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?”