INTRODUCTION to PROTEINS

INTRODUCTION TO PROTEINS

  • Proteins are naturally occurring polymers of amino acids linked by peptide bonds.
  • They are the most abundant organic molecules in living systems.
  • The term protein is derived from the Greek word proteios, meaning holding the first place.
  • They are nitrogenous organic compounds with large molecular weight consisting of one or more long chains of amino acids.
  • Proteins are made from 2020 a- amino acids; a single unit is a monomer; many monomers form polymers.
  • On hydrolysis, proteins give amino acids.
  • Found in every cell; involved in most body functions and life processes.

FUNCTIONS OF PROTEINS

  • Structural: provide strength and structure to cells, tissues, organs; examples include elastin & collagen.
  • Enzymes: catalyze biochemical reactions (increase rate).
  • Transport: transport metabolites (transport proteins); involved in respiration.
  • Regulatory: regulate metabolic pathways (e.g., insulin).
  • Defence: protect from infection/toxins (antibodies, immunoglobulins).
  • Muscle proteins: perform mechanical work.
  • Regulation of body fluids: pH, osmotic pressure, temperature, and electrolyte balance.

CLASSIFICATION OF PROTEINS

  • SIMPLE PROTEINS: composed only of amino acids.
    • Globular proteins: spherical/oval, water-soluble
    • Albumins and Globulins (e.g., serum albumin, ovalbumin, lactalbumin)
    • Glutelins (plants, e.g., wheat glutelin)
    • Prolamines (soluble in 70% alcohol, e.g., gliadin, zein)
    • Histones (basic, heat-coagulable, widely distributed)
    • Globins (often discussed with histones)
    • Protamines (basic, soluble in NH4OH; associated with nucleic acids; sperm proteins)
    • Lectins (carbohydrate-binding proteins; e.g., agglutinin)
    • Fibrous proteins: fibre-like, insoluble
    • Collagens (connective tissue; soluble on boiling)
    • Elastins (elastic tissues: tendons, arteries)
    • Keratins (skin, hair, nails)
  • CONJUGATED PROTEINS: contain non-protein prosthetic groups
    • Nucleoproteins: histones + RNA/DNA
    • Chromoproteins: conjugated with chromophoric group (haem, riboflavin)
    • Phosphoproteins: include phosphate group (e.g., milk casein)
    • Glycoproteins: amino sugars, sulfates, sugar acids
    • Lipoproteins: proteins + lipids (brain, membranes)
    • Metalloproteins: metals (Fe, Co, Zn) bound to protein
  • DERIVED PROTEINS: denatured or degraded products of simple/conjugated proteins
    • Primary-derived: coagulated proteins (e.g., albumin), proteans, metaproteins
    • Secondary-derived: degraded products from peptide bonds

PROTEINS & ITS STRUCTURE

  • Proteins are polymers of exta−extaminoacidsext{a}- ext{amino acids}.
  • Four levels of structure: Primary, Secondary, Tertiary, Quaternary.
  • A protein is defined as a polypeptide containing more than 5050 amino acids.
  • PRIMARY STRUCTURE: linear sequence of amino acids; backbone; determines function.
  • SECONDARY STRUCTURE: spatial arrangements via twisting/folding of the polypeptide chain.
    • $$egin{cases} ext{a-helix} \ ext{(alpha-helix)} \ eta ext{-sheet (beta-pleated sheet)} \ ext{hydrogen bonds between backbone} \ ext{can be parallel or antiparallel} \ ext{stability often lowest energy for a-helix} \ ext{H-bonds: } ext{N-H} \