Proteins: Polypeptides of
Defined Sequence
WHAT ARE PROTEINS?
Proteins are not just
polypeptides,
they are polypeptides of defined
sequence.
- complex natural compound: a complex
natural substance that has a globular or
fibrous structure composed of linked amino
acids.
Proteins are essential to the structure and
function of all living cells and viruses.
Meaning, every protein has a
defined order of amino acid residues.
As with the nucleic acids, this
sequence is referred to as the primary
structure of the protein.
When a living cell makes protein, the
carboxyl group of one amino acid is linked to
the amino group of another to form a peptide
bond. The carboxyl group of the second
amino acid is similarly linked to the amino
group of a third, and so on, until a long chain
is produced.
This chainlike molecule, which
may contain from 50 to several
hundred amino acid subunits,
is called a polypeptide. A
protein may be formed of a
single polypeptide chain, or it
may consist of several such
chains held together by weak
molecular bonds.
Each protein is formed according to a precise
set of instructions contained within the
nucleic, which is the genetic material of the
cell. These instructions determine which of the
20 standard amino acids are to be
incorporated into the protein, and in what
sequence.
The R groups of the amino acid
subunits determine the final shape of
the protein and its chemical
properties; an extraordinary variety
of proteins can be produced from the
same 20 subunits.
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