Protein structure is very complex because the different amino acids cause coiling and folding in different ways.  We should think about protein structure in two ways: 

1. Primary structure: simple two-dimensional chains of amino acids.

2. Secondary structure: chains that are coiled.

3. Tertiary structure: coiled chains that are folded. 

One way to get the idea is to cut a sheet of paper in half, lengthwise.  Mark a couple of colored spots on it, to represent "hot spots" or active regions of the protein. 

Depending on how the paper folds, the hot spots are in different positions. Looking at the picture on the left, a drug or hormone that needs to bind to two hot spots has to have a compatible shape that can bind. But that same drug could not bind if the protein were folded as on the right.

When clusters of different proteins clump together as sub-units of a much larger glob, this creates was is called quaternary structure.

For a more complete illustration of the four levels of structure, click here. (This website contains some good illustrations of the levels of protein structure.)

Some parts of a protein may be strongly bonded because of atomic bridges that link two regions. Sulphur-sulphur bonds commonly create such bridges in proteins. (These are the same bridges that get broken and reformed during a hair perm. They're what causes part of the hair perm smell too.)

Certain toxins damage cells because they impose extra bridges that do not normally occur (see our discussion of the hazards of hexane).

Primary Structure
  (each letter stands for a different amino acid) 

Secondary Structure (structure is now coiled, similar to a spring

Tertiary Structure

 (folds back and forth on itself, similar to a piece of string that has been twisted too much.)

Quaternary Structure

(several protein units form a big glob)