General properties of proteins

• The monomers from which proteins are made 
• Amino group -NH2
• Acid group -COOH
• 20 different R groups 
• Joined by condensation reactions, forming peptide bonds 
• 2 amino acids in a dipeptide
• 3 amino acids in a tripeptide
• Many amino acids in a polypeptide
• Protein is made from one or more polypeptides
• An infinite variety of proteins 

Primary structure

• Sequence of amino acids (e.g. valine, serine, proline, valine)
• Insulin is a small protein consisting of 51 amino acids (many proteins have hundreds)

Secondary structure

• Shape formed when part of the chain of amino acids becomes folded and coiled 
• The two most common structures are an a-helix (spiral shape) and a B-sheet (pleated like a folded sheet of paper)

Tertiary structure 

• Overall shape of the amino acid chain (i.e. the whole polypeptide)
• When in water, a wide variety of forces combine to twist, fold and bend the polypeptide into its most stable shape

Quaternary structure

• Occurs when the protein has more than one polypeptide chain

• Examples: enzymes, antibodies and hormones
• Functions include: holding your body together, allowing movement, digestions and absorption of food, controlling body chemistry, fighting off disease
• Proteins are large, complex molecules
• Always contain the elements: carbon, hydrogen, oxygen and nitrogen 

Globular proteins (rounded in shape, tend to have a chemical function)

• Enzymes
• Membrane proteins that control the movement of ions and other substances in and out of the cell
• Hormones such as insulin 
• Haemoglobin
• Antibodies to help fight disease

Fibrous proteins (long, thin molecules that generally have a structural function)

• Collagen - gives strength to tissues such as bone, tendon and ligaments
• Keratin - gives strength to skin, hair and nails 
• Actin and myosin - make muscles contract

Shape of globular proteins is maintaines by relatively weak forces such as hydrogen bonds and ionic bonds. Fibrous proteins rely more on strong disulfide bridges.

Biuret Test

• Biochemical test used for detecting the presence of peptide bonds
• First, disolve the test substance in water, then add the biuret reagent (mixture of copper sulfate and sodium hydroxide)
• Change in colour from blue to lilac/purple indicates the presence of protein