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STRUCTURE & FUNCTION OF PROTEIN

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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
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
PROTEIN FUNCTIONS
- Enzyme
- Transportation & storage
- Muscle Contraction
- Part of cell membrane
- Mechanical support
- Immune Protection