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Lecture 4
Protein Structure-II
Every protein has at least three levels of structural organization. Some of them may have
a fourth level giving rise to what is known as the quaternary structure of proteins where
monomeric subunits interact to form a multimeric protein. The most common example
that can be thought of is hemoglobin which is a tetrameric protein having two α-subunits
and two β-subunits. These assemble to form the tetrameric structure of hemoglobin.
Structure of human hemoglobin: The α and β subunits are in red and blue, and the ironcontaining heme groups in green. So hemoglobin is actually a dimer of dimers and can be
referred to as a heteromeric protein.
If the subunits are identical then it is a homomeric protein.
Fibrous
Globular
Membrane
Fibrous proteins:
Fibrous proteins are formed from long polypeptide chains that are arranged parallel or
nearly parallel to one another. Fibrous polypeptide chains form long strands or sheets and
because of many hydrophobic amino acid residues, they are water insoluble but strong
and flexible. These long fibers or sheets result in physically tough materials that form the
basis of structural proteins in the body. Examples of components that are comprised of
such proteins are muscle, hair, nails etc.
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Fibrous proteins contain polypeptide chains organized parallel along a single
axis, producing long fibers or large sheets.
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They are mechanically strong, play structural roles in nature;
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Difficult to dissolve in water;
Keratins and Collagen are examples of fibrous proteins
α-keratins are found in hair, fingernails, claws, horns and beaks;
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Sequence consists of long alpha helical rod segments capped with non-helical Nand C-termini
β-keratins are found in silk and consist of gly-ala repeat sequences;
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Ala is small and can be packed within the sheets
Globular proteins:
The polypeptide chain in this case folds into a compact structure close to a spherical
shape. Most of these proteins are water soluble and because of this feature are mobile in
the cell. They have diverse functions and act as enzymes and several regulatory proteins.
Since globular proteins are compact mobile proteins they are also the major proteins that
can function as transport proteins.
Globular proteins are classified according to the type and arrangement of secondary
structure
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Antiparallel alpha helix proteins
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Parallel or mixed beta sheet proteins
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Antiparallel beta sheet proteins
Membrane proteins:
These proteins are embedded in the lipid bilayer of the cell membrane and act as ion
channels for the transport of molecules and ions in and out of the cell. Since the proteins
interact with the non polar lipid bilayers, they have hydrophobic amino acid residues on
the surface. These proteins do not have stable structures in aqueous solution.
Have you ever wondered what would happen if fibrous proteins were water soluble?
Some protein functions are listed below:
Transport (hemoglobin)
Transmembrane (Na+/K+ ATPases)
Hormones (insulin)
Physico-Chemical Properties of Proteins
: reflects amino acid composition and levels of organization
1) AMPHOTERIC NATURE - ion transportation
2) BUFFERING ABILITY
- involves the imidazole group of HIS (pK = 6.1)
3) SOLUBILITY
- characteristic for each protein
- at the isoelectric point (pI), we have zero charge
- at pH on acid side of pI, then the net charge is (+)
- at pH on basic side of pI, then the net charge is (-)
4) SHAPE
- very important
- eg. enzyme recognition of a particular substrate