Quaternary structure refers to the arrangement of multiple protein subunits into a single protein complex. Hemoglobin is a common example that is made of two alpha and two beta subunits. The subunits interact through hydrophobic interactions, hydrogen bonding, and other bonds. Globular proteins tend to have quaternary structure that clusters the subunits into a spherical shape, while fibrous proteins form long coils or sheets through interactions between subunits. Quaternary structure allows proteins to take on specialized functions beyond what individual subunits could achieve alone.

1. Quaternary
Structure of
Protein
Fathima P.E.

2. Protiens:
Proteins were first described by the Dutch
chemist Gerhardus Johannes Mulder and
named by the Swedish chemist Jöns Jakob
Berzelius in 1838. Early nutritional scientists such as
the German Carl von Voit believed that protein
was the most important nutrient for maintaining
the structure of the body, because it was
generally believed that "flesh makes flesh."

3.  The amino acids in a polypeptide chain are linked by
peptide bonds.
 Once linked in the protein chain, an individual amino
acid is called a residue, and the linked series of carbon,
nitrogen, and oxygen atoms are known as the main
chain or protein backbone.
 The peptide bond has two resonance forms that
contribute some double-bond character and inhibit
rotation around its axis, so that the alpha carbons are
roughly coplanar.
 The other two dihedral angles in the peptide bond
determine the local shape assumed by the protein
backbone.
 The end of the protein with a free carboxyl group is
known as the C-terminus or carboxy terminus, whereas
the end with a free amino group is known as the N-
terminus or amino terminus.

4. Levels of Proteins:
Four levels of structure are frequently cited in
discussions of protein architecture.
 Primary Structure
 Secondary Structure
 Tertiary Structure
 Quaternary Structure

5. Quaternary Protein - Structure
 The quaternary protein structure involves the
clustering of several individual peptide or protein
chains into a final specific shape.
 A variety of bonding interactions including
hydrogen bonding, salt bridges, and disulfide bonds
hold the various chains into a particular geometry.
 There are two major categories of proteins with
quaternary structure - fibrous and globular.

6. Fibrous Proteins:
 Actually, the final beta-pleated sheet structure of
silk is the result of the interaction of many individual
protein chains.
 Specifically, hydrogen bonding on amide groups
on different chains is the basis of beta-pleated
sheet in silk proteins.
SILK

7.  Other fibrous proteins such as the keratins in wool
and hair are composed of coiled alpha helical
protein chains with other various coils analogous to
those found in a rope.
 Other keratins are found in skin, fur, hair, wool,
claws, nails, hooves, horns, scales, beaks, feathers,
actin and mysin in muscle tissues and fibrinogen
needed for blood clots.

8. Globular Proteins:
 On the other hand, globular proteins may
have a combination of the above types
of structures and are mostly clumped into
a shape of a ball.
 Major examples include insulin,
haemoglobin, and most enzymes.
INSULIN

9.  The most common example used to illustrate
quaternary structure is the hemoglobin protein.
 Hemoglobin's quaternary structure is the package
of its monomeric subunits.
 Hemoglobin is composed of four monomers. There
are two α-chains, each with 141 amino acids, and
two β-chains, each with 146 amino acids.
 Because there are two different subunits,
hemoglobin exhibits heteroquaternary structure. If
all of the monomers in a protein are identical, there
is homoquaternary structure

10.  Quaternary structure refers to the association of
multiple individual protein chains into a single
protein with multiple subunits. The arrangement of
the subunits gives rise to a stable structure, which
can usually be dissociated in the laboratory, but is
very strongly bound in vivo.
 The subunits may be identical or different
 When they are different, each subunit tends to
have a different function
STRUCTURAL DISCRIPTION

11.  A common shorthand for describing such proteins is
to use Greek letters for each type of subunit, and
subscript numeral to specify numbers of units.
 A protein designated a2bg consists of two a units
and one each of b and g.
 The subunits usually are held together by
hydrophobic interactions, the clustering serving to
reduce exposure of hydrophobic side chains to the
solvent.
 Occasionally, ionic interactions between
carboxylate and amino side chains may contribute.

12. Biological Function:
 Enzymes are generally globular proteins and range from
just 62 amino acid residues in size, for the monomer of 4-
oxalocrotonate tautomerase, to over 2,500 residues in
the animal fatty acid synthase.
 A small number of RNA-based biological catalysts exist,
with the most common being the ribosome; these are
referred to as either RNA-enzymes or ribozymes.
 The activities of enzymes are determined by their three-
dimensional structure.
 However, although structure does determine function,
predicting a novel enzyme's activity just from its structure
is a very difficult problem that has not yet been solved.

13.  Several proteins are actually assemblies of more
than one polypeptide chain, which in the context of the
larger assemblage are known as protein subunits.
 In addition to the tertiary structure of the subunits,
multiple-subunit proteins possess a quaternary structure,
which is the arrangement into which the subunits
assemble.
 Enzymes composed of subunits with diverse functions are
sometimes called holoenzymes, in which some parts may
be known as regulatory subunits and the functional core
is known as the catalytic subunit.
 Examples of proteins with quaternary structure include
haemoglobin, DNA polymerase, and ion channels. Other
assemblies referred to instead as multiprotein complexes
also possess quaternary structure. Examples
include nucleosomes and microtubules.

14. Classification of enzymes:
 Oxidoreductases: catalyze oxidation/reduction
reactions
 Transferases: transfer a functional group (e.g. a methyl or
phosphate group)
 Hydrolases: catalyze the hydrolysis of various bonds
 Lyases: cleave various bonds by means other than
hydrolysis and oxidation
 Isomerases: catalyze isomerization changes within a
single molecule
 Ligases: join two molecules with covalent bonds.

15. Protein structure determination
• X-Ray crystallography
• NMR spectroscopy
• Mass spectroscopy

16. Types of protiens:
 Conjugated protein
 Lipoproteins
 Glycoproteins
 Phosphoproteins
 Metalloprotein etc.

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