Proteins

Proteins

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.