STRUCTURE & FUNCTION OF PROTEIN

STRUCTURE & FUNCTION OF PROTEIN

1. AMINO ACID

   • All proteins are composed of amino acids
   • There are twenty common amino acids
   • Consists of:
     • -One central carbon atom
     • An amino group
     • An carboxyl group
     • A side chain (R)
   • At physiological pH =7.4 amino acid will form ZWITTERION ( +/- ion) as both base and acid are present
   • Difference in the side chain will distinguish the type of amino acids
     • Five categories of amino acids
       • Hydrophobic amino acid (C,N)
       • polar uncharged (O,N)
       • acidic (charged)
       • Basic (uncharged)
       • Aromatic
   • Acidic & basic properties
     • Amino acids in aqueous solution contain:
       • weakly acidic carboxyl group (-COOH)
       • weakly basic amino group (-NH2)
       • Ionizable side chain group
   • Act as BUFFER
     • Solution that resists the changes in the concentration of the hydrogen ions by the addition of an acid or a base
     • Acid Buffer: Mixture of a weak acid and its conjugated base
     • Basic Buffer: Mixture of a weak base and its conjugated acid
     • Henderson-Hasselbach Equation-predict buffering capacity

2. PROTEIN STRUCTURE

   • PRIMARY STRUCTURE
     • Linear sequence of amino acids
       • N-terminal
         • One end side of the polypeptide chain
       • C-terninal
         • Another end side of polypeptide chain
     • Consequences
       • Polymorphism
         • Vary in the primary sequence but still exhibit the same function
       • Mutation
         • Changes in the primary sequence causing sickle cell
   • SECONDARY STRUCTURE
     • Beta-pleated sheet
     • Alpha helix
     • Long chain of amino acids folds into repeating structure held by hydrogen bonding (amide hydrogen & carboxyl oxygen)
   • TERTIARY STRUCTURE
     • 3D structure of a poplypeptide
     • Stablized by side chain interactions
       • Hydrogen bonds
       • Hydrophobic interactions
       • Disulphide bond
       • Ionic interactions
   • QUATERNARY STRUCTURE
     • Subunit composition of a protein
       • Heteromeric
         • Different subunits by different genes
       • Homomeric
         • Same subunits by same genes
     • Held by hydrophobic interactions & electrostatic forces

3. PROTEIN FUNCTIONS

   • Enzyme
   • Transportation & storage
   • Muscle Contraction
   • Part of cell membrane
   • Mechanical support
   • Immune Protection

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