Abstract
The conductance-voltage (G-V) characteristic of a single-filing, multi-barrier, multi-occupancy channel depends in the limit of low ion concentration upon only two parameters: the voltage dependence of the entry step and the ratio of the rate constant for leaving the channel to that for crossing its middle (14,17,20). We show that the G-V shape in this low concentration limit can be measured accurately using a triangular wave, many-channel technique and demonstrate that the observed shape is incompatible with that expected if the only important rate limiting barrier at low concentration were at the channel mouth. Instead the central barrier turns out, surprisingly, in view of the markedly sublinear I-V shape at low concentration, to be even slightly larger than the exit barrier. Additionally, we find that it is not possible to fit both the G-V shape and the concentration dependence of the zero-current conductance simultaneously with a 3-barrier 2-site model. However, by adding additional sites to yield a 3-barrier 4-site model either of the type 3B4S" where the extra site in each channel half is external to the mouth of the channel or of the type 3B4S' where the extra site is internal to the mouth of the channel, we obtain good agreement. Additionally, using the flux ratio data of Procopio and Andersen (19) to discriminate between 3B4S and 3B4S" models, we find the 3B4S" model to be the only satisfactory one.
Highlights
AND THEORETICAL EXPECTATIONSThe shape of thecurrent - voltage (I-V) characteristic of an i o n channel gives information on i t s energy profile ( i . e . theheights and thelocationsintheelectricalpotentialfieldofthebarriers ) ; and t h e changes i n G-Vshape with ionconcentrationgive information about t h e depths and locations of thewells (e.g. t h e binding constants of thesites ) as w e l l as about shiftsin energy profilefordifferentloading states
W e show thattheG-V shape inthis low concentrationlimit can be measured accuratelyusingatriangular wave, many-channel technique and demonstrate that the observed shape is incompatible with that expected i f the only important rate limitingbarrierat low concentration wereatthe channel mouth
“supralinearly“; whereas i f the surface barriersweresufficientlyhigh compared withtheinternalbarriersthecurrent would i n crease “sublinearly”. Extending these considerationsto channels occupiable by a second cation, Hladky et a l . [15,16, 25] explainedthe dependence of t h e shape of t h e G-Vcharacteristic of t h e gramicidin channel by assuming thatat low ionactivitiestheentrystepwasrate determining ; whereas at high ion activity transfer across t h e middle of the channel was assumed t o become t h e slow s t e p
Summary
AND THEORETICAL EXPECTATIONSThe shape of thecurrent - voltage (I-V) characteristic of an i o n channel gives information on i t s energy profile ( i . e . theheights and thelocationsintheelectricalpotentialfieldofthebarriers ) ; and t h e changes i n G-Vshape with ionconcentrationgive information about t h e depths and locations of thewells (e.g. t h e binding constants of thesites ) as w e l l as about shiftsin energy profilefordifferentloading states. [15,16, 25] explainedthe dependence of t h e shape of t h e G-Vcharacteristic of t h e gramicidin channel (sublinear a t low ion concentrations and supralinear a t high) by assuming thatat low ionactivitiestheentrystepwasrate determining ( theflux being limited by t h e frequency w i t h which ionsarriveatthechannel ) ; whereas at high ion activity transfer across t h e middle of the channel was assumed t o become t h e slow s t e p These authors concluded [16] thattherewas good agreement between measured singlechannelcurrents and thosetheoreticallycalculatedaccordingtoa 3 - barrier 2-site (3B2S) model. Because t h e changes i n G-V shape are notparticularlystrikingfortheusualconcentrations ( 3 , 4 , 1 6 ) , it wasinitiallybelieved ( 1 7 ) thattheG-Vcharacter - istic of the single channel w a s only “slightly sublinear a t low ion concentrations and linearorslightly superlinear at saturating ion concentration”
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
