Peptide bonds linking amino acids are enzymatically formed by dehydration
synthesis. An oxygen atom is removed from the carboxyl group
of one amino acid together with two hydrogens from the amino group of
a second amino acid (Figure 2-4). This gives peptide chains polarity.
At one end is a free amino group, at the other, a free carboxyl group.
Oligopeptides are chains usually less than ten amino acids in length. The
term polypeptide is used for longer chains of amino acids, whereas
chains over 5,000 daltons are generally called proteins. Some proteins
consist of only a single polypeptide chain. In these cases, a complete
polypeptide chain would be synonymous with a functional protein. In
other instances, however, a functional protein may consist of two or more
Polypeptides may differ by the number and kinds of individual
amino acids they contain. The final structure can be described on four levels
of increasing complexity. The primary structure
of a functional protein consists of the linear sequence
of amino acids in each of its polypeptide chains.
There are two major kinds of secondary protein
structure: α-helix and β-pleated sheet. An α-helix
forms when a carbonyl (C=O) adjacent to one peptide
bond is linked by a hydrogen bond to an amino group
(NH) flanking a peptide bond in an amino acid about
four residues along the same chain. β-pleated sheets form when hydrogen
bonds form between amino acids on adjacent, parallel polypeptide
strands. The polypeptide chain may fold back upon itself, forming weak,
internal bonds (e.g., hydrogen bonds, ionic bonds) as well as stronger covalent
disulfude bonds that stabilize its tertiary structure into a precisely
and often intricately folded pattern. These bonds are formed from
the side chains of different amino acid residues. If two or more polypeptide
chains spontaneously associate, they form a quaternary structure.
|Figure 2-4 Dehydration synthesis of a dipeptide by the formation
of a peptide bond.
Proteins perform many enzymatic, structural, and other roles in living
systems. For example, they are a major structural component of ribosomes,
they may act as hormones that signal between different cell types, or they may assist in the movement of organelles within the cell
and movement of the cell itself.
An average polypeptide contains about 300 residues.
RNA has uracil in place of thymine.