G Proteins :: Biology Essays Research Papers

G Proteins Proteins play various important roles in inter-neuronal communication. Receptor sites are made up of proteins and the ion channels in the cell membranes are proteins. The link between the receptor sites and the protein channels sometimes is the guanine nucleotide-binding protein, better known as G Protein. (1) The basic structure and function of these shall be explored in the following. In order for neuron communication to occur, the post-synaptic neuron must have receptor sites for the neurotransmitters released by the pre-synaptic neuron. Also, these neurotransmitters, by binding on to the receptors, must bring about a change in the post-synaptic neuron, namely an EPSP (excitatory post-synaptic potential) or an IPSP (inhibitory post-synaptic potential), which may or may not lead to an action potential triggering in the post-synaptic neuron. EPSPs and IPSPs are produced in the post-synaptic neuron due to variations in either the Na+ or Cl- concentrations within the neuron. A change in concentration occurs when the protein channels which gate ion flow, permit Na+ or CL- to migrate across the cell membrane. The question now is, what causes the protein channels to open to Na+ or Cl-. In essence, there are three manners in which the ion flow can come about. The simplest way in which neurotransmitter-receptor binding can cause the opening of the protein channels is when the receptor is located immediately on top of the protein channel. Once a neurotransmitter binds on to the receptor, it causes the protein channel to permit ion flow. Receptors can also be acting on protein channels in more indirect fashion, via a second messenger system. A second messenger system is characterized by a G Protein's inclusion in the transduction of "signals from the transmembrane receptors to intracellular effectors." (1) That means, the binding of a neurotransmitter to a receptor activates a G Protein, which causes the protein channels gating ion flow to open. For this, two general mechanisms exist. Before they can be explained, however, the structure and dynamics of the G Protein must be considered. G Proteins are heterotrimic substances, i.e. they are composed of three subunits, alpha, beta and gamma. The alpha subunit of a G Protein is looked upon as the active subunit, as it binds GDP (guanine diphosphate) when it is inactive, but exchanges GDP for GTP (guanine triphosphat) when active (2) and acts as the "messenger" between the receptor sites and the effector. The beta and gamma subunits aid the alpha subunit to bind to membranes.