Protein 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 develops multiple levels of structure, which contributes 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 on 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 structures are hemoglobin and the sodium channels in the cell
membranes.