Proteins

Proteins are naturally occurring polypeptides. They:

• contribute to the mechanical structure of animals, including humans, e.g. keratin in hair and fingernails, and fibrous proteins such as collagen in tendons
• enable animals to move, e.g. myosin in muscle
• facilitate transport of smaller molecules around animals' bodies, e.g. haemoglobin
• control the types and rates of chemical reactions in living things; then they are called enzymes, e.g. amylase
• are important components of the human immune system, e.g. immunoglobins

The general structure of an amino acid as:

           R
           I
H2N--C--COOH
           I
          H

Amino acids are the building blocks (monomers) of proteins. Twenty different amino acids are used to make the body's proteins. Of these nine are called essential (meaning they can only be obtained from the food we eat) and eleven are non-essential (they may be synthesised in the body though they are usually obtained from food).

Amino acids have the general structural molecular formula -NH₂CHRCOOH. They have two important functional groups (a functional group means a group of atoms in a molecule that have characteristic chemical reactions regardless of the rest of the molecule):

carboxylic acid group	-COOH
amine group	-NH2

Condensation and the formation of peptide bonds linking together amino acids to form polypeptides.

Two amino acids can undergo a condensation reaction to form a dipeptide. Further condensation reactions result in a polypeptide. The amino acid units are linked by peptide bonds (sometimes called peptide links).

The relationship between primary, secondary, tertiary and quaternary structure, and protein function.

The sequence of amino acids in a protein is called its primary structure. Within a chain the atoms are held together by covalent bonds. Each protein has its own characteristic sequence of amino acids.

Three types of bonding can happen within a protein molecule (intramolecular bonding) and between protein molecules (intermolecular bonding):

• Hydrogen bonds
• Covalent bonds
• Ionic bonds

Protein chains arrange themselves to maximise the intra- and intermolecular bonding. The structure when protein chains are held in place is called the secondary structure. This may be:

• helical, e.g. keratin (the protein found in hair), or
• pleated sheet, e.g. fibroin (the protein found in silk)

These structures are held in place by hydrogen bonds.

Protein chains may fold into a globular shape. This is the tertiary structure of a protein. These globular proteins include enzymes and immunoglobins. The structures are held in place by hydrogen bonds, disulfide bridges and ionic bonds.

The precise structure of a globular protein is the key to specificity of enzymes. Similarly proteins that act as receptor sites on the cell surface can recognise specific molecules because of their shapes.

Finally some proteins have a quaternary structure. These contain more than one protein chain. Examples are insulin and haemoglobin.

www.johnkyrk.com/aminoacid.html

The biuret test for proteins

The biuret test is a chemical test used for detecting the presence of peptide bonds. In a positive test, a copper(II) ion is reduced to copper(I), which forms a complex with the nitrogens and carbons of the peptide bonds in an alkaline solution. A violet color indicates the presence of proteins.

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