The Heterotrimeric G-Proteins

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Describe the signalling pathways downstream of the heterotrimeric G proteins Gs, Gi and Gq
G-Proteins are one of the largest families of proteins in the body and are involved in many physiological pathways. G-proteins alter the levels of second messengers in the cell. Inactive G-proteins are always associated with the membrane. G-Proteins are heterotrimeric i.e. they have three different subunits: the largest sub unit, the alpha subunit (Gα); beta subunit (Gβ) and gamma subunit (Gγ). The Gβ and Gγ tend to stick together essentially forming a single sub unit in signalling pathways. Hence they are often referred to as the Gβγ subunit complex. The Gα of the G-Proteins binds to guanine nucleotides: in its active state the G-Protein is bound to
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Inactive receptors have many nearby inactive G-Proteins. These G-Proteins are bound to GDP. The receptor is activated by an agonist,a hormone or chemical mediator. When the agonist binds to the receptor it causes a conformational change in the receptor. This allows the receptor to bind to a nearby G-Protein. This binding then causes the G-Protein to undergo a conformational change. Due to the conformational change the receptor bound G-Protein will release its bound GDP and replace it with GTP. The Gα and Gβγ dissociate, leaving the receptor. A subunit will then move through the plasma membrane to go on to activate effector molecules, which are also associated with the plasma membrane, causing a number of physiological responses depending on the type of effector and G-Protein involved. The effector may be an ion channel or enzyme. Most often it is the Gα subunit that interacts with the effector however some effectors are activated by the Gβγ subunit complex. The G-Protein will continue to interact with the effector until the bound GTP is catalysed into GDP. The Gα and Gβγ subunits will then reassemble. This will cause the cycle to end as the G-Protein is now again in its inactive heterotrimeric, GDP bound state. Once the G-Protein has left the receptor, the receptor can then go onto activate other nearby G-Proteins. This leads to signal amplification as the receptor, when bound to an agonist, can activate multiple

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