Ø The structures of proteins determine
Their Functions:
·
Levels
of Structure in Proteins:
Biologically
active proteins are polymers consisting of amino acids linked by covalent
peptide bonds. Many different conformations (three-dimensional structures) are
possible for a molecule as large as a protein. Of these many structures, one
or, at most. A few have biological activity; these are called the native conformations.
Many proteins have no obvious regular repeating structure. As a consequence,
these proteins as are frequently described as having large segments of “random
structure” (also referred to as random coil). The term random is really a
misnomer, since the same nonrepeating structures found in the native
conformation of all molecules of a given proteins, and this conformation is
needed for its proper function. Because proteins are complex, they are defined
in terms of four levels of structure.
v Primary Structure:-
Primary structure is the order in
which the amino acids are covalently linked together, The peptide Leo-Glee-Thru-Val-Argo-Asp-His
(recall that the N-terminal amino acids is listed first) has a different
primary structure from the peptide Val-His-Asp-Leo-Argo-Thru, even though both
have the same number and kinds of amino acids. Note that the order of amino acids
can be written on one line. The primary structure is the one-dimensional first
step in specifying the three- dimensional structure of protein.
Two
three- dimensional aspects of a single Polypeptide chain, called the secondary
and tertiary structure, can be considered separately. Secondary structure is
the arrangement in space of the atoms in the peptide Backbone. The a-helix and
B-pleated sheet arrangements are tow different types of secondary structure.
Secondary structures have repetitive interactions resulting from hydrogen
bonding between the amide N-H and the carbonyl groups of the peptide backbone.
In my proteins, the folding of parts of the chain can occur independently of
the folding of other parts. Such independently folded portions of proteins are
referred to as domains or super-secondary structure.
v Tertiary Structure:-
Tertiary
structure includes the three-
dimensional arrangement of all the atoms in the proteins, including those in
the side chains and in any prosthetic
groups (groups of atoms other than amino acids).
A
protein is consisting of multiple polypeptide chains called subunits. The arrangements
of subunits respect to one another are the quaternary structure. Interaction
between subunits is mediated by monovalent interactions, such as hydrogen
bonds, electrostatic attractions, and hydrophobic interactions.
v Quaternary Structure of Proteins:
Each
chain is called a subunit. The number of chains can range from tow to more than
a dozen, and the chains may be identical of different. Commonly occurring
examples are dimmers, timers, and tetramers, consisting of two, three, and four
polypeptide chains, respectively. The generic term for such a molecule, made up
of a small number of subunits, is oligomer.
o
Hemoglobin:
Hemoglobin
is a tetramer, consisting of four polypeptide chains, tow a-chains and tow
B-chains (Figure 4.20). The overall structure of hemoglobin is A2B2 in Greek
letter notation. Both the a-and B-chains of hemoglobin are very similar to the
myoglobin chain. The a-chain is 141 residues long, and the B-chain is 146
residues long; for comparison, the myoglobin chain is 153 residues long. Manu
of the amino acids of the a-chain the B-chain and myoglobin are homologous;
that is, the same amino acid residues are in the same positions. The heme group
is the same in myoglobin and hemoglobin.
o
Myoglobin:
An Example of protein structure:-
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