Electron transport: Front-loading and More

We’ve seen that the cytochrome oxidase, otherwise known as haem-copper oxygen reductase, plays the key role in aerobic respiration. There are actually three different versions of these protein complexes, known as type A, B, and C. The type A version is the one that is found in our mitochondria.

Researchers from France recently took a close look at these oxidases, surveying 673 complete genomes from both the archaeal and bacterial domains (Brochier-Armanet C, Talla E, Gribaldo S. 2009. The multiple evolutionary histories of dioxygen reductases: Implications for the origin and evolution of aerobic respiration. Mol Biol Evol. 26:285-97). They retrieved 1,640 examples, which indicates that many prokaryotes carry multiple versions of these oxidases.

Through careful genomic analyses, the researchers were able to track the evolutionary history of these three types of oxidases.

They found that type B oxidases likely originated among the archaebacteria and some copies have been laterally transferred to bacteria. Type C oxidases likely originated among the proteobacteria and transferred to other types of bacteria. But what about the type A oxidases, the type that are found in eukaryotic mitochondria (which are themselves the product of an earlier endosymbiosis)?

A-O2Red are the most ancient O2Red because they likely originated prior to the divergence between archaea and bacteria, thus before the emergence of oxygenic photosynthesis in the cyanobacterial lineage and were largely maintained through the subsequent diversification of bacterial and archaeal phyla.


Importantly, clearly identifiable HGT events do not confound the subsequent evolutionary history of A-O2Red, which indicates that these enzymes were largely maintained throughout bacterial and archaeal diversification, suggesting a selection pressure for the conservation of the native copy during evolution.

This means that the cytochrome oxidase, which functions to transfer electrons to oxygen, was likely in existence prior to cyanobacteria terraforming the planet by creating a oxygenic atmosphere. What’s more, it has not changed that much since this time (for DM readers, think OMD). Echoes of front-loading.

In fact, the researchers also note:

Moreover, this implies that the ability for O2 reduction did not emerge in response to the growing availability of O2 produced by Cyanobacteria but rather that it was an important key preadaptation that would have allowed the very emergence of oxygenic photosynthesis.

It would seem to me the existence of these oxidases were key preadaptations that allowed the emergence of aerobic respiration.

But then again, aerobic respiration might have already been in existence. If you go back to this video, you’ll see that the other two important complexes are the cytochrome bc1 complex and the NADH dehydrogenase complex. Both of these complexes are widely distributed among both bacteria and archaebacteria. This is consistent with the presence of both complexes prior to the emergence of these two domains.

And given the recent finding that oxygenic photosynthesis may have already been in play 3.5 billion years ago, it becomes increasingly plausible that aerobic respiration either existed, or was poised to exist, around this time.

Not only does all this speak to the growing plausibility of front-loading, but it raises two other very interesting routes of inquiry.

1. The first cells on this planet may have been remarkably complex. Or, to put it in more conventional terms, the cells prior to LUCA may have been remarkably complex, capable of both respiration and photosynthesis.

2. The symmetry between photosynthesis and respiration extends beyond the reaction noted earlier and into the components of these processes.

As just one tease to think about, consider again the cytochrome bc1 complex that is the middle complex of aerobic respiration. It is composed of three subunits: the Rieske iron-sulfur protein (ISP), cytochrome b, and cytochrome c1. The functional core of this complex is the ISP and cytochrome b, where the cytochrome c1 functions more as an adaptor. So why bother with this detail? Photosynthesis also employs the ISP/cytochrome b core, it simply replaces the cytochrome c1 with cytochrome f. In photosynthesis, the complex is called the cytochrome b6f complex.

Take 3 minutes and watch this video to refresh your memory about the location/function of the cytochrome bc1 complex.

Now, sit back and watch this video to see if you can spot the location/function of the cytochrome b6f complex.

Hmmm. In respiration, it’s NADH dehydrogenase – cytochrome bc1 – cytochrome oxidase. In oxygenic photosynthesis, it’s photosystem II – cytochrome b6f – photosystem I.

And if that wasn’t enough for you, consider one more fact. In cyanobacteria, the cytochrome b6f complex is used in both photosynthesis and aerobic respiration.

Bunnah says, “Are ya thinkin’ toolkit, yet?”


2 responses to “Electron transport: Front-loading and More

  1. I’m afraid I’ll have to re-read this several times before I understand what you wrote.

  2. Hi Bilbo,

    Let me try again.

    You couldn’t hold your breath for several days. Why is this? Because the cells of your body need oxygen.

    Why do the cells of your body need oxygen? Because the mitochondria in these cells need oxygen. Your cells need functioning mitochondria because they generate the ATP, the energy currency of the cell.

    Why do the mitochondria need oxygen? Inside the mitochondria you will find a series of membrane proteins – the electron transport chain – that function like an assembly line passing electrons to each other. These electron handoffs are coupled to a second function – pumping protons across the inner mitochondrial membrane, which is something that the ATP synthases need in order to make ATP.

    Now, the last protein complex in that chain (cytochrome oxidase) needs oxygen to work, as it passes its electrons to oxygen.

    If it cannot pass its electrons to oxygen, it cannot receive electrons from the second complex in the chain (it’s “hands” are full).

    If the second complex cannot handoff its electrons, it cannot receive electrons from the first complex.

    If the first complex cannot unload its electrons, it cannot receive them from the electron carriers, NADH, which got their electrons from breaking down the sugar your consume.

    In other words, without oxygen, the whole assembly line comes to a stand still. And as a result, it stops pumping protons across the membrane. If that happens, the proton gradient dissipates, and if that happens, the energy needed to run the ATP synthase is gone and no ATP is produced.

    All of this points to the key role that oxygen plays and, along with it, the cytochrome oxidases (the last complex in the chain).

    Okay, for a long time, it was believed that this whole process (respiration) evolved after photosynthesis did, because photosynthesis would pump the oxygen into the air, creating an environment which would provide the selective benefit for the evolution of such a complex process.

    But photosynthesis, as a process that generates oxygen, is restricted to plants, algae, and cyanobacteria. However, since the plants and algae rely on chloroplasts that were once cyanobacteria, conventional thinking would have us place the origin of oxygenic photosynthesis with the origin of cyanobacteria, which are only one lineage among many bacteria. Also, photosynthesis is not found among archaebacteria, a group that is as different from bacteria as eukaryotes are. In other words, photosynthesis arose after bacteria and archaebacteria split from each other, and after cyanobacteria split from other lineages of bacteria.

    What these researchers found is that cytochrome oxidases, the type found in mitochondria, likely appeared before archaebacteria split from bacteria. In other words, prior to the last universal common ancestor of all life.

    So here is a key protein of aerobic respiration, that functions to pass electrons to oxygen, existing long before photosynthesis and oxygen appeared.

    At this point, one could argue that oxygen, and perhaps photosynthesis is much older than we think. A very intriguing thought.

    Or the cytochrome oxidases originally interacted with something other than oxygen (like nitric oxide).

    In that case, it looks as if the process of aerobic respiration came about as the result of preadaptation. In other words, front-loading.

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