PROTEINS

• C, H, O, N, (S)

• Polymers made from chains of amino acids

• 20 amino acids used

• Linked by a peptide bond

In addition fibrous proteins (collagen) form structural components in cells and tissues

Amino Acids

• Central carbon has attached:– Amine group– Acid group– Functional group (R) – determines nature of amino acid

• R groups fall into 4 categories• Non-polar - only carbons; chains or aromatic rings

(methionine has sulphur)• Uncharged Polar- carbons with amine groups (NH2) or

- hydroxyl groups (OH)• Acidic - carboxylic acid groups (COOH) ionizes to negative

charge COO-

• Basic - terminal amine groups (not next to C=O) ionizes to NH3

+

Peptide bond

• Amino acids joined by a peptide bond

• Condensation reaction between – COOH of 1st amino acid and NH2 of 2nd amino

acid•Chains are called peptides (short)/ polypeptides (longer)•Peptide bond is rigid•Bonds either side can rotate

–Introduces flexibility allowing proteins to take up variety of shapes.

Protein Structure

• 4 levels– Primary– Secondary– Tertiary– Quaternary

PRIMARY STRUCTURE

• Order in which amino acids are linked together– Written starting at the N (amino) terminus– e.g.

– Arg-Lys-Phe-Glu-Ser-Gly-

– R K F E S G

N C

N terminus

C terminus

SECONDARY STRUCTURE

• Two possible shapes in the protein chain each stabilised by Hydrogen bonds: -pleated sheet -helix

-pleated sheet

• Peptide chains arranged side by side• Held together by H-bonds between the two chains• Parallel (chains running same direction)

• N C

• N C

• Antiparallel (chains in opposite directions)• N C

• C N

-pleated sheet

• Silk– Resistant to stretch (very strong)

-helix

• Peptide chain coils into a helix– Held by H-bond between N-H group and the

C=O 4 residue away

• 4 residues per turn

-helix• Hair/Wool (keratin)

– Stretchy (er) -helices coiled together to form a superhelix

– For horn/hoof more disulphide bridges are present (covalent)

Tertiary Structure

• The overall folded shape of a protein held together by (usually) weak forces.– Hydrogen bonding which doesn’t form secondary

structure– Hydrophobic interactions

• Place non-polar amino acids inside protein• Polar amino acids on surface

– Van der Waals forces– Ionic interactions (strong) – Disulphide bridges (strong)

• Covalent bond between cysteine residuesMyoglobin

• With reference to bonding, explain why enzyme activity decreases as you increase the temperature above the optimum, and as you move pH away from the optimum.

Proteins fold to take up their shape

Shape is determined by primary structure –

order of hydrophobic/ hydrophilic amino acids & relative positions of polar/charged amino acids.

Loss of tertiary structure is called denaturation.

Lysozyme

Proteins are 3D

Primary structure determines tertiary structure

Mutation acid (polar) for non polar changes folding pattern

Prosthetic groups• Some proteins have permanently bound non

protein groups, called prosthetic groups– e.g. myoglobin & haemoglobin bind to a porphyrin

(haem) chelating an Iron atom– e.g. Chlorophyll has a similar prosthetic group

chelating Mg

• The protein without its prosthetic group is called an apoprotein, with its group it is called a holoprotein

Co-factors/ Co-enzymes

• Other proteins have inorganic ions temporarily bound to them– E.g. copper/ zinc on enzymes

• Others have carbon containing molecules temporarily attached– e.g. Coenzyme A, NAD, FAD

Quaternary Structure

• Only present if protein has more than one polypeptide chain

• Describes the shape adopted by the interacting polypeptide chains

Nucleic Acids

• DNA– Deoxyribonucleic Acid

• RNA– Ribonucleic Acid

• Video

Nucleic Acids

• Summary Knowledge– DNA deoxyribose sugar,– RNA ribose sugar– DNA double stranded (antiparallel)– RNA single stranded– DNA thymine,– RNA uracil– A double (hydrogen) bonds to T (A 2 T)– G triple (hydrogen) bonds to C (G 3 C)– G & A purines (small word, big molecule –A Giant)– C,T & U pyrimidines (big word, small molecule)