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Figure 2–30. Spiral configuration of the polypeptid chain (After Pauling, 1960).
Structurally, proteins are globular and spiral. Spiral proteins
can be built from one or several polypeptide chains. By X-ray analyses
the structural organization of a great number of protein molecules,
enzymes including has been elucidated — hemoglobin, myoglobin,
myosin, trypsin, ribonuclease, etc. Of special interest are the spiral
configurations of the polypeptide chains and the α-spiral of Pauling
(1960) in particular (Fig. 2–30), having triggered ideas on disclosure
of the spatial structure of DNA. All atoms building the molecule
skeleton have independent orientation. The structure resembles
a spiral staircase in which the “steps” are amino acid residues.
Each residue occupies 1.5 nm of the spiral axis. One coil is equal to
5.3 nm. It is stabilized by hydrogen bonds between the carboxyl group
of one residue with the amino group of the other.
Of the twenty amino acids (see Fig. 2–29) comprising the
various proteins all but one (proline) have a uniform structure
group (—COOH) are bound to the α-carbon atom.
Protein molecules are built from amino acids in a linear sequence
bound to one another by peptide bonds. Peptide bonds result from the
interaction of the α-amino group (—NH₂) of the amino acid with the α-carboxyl
group (—COOH) of the other thus releasing H₂O. The binding of amino
acids leads to the formation of polypeptide chains (Fig. 2–31).