The general structure of an amino acid. Condensation and the formation of peptide bonds linking together amino acids to form polypeptides. The relationship between primary, secondary, tertiary and quaternary structure, and protein function.

There are 20 different types of amino acid. In a polypeptide chain there are an average of 400 amino acids, the order of these will dictate which kind of protein it is, and therefore what its function is.

General structure of an amino acid
NH2 on the left is the amine group.

COOH on the right is carboxylic acid.

R represents the R group of the amino acid, the R group is what differs between amino acids. They can contain H, C, N and O but they can also contain Sulphur.

General structure of an amino acid

Primary structure of a protein
Amino acids join together by a condensation reaction: this produces water and a peptide bond (red line.) The oxygen and hydrogen of one amino acid, bond to the hydrogen of another forming H2O (water.) This leaves the carbon of one and nitrogen of the other free to make another bond; a peptide bond to each other.
When this is done to around 400 amino acids consecutively a polypeptide chain is formed (this is the primary structure of a protein.)
O=CNH is a peptide linkage; where as CN is the peptide bond.

Products of a condensation reaction between two amino acids

Secondary structure of a protein
Polypeptide chains bond together to form either alpha helices or beta pleated sheets.
In these structures every four amino acids has a hydrogen bond, this is a weak bond between the positive H (of the NH) and the negative O (of the CO.)

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Tertiary structure of a protein
The secondary structure is twisted and folded into an even more complex structure.
This can be held together in a variety of ways:

• Disulphide bridges can form if there are amino acids with sulphur in their R group
• Ionic bonds between carboxylic and amine groups that are not in a peptide bond
• Hydrogen bonds as in the secondary structure (but not regular)

Quaternary structure of a protein
This only happens sometimes, unlike the other stages which all have to be gone through to make a protein.
Several polypeptide chains at the tertiary level may join together to form a new protein.
An example of this is when four polypeptide chains bond around an iron making hemaglobin.