Medical Lecture Notes

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Tuesday, 9 January 2018



Definition and Function

Literal definition:  proteois = primary, highly important .

Protein have a diversity of functions:

  • Structural integrity: e.g. muscles, hair, bone teeth, etc,,,
  • Transport: carrier of vitamins (albumin), oxygen and carbon dioxide (haemoglobin & myoglobin).
  • Catalysis (enzymes).
  • Protection: immunoglobulins + colostrum (γ-globulin).
  • Movement: muscle contraction
  • Blood clotting: e.g. fibrinogen.
  • Control of pH in the body.
  • Regulatory proteins: peptide hormones.
  • Heredity: Histone
  • Storage: e.g. ferritin, and myoglobin.

The Four Orders of Protein Structure

  1. Primary structure
  2. Secondary structure
  3. Tertiary structure
  4. Quaternary structure
Primary structure:
  • The order in which amino acids joined together
  • Peptide bonds are formed between amino acids.
  • Is the backbone of the polypeptide
  • The DNA coding structure
Secondary structure:
  • Is the spatial relationships of neighbouring amino acids residue.
  • Hydrogen bonds are formed between the COO- group of one peptide bond and the NH- group of another nearby peptide bond.
  • It forms the α-helix:
  • The rode like structure.
  • The H bonds formed between peptide bonds in the same chain (four residues away).
  • It forms the β-bleated sheet:
  • It is bleated because the C-C bonds tetrahedral (not exist in straight chain).
  • The chains lie side by side because hydrogen bonds formed between the neighbouring chain.
  • The chains could be parallel β-sheet (run in the same direction) or anti parallel β-sheet (run in the opposite direction).
Tertiary structure:
  • Refer to the spatial relationship of more distant residues
  • The conformation results from:
  • Hydrogen bonding.
  • Ionic bonds.
  • Disulfide bonds (bridge between cysteine residues).
  • Hydrophobic interaction (non polar side chains of interior amino acids).
Quaternary structure:
  • Is the non covalent aggregation of tertiary structure e.g. haemoglobin (tetramer protein).
  • Form dimeric (homo or hetero), and tetrameric protein

Protein Denaturation

Disrupt (unfolding) secondary, tertiary, and quaternary structure of protein.
Due to:
  • Reagents such as urea and mercapto -ethanol.
  • Organic solvents (alcohols and acetone).
  • Mild H+ & OH- (rapture hydrogen and electrostatic bonds).
  • Extreme pH and temperature.
  • Heavy metals (lead and mercury).
  • Radiations (X-ray).
  • Lead to:
  •  loss of protein function.
  • Could be reversible.

Classification of proteins

While no universally accepted classification system exist, proteins may be classified on the basis of:
  • Overall shape:
  1. Globular proteins: enzymes, haemoglobin.
  2. Fibrous proteins: e.g. collagen, myosine , keratin.
  • Other Classifications
  • Simple proteins: e.g. albumin and globulin.
  • Conjugated proteins:

  1. Lipoproteins: HDL, VLDL, LDL, and chylomicrons.
  2. Nucleoproteins: histones.
  3. Chromoproteins: e.g. chlorophyll.
  4.  Derived proteins: e.g. egg yolk .  
  • Classification of proteins according to their biological values: 
  • Proteins of high biological value: e.g. meats, eggs and milk. 
  • Proteins of low biological value: e.g. wheat and maize.

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