Saturday, July 25, 2015

Two timelines

This is for Raphi. It's a slightly rough version of the post I lost yesterday. I've been through Nick Lane's timeline for the evolution of bioenergetics in The Vital Question as carefully as I can and it's set out below. I've coloured in green the lines which are unchanged in the timeline that I would suggest and in red the sections of both timelines which I have some trouble with. Here we go:


There is a proton gradient, FeNi catalysis, activated acetate, ATP (via SLP), no membrane.

Permeable membrane develops, Ech and ATP synthase develop and use the gradient across this membrane, both run on the vent H+ gradient

Membrane tightens to Na+.

Antiporter invented while membrane still proton permeable but Na+ opaque.

Antiporter provides a Na+ gradient in addition to the H+ gradient, this helps because Na+ (along with H+) can drive ATP synthase to produce ATP and Na+ (along with H+) can drive Ech (TVQ p144, line 10 and line 23) to produce reduced ferredoxin.

This is a pre adaptation to proton pumping, because a pumped proton drives the antiporter which maintains the Na+ gradient whenever the natural proton gradient of the vents diminishes. i.e. with an antiporter plus pumped protons a much smaller vent gradient is needed. A proton pump is developed, not specified where from but probably from Ech/antiporter.
Membrane progressively tightens to H+ and so Na+ pumping becomes progressively less important, it’s pumped protons which now return through ATP synthase.

No longer any benefit from Na+ pumping so everything uses pumped protons and these are pumped via Ech using reduced Fd- from electron bifurcation (acetogens) or via a modified anti porter powered by methylene-H4MPT (methanogens).

As I have commented before, I have trouble with the very early development of ATP synthase as a very complex nano machine providing ATP for a cell in the earliest periods of crude membrane formation. However, that does leave Ech, energy converting hydrogenase. The next few posts are on Ech but for now it is a primordial supplier of reduced ferredoxin to the protocell. It is very simple and works perfectly well on the primordial proton gradient.

However, having rejected ATP synthase I am left with the problem of ascribing the benefits of Na+ antiporting to the use of Na+ energetics by the developing Ech, this won't work. Ech, as a generator of reduced ferredoxin, is dependent on the pH gradient provided by the vents. It's a matter of redox potentials for H2 converting to 2H+ and 2e-. See next post, you can't do this with a Na+ gradient.

So that leaves me with energetic problems, no ATP synth and Ech needing a pH gradient. Hence the time line cobbled together below:

There is a proton gradient, FeNi catalysis, activated acetate, ATP (via SLP), no membrane.

Permeable membrane develops. This forces proto-Ech to develop as a protein encrusted FeNi enzyme, it runs on vent gradient to generate Fd- to generate activated acetate and ATP (still via SLP). 

Any membrane forces the development of an ATP driven translocase to allow spread of RNA/proteins through vents through membrane barriers.

Membrane tightens to Na+.

The translocase jams and ends up using ATP to pump Na+ ions rather than to translocate a protein. Modestly lowered intracellular Na+ improves proto-Ech function.

Antiporter invented while membrane still proton permeable but Na+ opaque.

The antiporter causes a marked drop in intracellular Na+ which forces the translocase-derived Na+ pump to flip in to reverse and so generate ATP from the suddenly hugely increased Na+ gradient. This is the origin of ATP synthase.

To leave the vents proto-Ech is adapted as a power supply to the antiporter (so that H+ gradient driven antiporting is no longer needed), becomes the true Ech at this stage. It always pumps Na+ outwards using Fd- from electron bifurcation either directly or indirectly via methylene-H4MPT.

Everything runs on Na+ energetics, there is no rush to generate a proton tight membrane, but no problem if/when it happens.

Cytochromes demand H+ energetics, most organisms convert to cytochromes at various different evolutionary times. Non-cytochrome microbes continue to run on Na+, even today.

The problems in this timeline, also in red, are the benefits to a protocell from lowered Na+, the limitation of Ech to pumping Na+ only, certainly until much latter and the evolution of cytochromes, and the idea that Methyltransferase, which pumps Na+ in methanogens, is a derivative of Ech. These are the subjects of my next couple of posts, along with where Ech came from and why it must have a proton gradient.


EDIT Methyltransferase is much more complex than an Ech derivative!


raphi said...

OK, much to think about.

I need to go over it a few times - and actually read Nick Lane's book haha!

Thanks for the valuable thoughts Peter

Passthecream said...

'The antiporter causes a marked drop in intracellular Na+ which ..."

The long-ago prototype of Maxwell's Demon ?!?


Peter said...

Raphi, the book is well worth a read. I just can't stop picking at the logic in this particular section...

Pass, yes, such a demon will feature in quite a few posts I think!