Proteins are not just polypeptides,
they are polypeptides of defined
- 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
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
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
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.