Amino Acids and Proteins

· Proteins are macromolecules (polymers)

· They are made up of monomers which are known as amino acids

structure of amino acids

1. The simplest amino acids has R = H as its side chain. à Glycine

2. There are 20 different amino acids involved in protein synthesis

3. Each amino acid has a different side chain;

a. If it is oxygen or nitrogenà polarity, hydrophilic

b. If it is hydrogen and carbon à hydrophobic

Peptide Bonds

· All proteins contain NITROGEN and some contain SULPHUR because amino acids do. This is significant to how proteins bond.

· Proteins contain peptide bonds

· These join amino acids together

· Many amino acids joined by peptide bonds form a macromolecule which is a polypeptide.

· If 100+ amino aids present = protein.

Condensation hydrolysis

Protein Structure

Primary Structure	Sequence of amino acids Sequence determined by genetic code in genes (DNA)
Secondary Structure	Chain of amino acids that coilà helix or pleated sheet Hydrogen bonds hold in structure on place (configuration)
Tertiary Structure	Secondary structure coils/ folds into complex 3D shape (v. precise) Held together by bonds between side chains
Quaternary Structure	Proteins of greater than one polypeptide have quaternary structures either GLOBULAR or FIBROUS

If primary structure is wrong the tertiary structure is wrong and hence the protein will not function correctly

Protein shape determines its function

4 Types of bonds hold protein structures together

1. HYDROGEN BONDS · Occur between polar groups. (any 2 polar amino acids) · Broken by pH and high temperature

 

2. DISULPHIDE BRDGES · Occur between cysteine molecules (type of amino acid) · S=S ; covalent bond; very strong · Broken by reducing agents

3. IONIC BONDS · Occur between ionized amines and carboxylic acid groups · Electrons from one atom leave its outer shell and enter the outer shell of another atom. · Gainer –> reduced –> -ve ion · Loser –>oxidised –> +ve ion · Form weak attractions between bonds (weaker than covalent bonds)

4. HYDROPHOBIC INTERACTIONS · Helps some proteins as water hating ‘hydrophobic’ groups point inwards, therefore hold structure together · Occurs between non-polar side chains

Protein shape related to function

Globular – e.g. Haemoglobin

· 4 polypeptide chains with disulphide bridges

· nearly spherical

· at the centre of each polypeptide à Fe containing haem group. This is a non-protein prosthetic group which combines with protein à conjugated protein

· present in red blood cells, carries oxygen from lungs to respiring cells as oxyhaemoglobin

Globular – e.g. Enzymes

· enzymes are a type of a protein- each enzyme has a specific shape, with an active site that locks onto the substrate molecule.

· amino acids with hydrophilic side chains towards the outside

· hydrophobic inwardly facing

· therefore, water molecules can surround the molecule hence it is soluble.

Globular – e.g. Hormones (Insulin)

· is a hormone that reduces blood glucose levels.

· Globular = good for transport

· Disulphide bonds hold the molecule in shape

· It’s a small molecule – so it’s easily transported and absorbed by cells

Fibrous – e.g. Collagen/Keratin/Elastin

· long strands

· insoluble

· chains are interlinked by strong covalent bonds

· polypeptides laid down in sheets

· collagen/Elastin –> SKIN

· Keratin –> nails and hair

· Collagen/elastin –> skin

· Actin/myosin –> muscle

Biuret Test

· Detects presence of proteins

· Amine groups in side chains of some amino acids of a protein are at the site of ionized peptide bonds react with copper ions to form a complex with a violet colour

· Biuret reagent 2 chemicals used in succession

1. dilute alkali (KOH, NaOH)

2. dilute Cu₂SO_4(aq)

· violet purple = +ve test