The latest animated video from Long Story Short explains the complex requirements for energy harnessing in life. Writing here, I provided an overview of the three steps to chemiosmotic coupling, the universal foundation of energy harnessing. Today, we will delve into the details and consider the proposed shortcuts for a naturalistic origin of life.
Membrane Respiratory Complexes
The first step of chemiosmotic coupling commonly involves the membrane respiratory complexes (more generally, the electron transport chain). This series of protein complexes serve to transfer electrons from the cell’s food source to an oxidizing agent. But direct transfer of electrons would circumvent energy harnessing, resulting in a raw and unproductive release of energy as heat — much like touching a wire between the positive and negative poles of a battery. Rather, a discrete chain of small redox steps is employed to harness the energy, ultimately resulting in 10 protons pumped across a membrane for each pair of electrons that progresses through the redox chain.
Each step must be extremely precise because loose electrons not only waste energy but can randomly damage bystander molecules such as DNA. In a simple prokaryotic life form such as Thermus thermophilus, the common pathway involves respiratory complexes I, III, and IV, which are comprised of 25 distinct proteins in total, each incorporating a precise arrangement of homochiral amino acids which are required for pinpoint handling of electron transfers down each redox step. The end result of the first step of chemiosmotic coupling is a proton gradient across a membrane — a membrane that will not allow protons to pass freely, even though much larger molecules must be transported across the same membrane (this is described by the prior Long Story Short video, here).
The second step in chemiosmotic coupling involves harnessing the proton gradient to rotate a transmembrane nanomachine known as ATP synthase. The rotation of ATP synthase combines ADP and phosphate into higher energy ATP. ATP synthase is made of at least eight distinct proteins and more than twenty protein units.
Nick Lane, an origin-of-life researcher, described ATP synthase as:
…the most impressive nanomachine of them all…This is precision nanoengineering of the highest order, a magical device, and the more we learn about it the more marvelous it becomes.1
However, Lane immediately recanted his insinuation of design, replacing it with a hand-waving explanation, crafted from obligatory naturalism:
Some see in it proof for the existence of God. I don’t. I see the wonder of natural selection.2
Nick Lane came to this conclusion without explaining how life succeeded in harnessing energy before the existence of ATP synthase, and he neglected to describe the additional complexity that is required for assembling ATP synthase proteins3, not to mention the incredible complexity of transcribing and translating DNA to produce such proteins.
We should also consider the interesting pathway that life employs to manufacture ADP — the starting molecule that must first be produced before it can enter the endless recycling pathway between ADP and ATP. As it turns out, producing one molecule of ADP requires at least seven molecules of ATP — a powerful example of circular causality. Producing ADP also requires a cadre of enzymes such as pyrophosphokinase, amidophosphoribosyltransferase, GAR synthetase, GAR transformylase, FGAM synthetase, AIR synthetase, AIR carboxylase, SAICAR synthetase, adenylosuccinate lyase, AICAR transformylase, IMP cyclohydrolase, and adenylosuccinate synthase.4 And of course, production of each of these enzymes requires a supply of ATP — more layers of circular causality.
The Molecules of Life
The third and final step of chemiosmotic coupling involves the generalized application of ATP to power hundreds of reactions to build, organize, and repair the molecules of life. Importantly, the requisite molecules to harness energy via ATP are themselves produced by molecules which require ATP to operate. So, again, ATP must have been available to produce the ability to manufacture ATP.
Origin-of-life researchers are understandably desperate to sidestep all of this energy-harnessing complexity and circular causality. Nick Lane’s attempt to explain the formation of ATP synthase through “the wonder of natural selection” is less than satisfying.
Origin-of-life researchers sometimes look to acetogens or methanogens, simple forms of life that generate a proton gradient across a membrane but replace the respiratory complexes with the acetyl-CoA pathway. However, this requires its own complex set of enzymes and “is anything but simple (even if one ignores the source of the complex cofactors) and is unlikely to represent the first biological economy.”5 Also, methanogens and acetogens still require steps two and three of chemiosmotic coupling. Rather than sidestepping all the complexity of energy harnessing, acetogens or methanogens take a partial deviation down another complex pathway.
Hope from Fermenters?
Others place their hope in fermenters that can produce ATP from a different (simpler) process: substrate-level phosphorylation. However, fermenters appear to have taken more of a degenerate pathway from the norm, rather than serving as pioneers in innovating toward the norm. Fermentation requires complex enzymes to make ATP and produces a large amount of partially oxidized end product,6 which inhibits further growth unless cleaned up by another form of life or by oxidative phosphorylation. Also, fermenters still require steps two and three of chemiosmotic coupling but, interestingly, they run ATP synthase in reverse, consuming ATP to produce a proton gradient. This facilitates active membrane transport6 and helps to expel the mess that they produce.
Another supposed pathway toward the evolution of energy harnessing comes from alkaline hydrothermal vents (“white smokers”). The vent’s natural pH gradient and ability to form small compartments provide hope that they may have jumpstarted the concept of proton gradients across a membrane. However, there are a host of reasons why alkaline hydrothermal vents are not up to the task: Brian Miller, writing in the journal Inference, has shown that the energy density required by life is about 100,000,000 times that which can be produced by the pH gradients of the vents.7 The small compartments in the rock structure have “membranes” that are far too thick for energy harnessing. And they would still require complex molecular machinery to make use of the free pH gradient.8
Energy harnessing in even the simplest forms of life requires extreme complexity and exhibits circular causality. Advocates for abiogenesis desperately seek to sidestep this complexity, but their best approach thus far requires placing blind faith in the wonders of natural selection.
- Lane N, The Vital Question. 2016, New York, NY: W. W. Norton and Company, p. 73.
- Ibid., p. 73.
- Vu Huu, K.; Zangl, R.; Hoffmann, J.; Just, A.; Morgner, N. Bacterial F-type ATP synthases follow a well-choreographed assembly pathway. Nature Communications 2022: 13; 1-13.
- Voet, D.; Voet, J. G.; Pratt, C. W. Fundamentals of Biochemistry: Life at the Molecular Level. Hoboken, NJ: John Wiley & Sons, 2013. Figure 23-1.
- Harold FM. In Search of Cell History: The Evolution of Life’s Building Blocks. 2014, Chicago, IL: University of Chicago Press, p. 76.
- Coleman JP, Smith CJ. Microbial Metabolism: Reference Module in Biomedical Sciences, Elsevier, 2014, https://doi.org/10.1016/B978-0-12-801238-3.05146-1.
- Miller, B and England, J. “Hot Wired.” Inference. May 2020. https://inference-review.com/article/hot-wired (accessed April 28, 2022).
- Jackson JB. Natural pH gradients in hydrothermal alkali vents were unlikely to have played a role in the origin of life. J Molecular Evolution. 2016: 83; 1-11.