3.1.4 Proteins

What are proteins made of

• Proteins are polymers made up of the monomers amino acids

• Protein polymer chains (peptides) are created on ribosomes then further folded and modified in the golgi apparatus

General structure of amino acids

• Central carbon

• Amine group (NH2)

• hydrogen atom

• carboxyl group (COOH)

• variable group (R group)

• Each amino acids has a different R group

Glycine

Simplest amino acid

2 sulfur-containing amino acids

• cysteine

• methionine

Types of proteins

• Basic - more bases than acids

• Acidic - more acids than bases

• Neutral - same number of acids and bases

How amino acids bond to form a protein

• Peptide bond formed between the carbon and nitrogen

• One condensation reaction

Dipeptides

= two amino acids bonded together formed by the condensation of two amino acids

Polypeptides

= formed by the condensation of many amino acids

Primary structure

• amino acids added to dipeptide forming polypeptide chain via condensation reactions

• proteins consist of one of more polypeptide chains folded in a specific 3D shape

• the sequence of amino acids is determined by DNA

• the order of amino acids will alter where bonds occur and how the protein folds

• primary structure determines the final 3D shape and protein’s function

Secondary structure

• the sequence of amino acids cause parts of a protein molecule to coil into an alpha helix or fold into beta pleated sheets

• hydrogen bonds form between the carboxyl groups of one amino acid and the amine group of another, these hold the secondary structure in place

Tertiary structure

• secondary structure folded into a precise, unique, globular 3D shape, held in place by hydrogen bonds, ionic bonds and disulfide bonds (which only occur between R-groups on sulfur-containing amino acids)

• the type of elements in the chain depend on the type of bonds that form to stabilise them. E.g. lots of hyrdrogen and oxygen atoms → hydrogen bonds, disulphate bridges form between sulfur atoms, oppositely charged groups form ionic bonds

Describing the tertiary structure of a protein:

• The further folding of the secondary structure

• To create a unique 3D structure

• Held in place by hydrogen, ionic and disulfide bonds

Quaternary structure

• a protein made up of more than one polypeptide chain

• hydrophobic interaction and same bonds as tertiary structure (hydrogen, ionic, disulfate) holds the quaternary structure together

• e.g. haemoglobin, has 4 polypeptide chains

Globular proteins

• regular, repetitive sequence of amino acids

• compact, spherical shape

• relatively unstable structure

• soluble

• plays an important role in metabolism e.g. enzymes, hormones, haemoglobin

Fiborous proteins

• long parallel strands

• stable structure

• cross linked chains as they provide additional strength

• insoluble

• for support and structural function

• e.g. keratin and collagen

Biuret test for proteins

1. Add biuret reagent to sample

2. It will turn from blue to purple if proteins are present

• For this test to work, there must be at least two peptide bonds present in any protein molecule, so if the sample contains only amino acids or dipeptides, the result will be negative