Levels of protein structure

When cells make a polypeptide, the chain folds spontaneously (i.e. associated with an increase in entropy) to assume the functional conformation of that protein. 4 superimposed levels of structure

Primary structure

- Order of amino acids along a chain
- Primary structure of protein is determined by the sequence of codons in DNA.
- Determines other structural levels
- Single changes in amino acid sequence may have profound impact on protein function(e.g.Sickle-cell anemia)
- Insulin was first protein to be sequenced by Sanger in the late 1940's
- Axiom: "each polypeptide has a specific primary sequence"

Secondary structure
local coils and folds resulting from H-bonds at regular intervals of polypeptide backbone (i.e. R-group not involved in H-bonding).
alpha helix: coil held together by H-bonding between every 4 a.a.

pleated sheet: chain folds back in parallel or antiparallel orientation, and H-bonds between parallel regions hold structure together.