BISC 421 1st Edition Lecture 7 Outline of Current LectureI.Ion Channels Continued II.Synaptic Transmission Current LectureIon Channels (continued) How do these channels “sense” voltage? Voltage-gated channels. Closed at resting MP. Depolarization opens these channels. Depolarization, the > Na+ influx. Structural changes that lead to gating.At rest, the cytosolic gate is closed. (+)-charged D-helices. Move towards the extracellular surface of the membrane.‐How do these channels sense voltage continuedRodMacKinnnonNobel Prize2003 Structure of a voltage-‐gated K+ channel•The structure of the different units when they are hyperpolarized and depolarized‐“Ball and Chan” inactivation mechanismThe Shaker channel was cloned from fruit flies. Flies with a mutation in this channel will robustly shake under ether anesthesia.Due to a mutant K+ channel in which part of the amino terminus is missing.If the N-term is removed, currents do not inactivate.If a synthetic peptide corresponding to the N-term is added, the mutant channels do inactivate.Inactivation happens more quickly. Lengthened?•Channel shaker from drosophila-‐ mutated flies have strange behavior-‐ shaker flies withanesthesia will begin to shake due to potassium inactivating under anesthesia and theyidentified which part of the K+ channel is missing-‐ the amino terminus•Experiment; normally record the wild type shaker K+ channel and can polarize cell to getcurrent and slower inactivation.The crystal structure of a simplebacterial K+ channel has been obtained. Made up of 4 subunits, or atetramer.1 pore domain- P segment.y In the membrane, the mergedsubunits form -segments make up the selectivity filter.•Ions make channels voltage dependent and activate or inactivate. The K+ channels thatactivate and what about them makes them selective for the ion•Look at crystal structure of K+ channels and recognize the pore domain using bacterialpotassium channel-‐ found that the components that make pore domains are the samefor all K+ channels•The protein above is the same as two transmembrane domains, two alpha helices equivalent to 5th and 6th domain and have pore loop between 5th and 6th domain•The P loop is selective for the ion•If looking at globular structure see that it forms and inverted TP structure and P forms a filter that is perfectly organized for K+ to come throughEvidence that the P segment controls selectivity.Sequence is similar amongst all K+channels. All contain a Gly-Tyr-Gly in pore.Mutations in pore disruptselectivity.Species replacement does not alterselectivity.What determines selectivity 2 from each P-segment.Entry is mediated by the low activation energy that is formed.Dehydrated Na+ cannot bind to the 8carbonyl oxygens. Too small, thus energetically unfavorable. K+ activation energy <1000 versus Na+.•Selective for K+ ion size but the problem is that Na+ is smaller than K+•Look at K+ in water and surrounded by 8 water molecules due to being hydrated by the carbonyl oxygens-‐ tightly compact due to sodium ions being a little smaller•What the selective filter has done is perfectly arranged in size and dimensions and the carbonyl oxygens come off each have same exact orientation distance apart from another as the water oxygens are if associated with potassium ionThe crystal structure reveals that K+ions are within pore. In positions 1 & 3, or 2 & 4.another ion to the next position.Steps in movement:In State 1, K+ ions are in positions 1 &3. Hydrated ion outside of pore.Each ion now moves one positionforward.Now in State 2. Ions now in positions2 & 4. Ion outside of pore loses 4 of its 8 waters. Moves intoposition 1.Ń Each ion moves forward, reverting toState 1.x Ion moving into vestibule picks up 8 waters.•Potassium ions surrounded by red oxygen from water and hydrated•One pore loops and second and third are behind screen and four is in front of screen-‐ as it becomes dehydrated it loses these and associated carbonyl oxygens come off pore domain and as far as potassium ion is concerned it still thinks its hydrated and doesn’t know is oxygen comes from water molecule or from protein ion channel•The key point is that it is surrounded by 9 oxygens – from water or protein depends onwhere cycle comes through•Reason it is selective is due to ions always in a hydrated state surrounded by oxygen but what happens is it gets dehydrated as it goes through and substituted for protein that water-‐ energetically favorable-‐ sodium never gets fully hydratedParoxysmal extreme paindisorder (PEPD) Erythermalgia,,Mutations in Nav1.7 cause persistent pain and a lack of pain sensation•Nulls for particular sodium channels do not make particular sodium ad look as familiarwhere mutation is prevalent and in this specific mutation the channel is stopped frombeing translated to protein so never makes this protein•People do not feel pain due to this channel•People with PEPD feel burning heat most of the time because Na+ channel does not inactivate and neurons sense pain due to mutationDiseases Caused by Altered Ion Channels•Some people with migraines-‐ voltage gated K+ channels with mutations•Night blindnessDiseases Caused by Altered Ion Channels (Continued)•Treatment is modifying channelsSynaptic TransmissionSynaptic Transmission-‐Electrical and ChemicalL•Now we have communication between two cells that occurs across the synapse•Where two cells come together•Electrical synapse-‐ straight forward communication and post synaptic cells make contat with one another such that there are pores to make connections between the two membranes intracellular environments•Structures are called gap junctions-‐ two large trans membrane domain proteins to formpore. Transmission in its fastest form•A lot of sodium can come in and depolarize the cell•Bidirectional•Systems that use this connection-‐ interneurons, some hormones in the hypothalamusElectrical SynapsesStructure and Function of gap junctions at electrical synapsesL •One network connects fast and records spontaneous spikes•Synapse electrically to the other