MadSci Network: Cell Biology

Re: how does membrane structure help in the synthesis of ATP in photosynthesis

Date: Fri May 14 15:29:06 1999
Posted By: Michael Maguire, Faculty,Case Western Reserve Univ.
Area of science: Cell Biology
ID: 926695701.Cb

The answer is in two parts:  the first describes some aspects of what we 
know about how membrane proteins interact with lipid membranes generally, 
the second describe questions that would be appropriate to ask, but for 
which we have no current answers.

First, the basic purpose of the membrane (for all proteins) is 3-fold.

 1)  It provides a hydrophobic (water-hating) environment for the parts of 
a protein that are likewise hydrophobic and would not be very soluble in 

  2)  It acts as a capacitor.  That is, because it is a non-aqueous and 
electrically non-conducting layer between two aqueous electrically 
conducting layers, it acts as an insulator.  If charge/energy builds up on 
one side of the membrane, it prevents that energy from dissipating and 
diffusing away into an (infinite) aqueous solution.  This is essentially 
the definition of a capacitor.  It is a system in which energy can be 
stored on one side of a barrier.  Then, proteins within that insulating 
barrier can allow some/all of that energy to flow through the membrane to 
the other side.  BUT, the point is that the proteins control that flow.

3)  The membrane provides vectorial space.  That is, it divides the 
universe into distinct sections or environments.  It provides sidedness.  
Thus, when proteins go into the membrane they could, in theory, be 
oriented either of two directions, 180 degrees apart.  HOWEVER, the 
machinery of the cell and the properties of the membrane make sure that 
proteins insert into the membrane in one and only one orientation, so that 
ALL molecules of a given protein face the same way.  (If, in the 
laboratory, we simply mix lipid vesicles with protein, the protein can 
insert into the vesicle and can function, but it inserts in both possible 
orientations because the machinery of the cell and the vectorial 
properties of the membrane are absent.)

Now for the other part.  Here's what we know and don't know about 
individual proteins, and specifically what we know and don't know about 
the synthesis of the F1F0-ATPase a.k.a. ATP synthase.  This enzyme makes 
that large, large majority of ATP in most cells and is the SOLE membrane 
enzyme that makes ATP.  It is present in all three kingdoms of life.  In 
the Bacteria and Archaea, it is present in the cell membrane.  In 
eukaryotic cells (yeast and plants to humans), it is present in the inner 
membrane of the mitochondrion or chloroplast (which is the equivalent 
membrane to the bacterial membrane and is logical since mitochondria and 
chloroplast used to be symbiotic bacteria that had invaded and taken up 
residence in another cell).  It is probably the single most important 
enzyme in an organism, certainly in animals.

We know that ATP synthase is vectorially inserted with the F1 part 
(fragment one, soluble in aqueous solutions) on the inside of the 
respective membranes.  The F0 part (fragment 0 (zero) inserted into the 
membrane.  This vectorial insertion is absolutely required.  Outside the 
membrane or mitochondrion, the pH is  acid (more protons) relative to the 
inside (less protons).  This establishes a chemical and potential energy 
gradient across the membrane directed inwardly (i.e., the protons flow out 
to in).  The passage of protons through the membrane from out to in allows 
the use of the energy they contain (and are dissipating by traveling down 
their energy gradient) to be captured by the F1F0-ATPase/ATP synthase.  
That energy is then used by the ATP synthase to attach a phosphate to ADP, 
making ATP.  So the membrane MUST be impermeant to protons (some membranes 
are slightly permeable).  That's the primary property it must have.  There 
are no specific lipids required, only that they can form a bilayer.  Some 
lipids work better than others, but most everything works at least a bit 
as long as it is sealed against proton.

So, that's a long-winded explanation of what we know (not much) and what 
we don't know (a lot).

Probably the ATP synthase interacts with some lipids to help maintain its 
structure, but we have no details of what might be happening.

Almost certainly in some cells the stoichiometry of the F0 part 
(number/ratio of the different proteins making up F0) varies BECAUSE in 
different cell types the lipid content/identity varies somewhat.

Membrane structure/lipid identity will differ depending on what
temperature(s) an organism lives at.

Indeed, in the Kingdom Archaea, these prokaryotes have a very different 
set of membrane lipids, sometimes a very thick, rigid membrane, composed 
of lipids of very different structure from that of Bacteria or 
Eukaryotes.  Yet, they also use F1F0-ATPase is just the same manner, the 
proteins adapting to the different lipids and the lipids adapting to the 

We clearly do not have a sufficient understanding yet of membrane 
structure and how it influences proteins inserted in it.

In some cells, sodium is used instead of proton to make ATP.  This is rare 
and is only used in some (but not all bacteria) that live in an alkaline 
(low proton, e.g., pH 10) environment.

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