Biochemistry of proteins

• Hydrophobic effect: Hydrophobic side chains are buried inside the protein to minimize contact with water molecules.
• Proteins


  The most abundant macromolecules in living cells
• The term 'protein' was first used


  1838
• Protein


  The most important of cell constituents, responsible for almost every function that occurs in the body
• Amino acids


  Linear chains that are linked together by covalent, peptide bonds
• Each protein has a specific and unique sequence of amino acids that defines both its three-dimensional structure and its biologic function
• There are approximately 300 amino acids present in various animal, plant and microbial systems, but only 20 amino acids are involved in the formation of proteins
• Peptide bond


  A covalent bond formed between the carboxyl group of one amino acid and the amino group of its next amino acid, with the elimination of one H2O
• Peptide bond formation


  1. Carboxyl group of one amino acid forms a covalent bond with amino group of another amino acid, eliminating H2O

  2. Repeating this process generates a polypeptide or protein of specific amino acid sequence
• Classification of amino acids


  • Chemical classification
  • Classification according to polarity of side chain
  • Nutritional classification
  • Metabolic classification
• Chemical classification of amino acids


  • Aliphatic
  • Aromatic
  • Neutral
  • Acidic
  • Basic
• Polar amino acids


  • Contain OH, SH, amide, or NH2 groups
• Non-polar amino acids


  • Alkyl hydrophobic groups that can't form hydrogen bonds
• Special amino acids


  • 4-hydroxyproline
  • 5-hydroxylysine
  • Carboxyglutamate
  • Phosphorylated amino acids
• Nutritional classification of amino acids


  • Essential
  • Semiessential
  • Nonessential
• Metabolic classification of amino acids


  • Ketogenic
  • Mixed ketogenic and glucogenic
  • Glucogenic
• Physical properties of amino acids


  • Solubility
  • Melting points
  • Taste
  • Optical properties
  • Ampholytes
  • Zwitterion
• Chemical properties of amino acids


  • Reactions due to COOH group
  • Reactions due to NH2 group
  • Reactions due to side chain
• Peptides


  Chains containing less than 50 amino acids
• Proteins


  Chains containing greater than 50 amino acids
• Classification of proteins based on biological function


  • Catalytic
  • Regulatory/hormonal
  • Structural
  • Transport
  • Immune
  • Contractile
  • Genetic
  • Storage
• Proteins can also be classified based on their shape
• Types of proteins


  • Storage proteins
  • Catalytic proteins
  • Regulatory/hormonal proteins
  • Structural proteins
  • Transport proteins
  • Immune proteins
  • Contractile proteins
  • Genetic proteins
• Catalytic proteins


  These are enzymes which may be simple or conjugated. For example: alkaline phosphatase, alanine transaminase.
• Regulatory/hormonal proteins


  Many proteins and peptides act as hormones. For example: insulin, growth hormone, thyroid hormone.
• Structural proteins


  Contribute to the structure of the tissue. For example: collagen, elastin.
• Transport proteins


  Serve to carry proteins. For example: transferrin carry iron, hemoglobin carry oxygen.
• Immune proteins


  Serve in defense mechanism. For example: Immunoglobins (antibodies) IgG, IgM, IgA, IgE.
• Contractile proteins


  Take part in muscle contraction. For example: actin, myosin.
• Genetic proteins


  Proteins present in combination with nucleic acid. For example: histone proteins
• Storage proteins


  Store proteins for nutritional purpose. For example: casein in milk, gliadin in wheat
• Globular proteins


  Axial ratio less than 10, spheroid in shape, enzymes are mostly globular in shape, subdivide into albumins (water soluble) and globulins (soluble in dilute salt solution)
• Fibrous proteins


  Axial ratio more than 10, helical strands mainly form fibers, long thread like molecules which are soluble in water, present where support is required. For example: collagen, elastin, keratin
• Types of simple proteins


  • Albumin and globulins
  • Globins (Histones)
  • Gliadines
  • Scleroproteins
• Albumin and globulins


  Proteins of high biological value, contain all essential amino acids and easily digested, present in egg, milk and blood
• Types of globulins


  • α1 globulin (e.g. antitrypsin)
  • α2 globulin (e.g. hepatoglobin)
  • β-globulin (e.g. transferrin)
  • γ-globulins (Immunoglobulins, antibodies)
• Globins (Histones)


  Basic proteins rich in histidine amino acid, present combined with DNA or heme to form hemoglobin
• Gliadines


  Proteins present in cereals
• Scleroproteins


  Structural proteins, not digested, include keratin, collagen and elastin
• α-keratin


  Protein found in hair, nails, enamel of teeth and outer layer of skin, α-helical polypeptide chain, rich in cysteine and hydrophobic amino acids, water insoluble