Protein Structure and Function
The amino acid sequence of proteins determines the three dimensional structure it will have. Protein structure varies in four distinct levels; primary, secondary, tertiary, and quaternary
Primary Structure:
A sequence of amino acids 
Secondary Structure:
The secondary structure diverges into two forms due to the hydrogen bond s between the repeating constituents of the polypeptide backbone. It is either the:
helix
pleated sheet 
Tertiary Structure:
The overall shape of a polypeptide resulting from interactions between the R groups. This 3 dimensional shape is also held together by weak hydrogen bonds but also by much stronger "disulfide" bonds between two amino acids of cystine ("covalent") disulfide "bridges" (linkages)
Quaternary Structure:
The overall protein structure that result from the aggregation of these polypeptide subunits
