Proteins

Proteins are made by the joining of amino acids by a peptide bond

Amino acids are made up of:

• a carboxyl group at one end (COOH)
• a amino group at the other end (NH2)
• A Hydrogen atom
• An R group which is alwways different

These groups surround a central Carbon atom.

The peptide bond form through a condensation reaction where by water is eliminated. It forms between the carbon of one amino acid and the Nitrogen of another.

Primary structure                                                                                                                 Primary structure is the unique sequence of amino acids that make up a polypeptide chain.           It is determind by the base sequence on the strand of the DNA molecule

• The order of amino acids in a polypeptide chain
• There can be upto 20 different types of amino acids
• The amino acids are joined together in any order and number
• There are a huge number of possible polypeptide chains

Secondary structure

• The secondary structure is the way in which the primary structure is folded or twisted to form a 3D shape
• The shape of the polypeptide chain forms as a result of Hydrogen bonding, it occurs between the =O on the -CO groups
• Hydrogen bond cause the polypeptide chan to twist into a 3D shape
• Common shapes include the alpha helix and the less common beta pleated sheet

Tertiary structure

This is the 3D structure of a protein resulting from the shaping of the secondary structure. The alpha helix or beta pleated sheet can be folded into a more complex and compact 3D shape.

The shape is maintained by different types of bonds including:

• Disulphide bridge- this occurs between two cystein amino acids, a strong double (S=S) bond is formed between the sulfur atoms
• Hydrogen bonds
• Ionic bonds forming between two oppositely chargred R groups 
• Hydrophobic and Hyrophilic interactions

Breaking the tertiary structue with heat causes the protein to be denatured. The molecules start to vibrate as the kinetic energy increases so that the bonds break.

Quaternary Structure

Not all proteins are functional on their own, as a result they combine with other polypeptide chains.

The 3D structures result in globular and firbrous proteins

Globular Proteins

• Globular proteins tend to form ball-like structures
• The hydrophobic parts face towards the centre
• The hydrohilic parts face toward the edges
• They are water soluble

Role                                                                                                                                They have metabolic roles such as enzymes and antibodies in mammals

Example of a globilar protein is haemoglobin

• Haemoglobins function is to carry oxygen in the blood
• Haemoglobin is a water soluble protein composed of 4 folded polypeptide chains
• at the centre is an iron-containing group, haem

Fibrous proteins

• Fribrous proteins are long fibres of repeated sequences of amino acids
• They are insoluble in water

Types of fibrous proteins

• They have structural roles such as collagen, cartalige and keratin
• Collagen is found in bone and tendons it provides the necessary strength and roughness
• Structure of collagen is formed from a single fibre which is made up of 3 of the same polypeptide chains, which twist around eachother like a rope. The 3 chains are joined by hydrogen bonds which gives it structural stability
• Keratin makes up hair and fingernails