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Proteins are made from long chains of amino acids.
Amino acids all have a similar structure, with carboxyl and
amino functional groups. What makes each amino acid unique
is the presence of a variable, or R group.
The R group gives each amino acid its chemical properties.
Amino acids are linked together by peptide bonds, forming long
polypeptide chains. As these chains form,
the polypeptide developments multiple levels of structure, which contribute to
its overall shape. These levels are called the primary,
secondary, tertiary, and quaternary structures.
The primary structure is the order of amino acids in the
polypeptide chain. Amino acids can be likened to letters in the alphabet.
Just as thousands of words can be constructed by varying
the 26 letters in our alphabet, so too can different proteins
be created by varying the number and sequence of just 20 amino
acids.
The secondary structure refers to the pleated sheet or helix
that a protein chain can form due to hydrogen bonding and other chemical
attractions between the R groups of nearby amino acids.
The overall shape of a polypeptide is called its tertiary
structure. This level of structure is created when the secondary
structure folds and twists upon itself. The tertiary
structure is held in place by a variety of bonds and interactions
that form between the R groups of the amino acids in the chain.
Once polypeptides have formed a tertiary structure,
they are commonly referred to as proteins.
In some cases, proteins can form quaternary structures,
which consist of interactions between multiple proteins.
These interactions often result in the formation of large
protein complexes. Examples of proteins that have quaternary
structure, are hemoglobin, and the sodium channels in the cell
membranes.