Handout7-new - G Proteins G proteins are so-called because...

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G Proteins G proteins are so-called because they bind the g uanine nucleotides G DP and G TP. They are heterotrimers (i.e., made of three different subunits) associated with the inner surface of the plasma membrane and transmembrane receptors of hormones, etc. These are called G-protein-coupled receptors ( GPCR s). The three subunits are: Gα, which carries the binding site for the nucleotide. At least 20 different kinds of Gα molecules are found in mammalian cells. How They Work Remember the equations in class: H.R + Gs.GDP + GTP H.R + Gs.GTP + GDP (etc) In the inactive state, Gα has GDP in its binding site. When a hormone or other ligands binds to the associated GPCR, an allosteric change takes place in the receptor (that is, its tertiary structure changes). This triggers an allosteric change in Gα causing GDP to leave and be replaced by GTP . GTP activates Gα causing it to dissociate from GβGγ (which remain linked as a dimer). Activated Gα in turn activates an effector molecule. In a common example (shown here), the effector molecule is adenylyl cyclase - an enzyme in the inner face of the plasma membrane which catalyzes the conversion of ATP into the "second messenger" cyclic AMP ( cAMP ) Activated Gα is a GTPase so it quickly converts its GTP to GDP. This conversion, coupled with the return of the Gβ and Gγ subunits, restores the G protein to its inactive state.
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s This type stimulates ( s = "stimulatory") adenylyl cyclase. It is the one depicted here. It is associated with the receptors for many hormones such as: 1. Adrenaline: also known as epinephrine synthesized and secreted by the adrenal medulla. The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland. The adrenal medulla releases: adrenaline (also called epinephrine) and noradrenaline (also called norepinephrine) Both are derived from the amino acid tyrosine, hence they are classified as amines Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors —transmembrane receptors in the plasma membrane of many cell types. As you have known, epi and nor-epinephrine bind to adrenergic receptors. Most of the
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This note was uploaded on 10/28/2010 for the course BIPN 100 taught by Professor French during the Winter '07 term at UCSD.

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Handout7-new - G Proteins G proteins are so-called because...

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