Th1L03 Amino Acids cont.

Th1L03 Amino Acids cont.

1. need to be obtained from the diet

   • Essential
     • Histidine
       • Contains imidazole group, an aromatic ring which can also be positively charged
         • pKa value near 6, can be uncharged or charged near neural pH depending on local environment
           • Often found in the active sites of enzymes, where the ring can bind and release proteins during the enzymatic reactions
             • positive charge, polar
     • Tryptophan
       • Has an indole group joined to a methyl group
         • Relatively simple, aromatic side chain
           • Hydrophobic, non-polar
       • Albinism
         • Defective tyrosinase (try > melanin)    
     • Lysine
       • Long side chains that terminate with groups that are positively charged at neutral pH
         • Capped with a primary amino group •
           • Polar, postive charge
     • Phenylalanine
       • Simple, aromatic (ring) side chain
         • Contains a phenyl ring attached in place of one of hydrogen atoms of alanine
           • Purely hydrophic
       • Phenylketonuria (PKU)
         • Defective phenylaline hydroxylase Phe > try                 Reduced tyr results in reduced dopamine and melanin production Protein rich food increases phenylalanine levels which can be toxic                   
     • Leucine
       • Contains an additional chiral centre
         • Only one isomer is found in protein
           • Hydrophobic, non-polar
     • Isoleucine
       • Hydrophobic, non-polar
         • Contains an additional chiral centre
           • Only one isomer is found in proteins
     • Methionine
       • Includes a thioether (-S-) group
         • Hydrophobic, non-polar
     • Valine
       • Contains an additional chiral centre
         • Only one isomer is found in proteins
           • Hydrophobic, non-polar
     • Thronine
       • Contains hydroxyl group attached to hydrophobic side chain
         • Version of valine with hydroxyl group instead of methyl group
           • Additional asymmetric centre
             • Polar, uncharged

2. can be synthesised in the body

   • Non-Essential
     • Proline
       • Its side chain is bonded to both the nitrogen and alpha-carbon atoms
         • Hydrophobic, non-polar
         • influences protein architecture because its ring structure makes it more conformationally than other amino acids
     • Aspartic Acid
       • Contain two acidic side chains
         • Side chains lack a proton at physiological pH and is present in acid form
           • Do not always accept protons, functionally important
             • Negative charge, polar
     • Glutamic Acid
       • Contain two acidic side chains
         • Side chains lack a proton at physiological pH and is present in acid form
           • Negative charge, polar
     • Glysine
       • Single hydrogen R group
         • Chiral
           • Hydrophobic, non-polar
     • Tyrosine
       • Contains hydroxyl group attached to hydrophobic side chain
         • Polar, uncharged
           • Epinephrine and dopamine synthesised from it
     • Cysteine
       • Similar to serine except it contains a sulfhydrl or thiol (-SH) group in place of the hydroxyl group which is more reactive
         • Polar, uncharged
     • Arginine
       • Long side chains that terminate with groups that are positively charged at neutral pH
         • Capped with a guanidinium group
           • Polar, positive charge
     • Glutamine
       • Uncharged derivative of glutamate
         • Contains carboxyamide in place of carboxyl acid
           • Polar, uncharged
     • Asparagine
       • Uncharged derivative of aspartate
         • Contains carboxyamide in place of carboxyl acid •
           • Polar, uncharged
     • Serine
       • Contains hydroxyl group attached to hydrophobic side chain
         • Version of alanine with hydroxyl group
           • Uncharged
     • Alanine
       • Methyl group side chain (-CH3)
         • Hydrophobic, non-polar

3. pKa of side groups

   • Measure of the ease with which groups give up their protons
   • At physiological pH the carboxyl and amino groups are attached to the alpha carbon and fully ionised
   • The lower the pKa the stronger the acid, the lower the pH it will dissocitate
   • Only the pKA of R-groups are relevant
   • Carobxyl group
     • pKa = 2.3
   • Amine group
     • pKa = 9.8