Proteins from Amino Acids

1. Proteins from Amino Acids • Two amino acids joined together make a dipeptide, as more and more join together by peptide bonds, a polypeptide is formed The amino acids in a polypeptide chain are sometimes referred to as residues because part of the molecule is lost in the condensation reaction that produces the peptide bond

2. Protein Synthesis • Polypeptides are made (synthesised) inside cells on the ribosomes. • Messenger RNA (mRNA) puts the amino acids in the right order to make a specific polypeptide chain

3. Protein Synthesis • As the mRNA passes through the ribosome, the amino acids are joined one at a time by a condensation reaction forming a peptide bond • Eventually a longer and longer chain of amino acids is formed • The mRNA determines the sequence of amino acids and therefore the type of protein

5. Forming Different Proteins We will look at a peptide chain that is 3 amino acids long. The R group of an amino acid may be any one of the 20 available. To calculate the total number of different possibilities, we need to multiply the total number of possibilities at each point. In this case: 20 x 20 x 20 = 8000 This means that 8000 different sequences of four amino acids are possible- 8000 different proteins can be made. Given that most proteins are 100 amino acids long, in theory the number of different possible proteins is extremely large

6. Protein Structure Task • We can look at proteins in various levels of detail. The first is their primary structure, this is followed by the secondary, tertiary and quaternary structures • Task: Using Cambridge Biology p110- 113, and a two page spread in your book, summarise the 4 levels of protein structure (including the bonding involved)

7. Primary Structure The sequence of amino acids that forms the protein

8. Secondary Structure • The chain of amino acids coils or folds to form an alpha helix or a beta pleated sheet • Hydrogen bonds hold the coils in place weakly, but as so many are formed they give stability to the protein molecule

9. Tertiary Structure • When the coils and pleats themselves coil or fold • Held together by: • Disulphide bonds: a covalent bond between 2 sulphur atoms • Ionic bonds: the R groups are sometimes charged (+ve or –ve) so they attract each other • Hydrogen bonds: +ve hydrogen atoms and –ve oxygen atoms attract • Hydrophobic and hydrophilic interactions: hydrophobic amino acids will be drawn into the centre of the molecule, and hydrophilic amino acids will be found on the outside.

12. Heating Proteins • Heating a protein increases the kinetic energy in the molecule • This causes the molecule to vibrate, breaking the bonds