Saturday, October 17, 2009
G proteins, short for guanine nucleotide-binding proteins, are a family of proteins involved in second messenger cascades.
G proteins are so called because they function as "molecular switches," alternating between an inactive guanosine diphosphate (GDP) and active guanosine triphosphate (GTP) bound state, ultimately going on to regulate downstream cell processes.
G proteins were discovered when Alfred G. Gilman and Martin Rodbell
G proteins are important signal transducing molecules in cells. In fact, diseases such as diabetes, blindness, allergies, depression, cardiovascular defects and certain forms of cancer, among other pathologies, are thought to arise due to derangement of G protein signaling.
The human genomes encodes roughly 350 G protein-coupled receptors, which detect photons (light), hormones, growth factors, drugs, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have unknown functions.
types of Gprotiens:
there are different types of G protiens that play an important role in the process of cell signalling
has its role in adenylcyclase activation which plays a role in the process of cAMP mediated cell signalling mainly for the transfer of phosphate molecule from ATP which leads to the formation of cAMP.
it usually inhibite the action of adenyl cyclase.
Gt: g transducin helps in the process og cGMP mediated cell signalling mainly in the process of photo reception process
it is agprotien that plays an important role with the olfactory signalling process
the action involves:
when a particular oudour acts as aligand the ligand receptor domain gets activated which leads to teh activation of the particular Gprotiens whicha re nonother than this Golf protiens and lets that to an on condition that ids the cells ready for the process of signalling.
the activated protiens in turn activates the protien kinases which help in the process of phosphorylation. this phosphorylation process actiavtes the cyclic phosphates medaited signalling and allows the influx of NA+ ions which leads to the depolarisation process through which the signals are carried to brain and helps to recognize thart particular odour.
the same happens with the pheromones which are mating harmones in prokaryotes as well as some mice
if the switch is off/blocking by G protiens:
this is the off condition of molecular switches that are nothing but the G protiens which allows the dephosphorylation by GRK that is Gprotien receptor kinase which leads to the activation of substance s called arrestins which actiavtes the regulatory sites they finally leads to the blockage of catalytic sites thus stops the process of cellsignaling.
Activation process with the subunits:
When a ligand activates the G protein-coupled receptor, it induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP on the Gα subunit. In the traditional view of heterotrimeric protein activation, this exchange triggers the dissociation of the Gα subunit, bound to GTP, from the Gβγ dimer and the receptor. However, models that suggest molecular rearrangement, reorganization, and pre-complexing of effector molecules are beginning to be accepted. Both Gα-GTP and Gβγ can then activate different signaling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein.