Proteins for Membrane Function

• Proteins for membrane function

  • Types:
  • Integral/transmembrane (from one side to other)
  • Peripheral (on one surface); can slide around membrane
  • Hydrophilic regions hold protein to membrane
  • Hydrophobic regions stabilise proteins in membrane
  • Glycoprotein: Short sugar molecule chain attached to membrane protein
  • Glycolipid: Short sugar molecule chain attached to phospholipid molecule
  • Sugar chains project from bilayer’s external surface only
  • Act as antigens/recognition sites for specific chemicals
  • Maintain membrane stability
  • Aid cell attachment (to form tissues)
• Functions:

  • Receptor Sites
  • Must be on outside surface
  • Have specific binding site for hormones, enzymes, or other complementary-shaped chemicals
  • Binding triggers other events in; cell signalling
  • Transporters
  • Integral
  • Either form a channel or carrier
  • Enzymes
  • Catalyse reactions in cytoplasm or outside cell
  • Attached either inside or outside
  • Structural
  • Inside surface
  • Attached to cytoskeleton
  • Help maintain cell’s shape or change it for mobility
  • Antigens
  • On cell surface identified by immune system cells
  • Often have attached carbohydrates (glycoprotein)
• Transporters

  Carrier Protein

  • Substance bind on side it is high concentration
  • Release where it is low concentration
  • Flips between two states alternately opened on sides
  • Na^+/ glucose co-transport
  • Glucose transporters
  • Use energy from downhill sodium gradient
  • Transport glucose against concentration gradient
  • Still passive
  • No ATP breakdown

  Channel Protein

  • Water-filled pore
  • Allow diffusion of charged substances
  • Change permeability to certain ions
  • Most gated (open/close), types:
  • Voltage-gated
  • Ligand-gated
  • Mechanically-gated
  • Aquaporins:
  • Water transport
  • Hydrophilic inside