Proteins

 Proteins: 

• Primary structure - The amino acid sequence
• Secondary structure - Folding of the polypeptide chain, held by hydrogen bonds, (alpha helix and beta sheet)
• Tertiary structure - Further folding of the secondary structure, held by hydrogen bonds, ionic bonds, and possibly disulphide bonds.
• Quaternary structure - Two or more polypeptide chains join together, 

Chains of amino acids are formed by condemnation reactions, producing peptide bonds, and releasing a water molecule. 

Globular proteins: 

Most proteins are globular, meaning they have compact ball-shaped structures. These include enzymes, membrane proteins, receptors and storage proteins.

• They have complex tertiary and sometime quaternary structures,
• They are folded into spherical (globular) shapes,
• They are usually soluble as the hydrophobic chains are in the centre of the structure,
• They play roles in metabolic retains,
• For example, enzymes and haemoglobin in the blood, 

Fibrous (or filamentous) proteins:

Fibrous proteins are long and thin, they tend to have more structural roles, such as collagen (bones), Keratin (hair), tubulin (cytoskeleton) and actin (muscle). They are always composed of many peptide chains.

• Little or no tertiary structure,
• long parallel polypeptide chains, 
• cross linkages at intervals forming long fibres or sheets,
• usually insoluble
• many have structural roles,
• for example, hair and outer layer of skin, collagen (a connective tissue)