Amino Acids + Proteins

Amino Acids + Proteins

1. Proteins

   • Functions
     • Catalysis (enzymes)
     • Motion (flagella, contratile tissue etc.)
     • Mechanical support (skin, bone)
     • Transport/storage
     • Communication
     • Regulation (hormones)
     • Defence/attack
   • Secondary Structure
     • a-helix
       • Right handed, rod like structure
       • Main chain inner part of rod, side chains extend outwards
       • Intrachain H-bonds, 4 residues ahead (e.g. C1 w/ C5)
       • 3.6 residues per turn
       • High content of A.A = Ala, Leu, Met, Phe, Glu, Gln, His, Lys, Arg
       • Disrupted by proline!
       • Disulphides
         • in a-keratin increases the strength of the super-helices
           • determines extent of curl in hair
     • B-pleated sheet
       • (Anti)-parallel
       • Arrows pointing towards Carboxyl group
       • Zig-zag formation

2. Amino Acids

   • Have H atom, a-amino group, a-carboxyl group, distinct R group
   • All A.As chiral bar gylcine (R-group is H)
     • A.As are only in the L- form
       • The CORN rule - L = Clockwise
   • Properties of A.As (caused by R-groups)
     • Size (large/small)
     • Shape (aliphatic/aromatic)
     • Hydrophobicity ((Non)-Polar)
     • Charge (acidic/basic)
     • Sulphur Containing (cysteine + Methionine)
     • Imino (proline is NOT an A.A)
   • pH
     • Charge dependent on pH
       • Zwitterions
         • Overall charge = 0
           • +ve charged a-amino group
           • -ve charged a-carboxyl group
     • High pH (low H+ conc)
       • Amino group loses its +ve charge by losing a proton - NH3+ --> NH2
     • Low pH (high H+ conc)
       • Carboxyl group loses -ve charge by gaining a proton - COO- --> COOH
     • Acids/Bases
       • Groups that lose protons =acids
       • Groups that accept protons = bases
       • Strong (inorganic) acids/bases - lose/gain protons easily
       • Weak (organic) acids/bases - only partially dissociated
         • All A.As have weak acid (COOH) and basic (NH2) groups
           • Act as Buffers
             • Weak Bases 'mop up' added H+ - become acids (reaction goes to left)
             • Weak Acids release protons to 'mop up' added OH- - forms H2O (reaction goes to right)
             • K eq = [H+] therefore pH = pK
     • pH = - log 10 [H+]
       • Therefore H+ becomes larger the pH becomes smaller