chain points away from the helical axis and
are free to interact.

13. BETA-PLEATED SHEET
 In this structure, two different regions of a polypeptide
chain lie side by side and are bound by hydrogen bonds.
 Types-parallel beta-pleated sheets and antiparallel
beta-pleated sheet.
 If two beta-strands run in the same direction, then it is
a parallel beta-pleated sheet, and
 if they run in opposing directions, then it is an
antiparallel beta-pleated sheet

14.  The R groups of the amino acids in a β-pleated sheet point out
perpendicular to the hydrogen bonds holding the β-pleated
sheets together, and are not involved in maintaining the β-
pleated sheet structure
 Another less commonly known secondary structure is the beta-
barrel. This structure is composed of antiparallel beta-strands,
and it is twisted and coiled into a barrel so that the first strand
hydrogen bonds to the last strand.
 One common example is aquaporins, which selectively allow
water molecules in and out of a cell while preventing the
passage of other solutes and ions.

16. Figure: Silk fibroin

17. SUPER-SECONDARY STRUCTURE
 Intermediate form between secondary and
tertiary structure
 Alpha helices and beta sheets are connected
with the help of a loop.
 They are known as motifs.
 Motifs don’t retain their functions when they
are separated from their larger protein part as
they lose their structures.