An Elec­tri­fy­ing Chemios­motic Ex­change

by Elio

Photo credit: Er­win Wod­icka. Source

This whole thing started with an email from Mar­garet Lee, a pro­fes­sor at Ohlone Col­lege in Fre­mont and Newark, CA, ask­ing for an ex­pla­na­tion re­gard­ing the chemios­motic the­ory. Not be­ing com­fort­able with things elec­tri­cal, I turned to my friend and es­teemed col­league, Frank Harold for a more rea­soned re­sponse. Frank is a pro­fes­sor emer­i­tus from Col­orado State Uni­ver­sity and an Af­fil­i­ate Pro­fes­sor at the Uni­ver­sity of Wash­ing­ton in Seat­tle. He is one of the main con­trib­u­tors to the un­der­stand­ing and elu­ci­da­tion of Mitchell's chemios­motic the­ory.

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Dear Dr. Schaechter (Elio),

I lis­ten to TWIM and read Small Things Con­sid­ered reg­u­larly, ever since I started teach­ing Mi­cro­bi­ol­ogy at the lo­cal com­mu­nity col­lege about a year ago. I'm not a mi­cro­bi­ol­o­gist by train­ing, though now I wish I had been, as I find it be such a fas­ci­nat­ing area of bi­ol­ogy.

I was read­ing your re­cent blog about Left­over Sci­ence and your work with bac­te­r­ial mem­branes. I'm not able to com­ment specif­i­cally on the re­search you de­scribed there– it's a little/or a lot over my head. How­ever, I have had a more ba­sic ques­tion about mem­branes for a while, and can't seem to find a sat­is­fac­tory an­swer when I search the in­ter­net. I hope you don't mind if I ask. How do gram pos­i­tive bac­te­ria carry out chemios­mo­sis with a seem­ing lack of an en­closed mem­brane space? In some pa­pers I see ref­er­ences to a very small periplas­mic space in gram pos­i­tives. Does that mean that the thick pep­ti­do­gly­can layer is able to serve as a not-so-porous outer bound­ary?

Thanks so much for your help.

Sin­cerely,

Mar­garet Lee

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Hello Mar­garet,

Elio for­warded your in­quiry to me, since he still be­lieves that I un­der­stand this stuff. You raise a good ques­tion, I re­mem­ber that a ver­sion of it gave me con­nip­tions when I was strug­gling to mas­ter Mitchell nearly 50 years ago.

The an­swer is that the cell wall has noth­ing to do with en­ergy cou­pling by a pro­ton cur­rent. Mem­brane vesi­cles are very good at link­ing the res­pi­ra­tory chain to var­i­ous trans­port sys­tems, but have no cell wall what­so­ever. The way it works is that the res­pi­ra­tory chain pumps pro­tons out of the cy­to­plasm, an en­closed space bounded by an im­per­me­able mem­brane. The pro­ton car­ries a pos­i­tive charge, so its ex­pul­sion leaves its for­mer part­ner be­hind hold­ing a neg­a­tive charge. Glob­ally, this means that there is now an elec­tri­cal po­ten­tial across the plasma mem­brane, in­te­rior neg­a­tive. That mem­brane po­ten­tial draws pro­tons (in­deed, any pos­i­tive charges) back into the cy­to­plasm. That is the pro­ton-mo­tive force that cou­ples the ex­tru­sion of pro­tons to use­ful work. Now, the par­tic­u­lar pro­ton that was ex­pelled has long since been lost in the ocean out­side the cell, but that ocean makes an in­fi­nite reser­voir of pro­tons (mostly vir­tual, avail­able from the dis­so­ci­a­tion of wa­ter). Any pro­ton out there will feel the pull, and if it finds it­self near a path­way that al­lows it to flow in, it will do so. If that hap­pens to be a ATP syn­thase, the down­hill flux of pro­tons can be har­nessed to the syn­the­sis of ATP. But a K+ ion will do too, help­ing to ac­cu­mu­late potas­sium.

NO, it isn't easy, but once you mas­ter it all of bioen­er­get­ics comes to­gether to make sense. You will find longer dis­cus­sions in some of my works (e.g. Harold, F M, The Way of the Cell, or The Vi­tal Force), in David White' The Phys­i­ol­ogy and Bio­chem­istry of Prokary­otes, or in Elio Schaechter et al.'s Mi­crobe. If I can be of help, please feel free to con­tact me.

With best wishes,

Frank Harold

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Hello Harold,

I much ap­pre­ci­ate the time you took to an­swer my ques­tion. Your ex­pla­na­tion does make sense. I didn't think the cell wall had any­thing to do with the process, since text­books say they are fairly porous. (I am not sure if the te­i­choic acids play a role in af­fect­ing ion movement–this was not part of my ini­tial ques­tion.) So, it ap­pears that you don't re­ally need a mem­brane-en­closed com­part­ment such as the mi­to­chon­dr­ial in­ter­mem­brane space or gram neg periplas­mic space to get a pro­ton mo­tive force; the cy­to­plas­mic space rel­a­tive to the out­side of the cell will work as well.

Thank you very much!

Mar­garet Lee

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Hi Again, No, that's not right, you ab­solutely do need a closed com­part­ment. I don't know how to draw a di­a­gram on the com­puter, but if you send me a mail­ing ad­dress I shall send you one. Or else, look at a printed dis­cus­sion. The mi­to­chon­drion works be­cause it is a closed com­part­ment: pro­tons are pumped out of it, gen­er­at­ing an elec­tri­cal po­ten­tial across the mem­brane, with the in­te­rior neg­a­tive. If you poke a hole in the mem­brane, the po­ten­tial col­lapses (there are sub­stances that do this). It is just that the out­side does not mat­ter.

Best re­gards,

Frank Harold

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Dear Dr. Harold,

I did think a closed com­part­ment is needed for the ATP syn­thase to work, as that's how the text­books teach it. Most bi­ol­ogy text­books men­tion the bac­te­r­ial cell mem­brane, but the dis­cus­sion and the fig­ures are usu­ally with the mi­to­chon­drion. This fig­ure in the link be­low shows ATP syn­thases in mi­to­chon­drion, chloro­plast, and gram neg bac­te­ria. My orig­i­nal ques­tion was, if you need a closed com­part­ment for this kind of chemios­mo­sis syn­the­sis of ATP, what/where is the closed com­part­ment in the gram pos­i­tive bac­te­ria mem­brane? (Is it that in this case, "out­side does not mat­ter.")

Much ap­pre­ci­ate your help.

Mar­garet Lee

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Mar­garet,

in this case what's good for mi­to­chon­dria is also good for a Gram-pos­i­tive bac­terium. The closed com­part­ment is the whole cell, bounded by its im­per­me­able plasma mem­brane. Pro­tons are ex­truded from the cell, and the pro­ton-mo­tive force acts across the plasma mem­brane. The cell wall and periplas­mic space, if any, have noth­ing to do with trans­port across the plasma mem­brane, nor with chemios­motic en­ergy cou­pling.

Re­gards,

Frank Harold

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Frank,

Thank you: your an­swer leaves no fur­ther ques­tions on this mat­ter. I much ap­pre­ci­ate your help over these emails!

Mar­garet Lee