Receptors in General

Since the receptors for hormones, neuropeptides and neurotransmitters are all structurally similar, they are studied and discussed together. 

Cross references: 
Receptor Evolution, Receptors Evolution TimelineLigands,  
Ion Channels,    Ligand-gated Ion Channel , G-Protein Coupled Receptors ,
 Transmembrane Transport Evolution,   Transmembrane Signaling Evolution

Note that, with the exception of Ligand-gated Ion Channel, receptors are involved with transmembrane signaling rather than with  transmembrane transport. 

IUPHAR DATABASE OF RECEPTORS AND ION CHANNELS 
This database includes all receptors, both signaling and transport. 
 

Receptor (Wiki) 

     "In biochemistry , a receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signaling molecules may attach. A molecule which binds (attaches) to a receptor is called a ligand , and may be a peptide (short protein) or other small molecule, such as a neurotransmitter , a hormone , ...
     "Each cell typically has many receptors, of many different kinds.
     "The final biological response (e.g. second messenger cascade or muscle contraction), is only achieved after a significant number of receptors are activated."
     "Depending on their functions and ligands , several types of receptors may be identified: Some receptor proteins are peripheral membrane proteins .  Many hormone and neurotransmitter receptors are transmembrane proteins :  

Click on image to enlarge

Transmembrane receptor:
E=extracellular space (red)
P=plasma membrane (green)
I=intracellular space (yellow);

     "A receptor which is capable of producing its biological response in the absence of a bound ligand is said to display 'constitutive activity'. 
     "Not every ligand that binds to a receptor also activates the receptor. The following classes of ligands exist:
  • (Full) agonists are able to activate the receptor and result in a maximal biological response. Most natural ligands are full agonists.
  • Partial agonists do not activate receptors thoroughly, causing responses which are partial compared to those of full agonists.
  • Antagonists bind to receptors but do not activate them. This results in receptor blockage, inhibiting the binding of other agonists.
  • Inverse agonists reduce the activity of receptors by inhibiting their constitutive activity."   
My comment
     Read the full Wikipedia article for more details.   At the end of the article there are six expandable menus which provide links to well over a hundred different receptors, and the links to the more important receptors are quite informative. 


Transmembrane Receptors (Wiki) 
http://en.wikipedia.org/wiki/Transmembrane_receptor 
     "Transmembrane receptors are specialized integral membrane proteins that take part in communication between the the cell and the outside world. Extracellular signaling molecules (usually hormones, neurotransmitters, cytokines, growth factors or cell recognition molecules) attach to the receptor, triggering changes in the function of the cell."  Note that the "extracellular signaling molecules" mentioned above are the Ligands discussed in the link, above, by that name.   
     "
Based on structural and functional similarities, membrane receptors are mainly divided into 3 classes: The ion channel-linked receptor; The enzyme-linked receptor and G protein-coupled receptor.
My comment
     Note that the 'ion channel-linked receptor' links to a mechanism for transmembrane transport while both the 'enzyme-linked receptor' and the 'G protein-coupled recptor' link to mechanisms for transmembrane signaling. 


Mechanism of Action: Hormones with Cell Surface Receptors (Goog) 
Full length book available online for free. 
    "
Several distinctive variations in receptor structure have been identified. As depicted below, some receptors are simple, single-pass proteins; many growth factor receptors take this form. Others, such as the receptor for insulin, have more than one subunit. Another class, which includes the beta-adrenergic receptor, is threaded through the membrane seven times.


    "
Currently, four second messenger systems are recognized in cells, as summarized in the table below. Note that not only do multiple hormones utilize the same second messenger system, but a single hormone can utilize more than one system. Understanding how cells integrate signals from several hormones into a coherent biological response remains a challenge.
Second Messenger Examples of Hormones Which Utilize This System
Cyclic AMP Epinephrine and norepinephrine, glucagon, luteinizing hormone, follicle stimulating hormone, thyroid-stimulating hormone, calcitonin, parathyroid hormone, antidiuretic hormone
Protein kinase activity Insulin, growth hormone, prolactin, oxytocin, erythropoietin, several growth factors
Calcium and/or phosphoinositides Epinephrine and norepinephrine, angiotensin II, antidiuretic hormone, gonadotropin-releasing hormone, thyroid-releasing hormone.
Cyclic GMP Atrial naturetic hormone, nitric oxide

My comment
     Click on the link to see a really cool animation of a tyrosine kinase receptor. 
  

The Venus Flytrap of Periplasmic Binding Proteins (PubMed) 
http://www.ncbi.nlm.nih.gov/pubmed/11741199?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_SingleItemSupl.Pubmed_Discovery_RA&linkpos=5&log$=relatedreviews&logdbfrom=pubmed 
Abstract only. 
     "
Located between the inner and outer membranes of Gram-negative bacteria, periplasmic binding proteins (PBPs) scavenge or sense diverse nutrients in the environment by coupling to transporters or chemotaxis receptors in the inner membrane...  PBPs consist of two large lobes that close around the bound ligand, resembling a Venus flytrap... 
In the process of evolution, genes encoding the PBPs have fused with genes for integral membrane proteins.

The change in shape of the receptor protein can result in either transmembrane transport or transmembrane signaling.  Both processes can be triggered by ligands in the external environment.  These ligands can be either hormones that circulate in the blood or neurotransmitters that convey messages from one nerve to another at a synapse.  There are even a few organic molecules that can serve either purpose, depending on the circumstances. 

Thus, diverse mammalian receptors contain extracellular ligand binding domains that are homologous to the PBPs; these include glutamate/glycine-gated ion channels such as the NMDA receptor, G protein-coupled receptors, including metabotropic glutamate, GABA-B, calcium sensing, and pheromone receptors, and atrial natriuretic peptide-guanylate cyclase receptors.
"  

If I understand correctly, PBPs form the outer portion of both the outside-in transmembrane transporters and the transmembrane signal receptors.  The difference between the outside-in transporters and the signal receptors is in what the PBPs connect to. 

In the examples cited above, the 'glutamate/glycine-gated ion channels are outside-in transmembrane transporters; they open ion channels which allow the passage of ions across the membrane.  In contrast, the metabotropic glutamate
, GABA-B, the calcium sensing, and the pheromone receptors are all transmembrane signal receptors which cause a change inside the cell without allowing the passage of ions or other molecules across the membrane. 

Note that only Gram-negative bacteria have PBPs, since only Gram-negative bacteria have a periplasmic space.   

"Gram-positive microorganisms lack a periplasm such that their binding protein is often a lipoprotein bound to the external face of the cell membrane.
"

As a reminder,
Prokaryotes offers a brief summary of the difference between:
Gram Positive Bacteria (Wiki) 
http://en.wikipedia.org/wiki/Gram-positive_bacteria 
and Gram Negative Bacteria (Wiki) 
http://en.wikipedia.org/wiki/Gram-negative_bacteria 





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