PROTEINS

Proteins are large, complex molecules composed of 20 commonly occurring amino acids.
Amino acids are the building blocks of proteins and join together by the formation of peptide bonds in a dehydration synthesis reaction.

Amino acids have an amino group and a carboxyl group simplest example is Glycine:

or more complex tryptophan

Peptide bond formation

In the example above, a dipeptide has been formed but there is still an amino and a carboxyl group available to bond with other amino acids or peptide chains => polypeptide.

There is potential for a huge array of combinations as 20 amino acids each with different R groups.

Proteins are complex, organised molecules and they are described as having 4 levels of structure:

Primary structure

This refers to the identity and sequence of amino acids making the polypeptide chain.
Eg: glycine to proline to cysteine to glycine to tryptophon etc.

Secondary structure

Bonds form certain stable angles and polypeptide chains have a limited number of recurrent structural patterns.

• Helix – clockwise helical structure (the telephone cable)

These spirals are often stabilised by hydrogen bonds between adjacent turns.

Tertiary structure

These spirals can then fold. The structure that then forms can be stabilised by further h-bonds are disulphide bridges which is a covalent bond that forms between the sulphur atoms in pairs of cysteine units.

Quaternary structure

This describes the structure of proteins that are made up of more than 1 polypeptide chain. Eg: haemoglobin, which has 4 chains.

The tertiary and quaternary structures are vital for its function. Enzymes are proteins whose shape must be exact in order to fit its particular substrate. If only 1 amino acid were out of place the whole structure of the protein would alter.