The role of the receptor

  • These are Globular proteins
  • Located mostly in the cell membrane
  • Receive messages from chemical messengers coming from other cells (CNS)
  • Transmit a message into the cell leading to a cellular effect
  • Different receptors specific for different chemical messengers
  • Each cell has a range of receptors in the cell membrane making it responsive to different chemical messengers


  • Chemicals released from nerve endings which travel across a nerve synapse to bind with receptors on target cells, such as muscle cells or another nerve. 
  • Usually short lived and responsible for messages between individual cells


  • Chemicals released from cells or glands and which travel some distance to bind with receptors on target cells throughout the body

Note:  Chemical messengers ‘switch on’ receptors without undergoing a reaction

  • Receptors contain a binding site (hollow or cleft on the receptor surface) that is recognised by the chemical messenger
  • Binding of the messenger involves intermolecular bonds
  • Binding results in an induced fit of the receptor protein
  • Change in receptor shape results in a ‘domino’ effect
  • Domino effect is known as signal transduction, leading to a chemical signal being received inside the cell
  • Chemical messenger does not enter the cell. It departs the receptor unchanged and is not permanently bound

The Binding Site

  • A hydrophobic hollow or cleft on the receptor surface – equivalent to the active site of an enzyme
  • Accepts and binds a chemical messenger
  • Contains amino acids which bind the messenger
  • No reaction or catalysis takes place
  • Binding site is nearly the correct shape for the messenger
  • Binding alters the shape of the receptor (induced fit)
  • Altered receptor shape leads to further effects – signal transduction

Induced Fit

  • Binding interactions must be strong enough to hold the messenger sufficiently long for signal transduction t o take place Interactions must be weak enough to allow the messenger to depart Implies a fine balance Designing molecules with stronger binding interactions results in drugs that block the binding site – antagonists

Main Types of Receptor


Ion Channel Receptors

  • Receptor protein is part of an ion channel protein complex
  • Receptor binds a messenger leading to an induced fit
  • Ion channel is opened or closed
  • Ion channels are specific for specific ions (Na+, Ca2+, Cl–, K+)
  • Ions flow across cell membrane down concentration gradient
  • Polarized or depolarized nerve membranes
  • Activates or deactivates enzyme catalyzed reactions within cell


  • Chemical messenger binds to receptor binding site
  • Induced fit results in further conformational changes
  • TM2 segments rotate to open central pore
  • Fast response measured in msec
  • Ideal for transmission between nerves
  • Binding of messenger leads directly to ion flows across cell membrane
  • Ion flow = secondary effect (signal transduction)
  • Ion concentration within cell alters
  • Leads to variation in cell chemistry

G Protein–Coupled Receptors

  • That Activate or Inhibit Adenylyl Cyclase
  • the very large group of cell surface receptors that are coupled to signal-transducing trimeric G proteins.
  • All G protein–coupled receptors (GPCRs) contain seven membrane-spanning regions with their N-terminal segment on the exoplasmic face and their C-terminal segment on the cytosolic face of the plasma membrane .
  • The GPCR family includes receptors for numerous hormones and neurotransmitters, light activated receptors (rhodopsins) in the eye, and literally thousands of odorant receptors in the mammalian nose.
  • The signal-transducing G proteins contain three subunits designated α , β, and ϒ. During intracellular signaling the β  and ϒ subunits remain bound together and are usually referred to as the Gβϒ subunit. The Gα subunit is a GTPase
  • switch protein that alternates between an active (on) state with bound GTP and an inactive (off) state with bound GDP.
  • Stimulation of a coupled receptor causes activation of the G protein, which in turn modulates the activity of an associated effector protein.
  • Although the effector protein most commonly is activated by Gα ·GTP, in some cases it is inhibited. Moreover, depending on the cell and ligand,
  • the Gβϒ subunit, rather than Gα·GTP, may transduce the signal to the effector protein.
  • In addition, the activity of several different effector proteins is controlled by different GPCR-ligand complexes.
  • All effector proteins, however, are either membrane-bound ion channels or enzymes that catalyze formation of second messengers (e.g., cAMP, DAG, and IP3).
  • These variations on the theme of GPCR signaling arise because multiple G proteins are encoded in eukaryotic genomes.
  • The human genome, for example, encodes 27 different Gα, 5 Gβ, and 13 Gϒ subunits. So far as is known, the different Gβϒ subunits function similarly. 

Bacteriorhodopsin & Rhodopsin Family

  • Rhodopsin = visual receptor
  • Many common receptors belong to this same family
  • Implications for drug selectivity depending on similarity (evolution)
  • Membrane bound receptors difficult to crystallize
  • X-Ray structure of bacteriorhodopsin solved – bacterial protein similar to rhodopsin
  • Bacteriorhodopsin structure used as ‘template’ for other receptors
  • Construct model receptors based on template and amino acid sequence
  • Leads to model binding sites for drug design
  • Crystal structures for rhodopsin and b2-adrenergic receptors now solved – better templates

Tyrosine kinase – linked receptors

  • Bifunctional receptor / enzyme
  • Activated by hormones
  • Protein serves a dual role – receptor plus enzyme
  • The receptor binds messenger leading to an induced fit
  • Protein changes shape and opens active site
  • The reaction catalyzed within cell
  • Overexpression related to several cancers

Epidermal growth factor receptor  (EGF- R)

  • Active site on one half of dimer catalyzes phosphorylation of Tyr residues on other half
  • Dimerisation of receptor is crucial
  • Phosphorylated regions act as binding sites for further proteins and enzymes
  • Results in activation of signaling proteins and enzymes
  • Message carried into cell

Intracellular receptors

  • Chemical messengers must cross the cell membrane
  • Chemical messengers must be hydrophobic
  • Example-steroids and steroid receptors

Cyclic ADP-ribose (cADPR),

  • The receptor (which may be composed of four subunits) leads to the opening of Ca2+ channels and an influx of Ca2+ in to the cytoplasm from the vacuole and the ER


  • The ethylene(G) regulates ripening, germination, elongation, senescence, and pathogen responses.
  • ETR1, a 79-kDa protein
  • ETR1 exists as a dimer in the plasma membrane.
  • Ethylene joins the two monomers together and permits intermolecular phosphorylation.
  • Mutations in ETR1 (designated efr1) lead to loss of physiological sensitivity to ethylene.

Auxin-binding protein (ABP1)

  • IAA
  • Growth regulator
  • Functions in cell division and   expansion.


  • Phytochrome is a photoreceptor, a pigment that plants and some bacteria and fungi use to detect light.
  • It is sensitive to light in the red and far -red region of the visible spectrum
  • In other word a blue green pigment found in many plant in which it regulates various development processes.
  • The long day plant is flower after short exposure to red light the red light is absorbed by phytochrome 660 nm which converts to P730 nm and induces flowering.
  • Short day plants absorb light p730 which is converted to p660nm. This is a much slover and needs a long dark period.
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