Protons: Where is FeS cluster N-1a?

I picked up this paper from one of Nick Lane's books, can't remember which one but probably PSS. The paper itself is very, very interesting and delves deeply in to redox potentials, especially within the iron-sulphur centres of complex I, to the sort of level which is way beyond my ability to critique. So I'm accepting their findings pretty much as presented, with the usual caveats about mitochondial studies. In particular it worries me that we can study superoxide generation in mitochondria at partial pressures of oxygen around 100mmHg (room air). No mitochondria see this concentration of oxygen in vivo.

The group make a pretty good case that the FeS centre N-1a is the primary source of superoxide generation from complex I when mitochondria are being fed on NADH generating substrates, in particular when there is a large excess of NADH per unit NAD+.

It also appears to be the source of superoxide under succinate driven activation of complex II, probably secondary to reverse electron flow up the FeS clusters secondary to markedly reducing the CoQ couple.

The group didn't look at activated fatty acids but the F:N ratios would suggest to me that palmitic acid and upwards (chain length of fully saturated fats) would behave much as succinate does, by reducing the CoQ couple in a very similar manner. I've argued for some time that superoxide is the physiological switch to turn off glucose metabolism (ie trigger insulin resistance at the individual cell level) when generous levels of saturated fats predominate for oxidation.

This leaves open the simple question of what, exactly, is the N-1a FeS cluster of complex I? What does it do? What does complex I look like anyway? How much iron-sulphur does it contain? I did a quick Google image search for the structure of iron-sulphur chain of complex I and it turned up some beautiful pictures. I rather like this one from the Netherlands:



Look at the string of FeS clusters, shown in red, running through the protein bed. Cluster N-1a is the one right adjacent to FMN, probably set behind it rather than as close as it looks. The one off to the right is N7. Modern complex I almost certainly never originated as a proton pump but its deepest ancestor rather looks to have been a layer of iron pyrites, FeS. Up at the top of the picture, in yellow, is the flavin mono nucleotide which transfers electrons from NADH to the FeS chain. At the bottom, in pink, is ubiquinone of the CoQ pool, ready to transfer electron equivalents to complex III. The CoQ pool is fascinating as having it reduced (in this paper by supplying succinate to complex II) appears to cause reverse flow up the FeS chain as far as (and perhaps beyond) the side branch to N-1a, with superoxide generation as the result. Palmitic acid probably does the same but obviously reduces the CoQ pool using electron-transferring-flavoprotein dehydrogenase (which lacks a catchy name) rather than succinate dehydrogenase (complex II), as we've discussed previously.

Both NADH and CoQ move electron equivalents around on a macroscopic scale by moving physically. The FeS clusters shuttle electrons by quantum tunnelling. This is a very short distance phenomenon. Below are the sorts of distances between the various FeS clusters in complex I. If anyone thinks the arrangement of FeS clusters is in any way a random set up I suspect they have no concept of what 4 billion years means.




We can ignore the N7 cluster as it's not really in the FeS chain at all, being over 20Å out to one side and it is not highly conserved. Cluster N-1a is highly conserved but it too is not really part to the chain of FeS clusters. It also sits out to one side, about 11Å out from FMN. This is a perfectly reasonable tunnelling distance to transfer electrons to and from FMN, but to hand an electron on to the N3 cluster (and so to the rest of the FeS chain) is over 19Å, a relatively low probability electron transfer. So N-1a looks as if it might be functioning as a reflection of the redox state of FMN, which reflects the reduction of the NADH/NAD+ couple. With a highly reduced NADH/NAD+ couple there is a spare electron on N-1a just waiting for an oxygen molecule to convert to superoxide and signal "no more" at the macroscopic (cellular) level... It will be interesting to see how this message is carried from N-1a to wherever it affects cell function but for the time being it's the superoxide generation which I find interesting per se.

I think it is worth pointing out that N-1a is directly sat at the very first step of the first complex of the electron transport chain. Where else would you put the control point? Very neat...

This gives us a nice view of complex I and N-1a linked superoxide generation which sets us up to look at what happens when you destroy complex I by growing mice with the TFAM gene knocked out in their adipocytes... Let's go there soon, if not next.

Peter