What Is a G-Protein-Linked Receptor?
In biological membranes the signal detection and transmission is initiated by the interaction of a chemical or physical stimulus with a specific membrane receptor which, in turn, becomes activated and initiates a chain of intracellular reactions that result in modulation of target protein activity. G-Protein Coupled Receptors are a superfamily of such membrane proteins that transmit a signal by coupling to heterotrimeric-binding proteins, which consist of three subunits (α, β and γ). Rhodopsin is a member of GPCRs superfamily which is pharmacologically important.
G-protein-coupled receptors (GPCRs) mediate most of our physiological responses to hormones, neurotransmitters and environmental stimulants, and so have great potential as therapeutic targets for a broad spectrum of diseases. They are also fascinating molecules from the perspective of membrane-protein structure and biology. Great progress has been made over the past three decades in understanding diverse GPCRs, from pharmacology to functional characterization in vivo. Recent high-resolution structural studies have provided insights into the molecular mechanisms of GPCR activation and constitutive activity. The past two years have seen remarkable advances in the structural biology of G-protein-coupled receptors (GPCRs). Highlights have included solving the first crystal structures of ligand-activated GPCRs—the human β2 adrenergic receptor (β2AR), the avian β1AR and the human A2A adenosine receptor—as well as the structures of opsin and an active form of rhodopsin. These successes followed decades of effort by many laboratories across the world, and are of great interest from the perspectives of membrane-protein biophysics, cell biology, physiology and drug discovery. GPCRs are the largest family of membrane proteins and mediate most cellular responses to hormones and neurotransmitters, as well as being responsible for vision, olfaction and taste. At the most basic level, all GPCRs are characterized by the presence of seven membrane-spanning α-helical segments separated by alternating intracellular and extracellular loop regions. GPCRs in vertebrates are commonly divided into five families on the basis of their sequence and structural similarity.
G-protein coupled receptors are the most diverse and largest of the membrane receptors in eukaryotes. These are known as cell surface receptors, which act as an inbox for messages in the form of peptides, sugars, energy, lipids and proteins. These messengers inform cells about the present/absence of light or life-sustaining proteins in their environment. GPCR’s are known for conveying information sent by others and take part in a lot of important roles in the human body. “It is estimated by research that between one-half of all marketed drugs act by binding to GPCRs.” In the human body there are hundreds of different types of GPCR and a diverse type of ligand which bind to these receptors. The receptors might be odor molecules, neurotransmitters, peptide hormones or others. All these structures have a similarity, which is its seven transmembrane domains structure (Kristiansen, K. (2004, July) The purpose of GPCR is to couple the binding of agonists to the activation of specific heterotrimeric G proteins, leading to the modulation of downstream effector proteins. GPCR can also be found in yeast, choanoflagellates and other animals. Studying GPCR’s in their crystalized state was difficult due to the limited amount of technology present, this was immensely challenging for all the scientist. For crystallographer’s, membrane proteins have always been a nuisance and GPCR’s were especially defiant. In the 90’s there were a series of breakthroughs which allowed the detailed structural characterization of GPCR. The beta-adrenergic receptor (BAR) was a model organism of the GPCR, which was bounded to adrenaline (Katritch, V., Cherezov, V. & Stevens, 2012). This brought about the “fight-or-flight” response among other things. Robert Lefkowitz, in the 80’s introduced “the modern study of GPCRs by first cloning and sequencing the genes for the BARs.”
In sum, acquired mutations in certain GPCRs cause abnormal increases in receptor activity and expression in cell membranes, which can give rise to cancer. Because GPCRs play specific roles in human disease, they have provided useful targets for drug development. The antipsychotic agents clozapine and olanzapine block specific GPCRs that normally bind dopamine or serotonin. By blocking the receptors, these drugs disrupt the neural pathways that give rise to symptoms of schizophrenia. There also exist a variety of agents that stimulate GPCR activity. The drugs salmeterol and albuterol, which bind to and activate beta-adrenergic GPCRs, stimulate airway opening in the lungs and thus are used in the treatment of some respiratory conditions, including chronic obstructive pulmonary disease and asthma.
Croeze, W. K., Sheffler, D. J., & Roth, B. L. (2003, December 15). G-protein-coupled receptors at a glance. Retrieved December 1, 2018, from http://jcs.biologists.org/content/116/24/4867
Kristiansen, K. (2004, July). Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: Molecular modeling and mutagenesis approaches to receptor structure and function. Retrieved December 1, 2018, from https://www.ncbi.nlm.nih.gov/pubmed/15251227
Jogalekar, A. (2012, October 10). G Protein-Coupled Receptors (GPCRs) win 2012 Nobel Prize in Chemistry. Retrieved December 1, 2018, from https://blogs.scientificamerican.com/the-curious-wavefunction/g-protein-coupled-receptors-gpcrs-win-2012-nobel-prize-in-chemistry/
Answers Ltd. (2018, November 22). G Protein Coupled Receptors | Essay. Retrieved December 1, 2018, from https://www.ukessays.com/essays/biology/g-protein-coupled-receptors-biology-essay.php
Katritch, V., Cherezov, V. & Stevens, R. C. Diversity and modularity of G protein-coupled receptor structures. Trends Pharmacol. Sci. 33, 17-27 (2012)