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the
molecule together. Hydrophobic groups tend to cluster together in
the center of globular proteins, while hydrophilic groups remain
in contact with water on the periphery. The thiol groups in
cysteine form sulfur-sulfur bonds, which are crucial in the
formation of primary structure. The hydroxyl and amino groups of
amino acids can get attached to oligosaccha-ride side chains which
are a feature of many mammalian proteins. Histidine and the
dicarboxylic amino acids (glutamate and aspartate) are critical
ion-binding proteins, such as the calcium-binding proteins and
iron - binding proteins .
Certain
amino acids achieve their final structure only after their
precursors are incorporated into the polypeptide, e.g.
hydroxyproline and hydroxylysine residues of the collagens, and
methy-lated histidines and lysines of actin and myosin proteins.
Hydroxyproline and hydroxylysine are critical components of the
cross-linking of collagen chains, leading to their rigid and
stable structures. The role of the methylated amino acids in
contractile protein function remains to be elucidated. Eight of
the 20 amino acids found in proteins are nutritionally essential
because the structures of these amino acids cannot be made in the
body of animals, and therefore must be provided in the diet.
The
amino acid sequences of proteins can be determined by fragmenting
them into smaller pieces using specific reagents, and then
establishing the amino acid sequence of each fragment by the Edman
degradation method. By placing the peptide fragments in the
correct order by finding sequence overlaps between fragments
generated by different methods, the amino acid sequence of the
original polypeptide chain can be established.
Homologous
proteins from different species of organisms show sequence
homology, i.e. certain positions in the polypeptide chains contain
the same amino acids, regardless of the species, though in other
positions the amino acids may differ. The invariant residues of
amino acids are evidently essential to the function of the
protein. The degree of similarity between amino acid sequences of
homologous proteins from different organisms correlates with the
evolutionary relationship of the species .
Each
protein has a unique three-dimentional structure, reflecting its
function. The protein structure is stabilized by multiple weak
interactions. Primary protein structure is established by amino
acid sequence and location of disulfide bonds, secondary structure
refers to the spatial relationship of adjacent amino acids, and
tertiary structure is the three-dimentional conformation of an
entire polypeptide chain. The quaternary structure of protein
involves the special relationship of multiple polypeptide chains.
Proteins
are classified as fibrous and globular proteins. Fibrous proteins
primarily have structural roles. The stability of structural
proteins, forming alpha-helix or beta-conformation, is established
by their amino acid content and by their relative placement in the
sequence. In structural proteins such as keratin and collagen, a
single type of secondary structure ( alpha-helix,
beta-conformation, or beta bend) predominates. The polypeptide
chains are supertwisted into ropes and combined in larger bundles
to provide the needed strength. The structure of elastin also per-mits
stretching.
Globular
proteins have more complicated tertiary structures and may contain
several types of secondary structure in the same polypeptide
chain. Globular proteins are compact, with their hydrophobic amino
acids located in the protein interior. Different proteins often
differ in their tertiary structure. Three-dimensional protein
structure can be destroyed by treatments ( e.g., heat) that
disrupt weak interactions, a process called denaturation, which
also destroys protein function. Thus, protein structure and
function have a very close relationship. Certain denatured
proteins can renature spontaneously to produce functional protein
( e.g., ribonuclease ), which shows that the tertiary structure of
a protein is determined by its amino acid sequence.
The
quaternary structure refers to the interaction between the
oligomeric subunits of proteins or large protein assemblies, e.g.
four subunits of hemoglobin exhibit cooperative interac-tions on
oxygen binding, these effects being mediated by subunit
interactions and conformational changes .
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