Page 52 - Cell biology PDC 2024

• Quaternary (4°) Protein Structure

Twisting around various bonds in the polypeptide backbone gives
proteins a variety of shapes. Bond angles give rise to secondary
structures. Then, the secondary structures help drive the peptide folding
that gives rise to tertiary structure.

1) The primary structure: it is the linear sequence of amino acids.
2) The secondary structure: Pauling and Corey proposed two secondary

structures in proteins many years before they were actually proven:
alpha – helix and beta – sheet
Both of these secondary protein structures are stabilized by hydrogen
bonding between the carbonyl oxygen atoms and the nitrogen atoms
of amino acids in the protein chain
In α helix, the carbonyl (C=O) of one amino acid is hydrogen bonded
to the amino H (N-H) of an amino acid that is four down the chain.
(E.g., the carbonyl of amino acid 1 would form a hydrogen bond to the
N-H of amino acid 5) resulting in a helix structure like a ribbon. Note
that the R groups of the amino acids stick outward from the α helix,
they don't interact.
In the β pleated sheet, two or more polypeptide chains stand up next
to each other, forming a sheet-like structure held together by hydrogen
bonds. The hydrogen bonds form between carbonyl and amino groups
of backbone, while the R groups extend above and below the plane of
the sheet (they don’t interact). The strands of a β pleated sheet may be
parallel or anti parallel.
3) Tertiary (3°) Structure of Protein

It is the three-dimensional structure of a polypeptide that is due to
interactions between the R groups of the amino acids that make up

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