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I'm determining some of the biochemistry for some artificial organisms created for various commercial tasks, from industrial to military use. I'm trying to figure out if augmenting the energy molecules could make the metabolism any more efficient. The main reason why I'm focusing on modifying ATP into Ap4 (adenosine tetra-phosphate) or even adenosine penta-phosphate (which I will call Ap5 for ease) is because the biochemistry of these organisms is going to be relatively similar to that of known life, so molecules such as phosphoenolpyruvic acid (PEP) may have other functions like signaling or producing ATP itself that using them in the same way that ATP is used could interfere with, though I could be wrong on that.

I'm thinking that the compounds with "extra" phosphates will help in times of high energy need, such as during exertion or stress. Most of the time, when the creatures are at rest or not under exertion, they would primarily use ATP for energy. But because some of the created organisms have extra dense muscle or complex structures such as internal computers that require extra energy to use, I figured that adding extra phosphates could prolong the time that the Ap4 or Ap5 can be used, as instead of going from ATP → ADP + phosphate, or ATP→AMP + PPi, there could be longer reactions such as Ap4→ ATP + phosphate that can energize more locations, or even larger bursts of energy at once coming from Ap4→ AMP + PPPi, which could provide greater amounts of power to more demanding structures.

Ap4 and Ap5 occur in nature already, as it has been observed to be synthesized by yeast Acetyl CoA. Ap4 has also been found in rabbit eyes and rat aortas.

My main concern is that this is not actually that effective. Dephosphorization of ATP to ADP and ATP to AMP yield about -31kJ/mol and -38kJ/mol in Gibbs free energy respectively, and dephosphorylating ADP leads to about -31kJ/mol as well. But would it be as energetically valuable or even worth it to attached on an additional one or two phosphates? I'm not a chemist (obviously), so I don't want to just go right out and say that for example, dephosphorylating Ap4 into ATP leads to -31Kj/mol, Ap4 into ADP leads to -38kJ/mol, and Ap4 into AMP leads to -45kJ/mol (my unscientific assumption that each additional phosphate bond adds 7kJ/mol afterwards, though this could probably be a lower amount.)

I think that the conditions for equilibrium would be the major factor, as in theory, the extra phosphates will make the molecule less stable and easier to accidentally hydrolyze. Perhaps with exertion, the cellular chemical equilibrium can shift, allowing for these extra phosphates to come into play?

My first thought was that raising the pH of the cells that need energy to around 7.1-7.4 as exertion progressively increases, as mammalian skeletal muscle cells tend to have lower pH values around 6.8-7.1, and human muscle cells at rest have been recorded at 5.99. The reason why I hypothesize raising the pH could make Ap4 or even Ap5 more stable is because Ap4 has been known to occur in the aqueous humor in the eyes of rabbits. At least in humans, the pH of the aqueous humor is around 7.1-7.4.

Not sure how well this could hypothetically work, or if it even makes sense. Raising the pH of various cells would definitely require some overhauls of other parts of the cell such as in enzymes or signaling molecules, particularly in muscle cells, which for my purposes is totally fine because these critters need to be built (almost) from the ground up and use artificial directed evolution in addition to computer simulation to make "blueprints."

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    $\begingroup$ I’m voting to close this question because it seems like a question for chemistry Stack Exchange. Maybe migrate it? $\endgroup$ Feb 18, 2021 at 18:29
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    $\begingroup$ After 4.5 billion years, if it hasn't been done, the answer is no. ATP is incredibly important in cell biology. If there was a way to improve it it would've evolved already. Either ATP is optimal, or the cell machinery for producing it can't make a better molecule. $\endgroup$
    – stix
    Feb 18, 2021 at 19:57
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    $\begingroup$ You are rapidly diving into the "we don't actually know how a lot of biology works in detail" area, so you can probably get away with just saying that it works. Most people reading won't be chemists, and the chemists won't be unduly bothered $\endgroup$
    – Sol
    Feb 18, 2021 at 22:50
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    $\begingroup$ Aside from the two answers, molecule size is also an issue. Cells are limited in their maximum storage capacity of ATP to just a few seconds. The molecules of higher phosphates might simply not be efficient or even fit well into the cell structures. You might have to redesign mitochondria from the ground up and possibly make them larger to fit such molecules and build them properly. $\endgroup$
    – Demigan
    Feb 19, 2021 at 11:16

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You are correct that ATP is not particularly energy dense. For its primary use, that is good thing.

Lowest common denominator

Its the rough equivalent of a one dollar cash. Usable for all your day to day financial transactions. As much as you don't want to use gold bars or stock certificates for day to day transactions. Cells don't use long chain fatty acids and carbohydrates directly to pay the energy cost of processes.

Its like you are asking if we use hundred dollar bills instead of one dollar bills, can you make bigger purchases? Currently cell processes just use more one dollar bills/ATP as needed. If you increase the minimum bill size the increase the minimum energy cost. An efficiency penalty.

Bodies will overheat before energy limits hit

You seem to be concerned about high energy demand systems. It is my understanding that thermal dissipation will be concern long before lack of ATP for said process becomes an issue. Cells (as a large group) can use enough energy to cook other cells(ie fever), The cash(ATP) in circulation, is not the limiting factor for cell processes.

Not impossible, but inefficient, and not the limiting factor

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This may be not the answer you are looking for but I will approach the problem from a worldbuilding perspective, as I'm too rusty and there are chemistry and biology SE's. Plus, there is a big hole in your plot which I would like to address.

You may be heading in the wrong direction

When we talk about high metabolic rates, the first thing which comes to mind is birds. Google for "bird metabolic rates", and sure enough there are different studies that investigate different aspects of avian metabolism, including the factors which affect it to be higher, among others.

On top of my search results, I see Avian Energy Balance & Thermoregulation, which is simple enough and can be useful for the case. Let it be our source of data.

Birds have high basal metabolic rates & so use energy at high rates. Among birds, songbirds (passerines) tend to have higher basal metabolic rates than nonpasserines. And, of course, the smallest birds, hummingbirds, have the highest basal metabolic rates of all birds. In general, basal metabolic rate (or BMR) is related to mass, with larger birds expending less energy per unit weight than smaller birds.

enter image description here enter image description here

As you can see, small birds have 10-30 times faster metabolic rates than humans. So if you dive deeper into the topic of what makes them be so energetic, you can draw scientifically accurate reasons for your artificial soldiers or creatures.

Some Fundamentals

To be more specific to your question, you are missing some vital parts which have a greater impact.

Let's look at Krebs cycle

enter image description here

The details are not important for us, but the overall picture is relevant. Observe that there are 10-11 reactions at least - and each of those can be a bottleneck. So your most significant mistake is to consider that ATP part is the only source of the bottleneck, and if one looks at it from the perspective of excursion power, it may look true but it's more like burst power at best. More important is how fast AMP is restored to ATP and that is defined by Krebs and(or) others.

As we saw on Hummingbird, the difference it can make is over the order of magnitude of continuous power. And if there is need for more burst power, changes in concentrations of ATP can have more impact than Ap4 or Ap5, which may or may not bring +20% of something, when a simple 2 times increase of concentration brings you +100%.

Reactions of metabolic cycles have low potential barriers (not sure how it is called in English, energy to initiate the reaction is low, relatively) and for this reason, small changes in temperatures have a good impact on the intensity of the reaction - but it has to be understood that it is a function of many factors, and there is a limit to how far we can ride the vehicle.

Difference between Artificial and Natural

Are birds fundamentally different from others since they have metabolic rates higher by an order of magnitude? Unless there is data indicating otherwise, we probably have to stick to no, because of evolution and all that.

With naturally evolved species we have to understand that metabolic rate is not the goal, survival is. Therefore, those rates are a tool and are a function of means, environment, and seasonal changes in which the birds have to survive. And burning at max is not necessarily beneficial to survival.

So naturally evolving species have to compromise. Even a 0.01% improvement that shines in some specific situation that doesn't necessarily occur all the time, can be beneficial over generations and population numbers.

Hence the food sources, nutrition values, scaling down the concentrations of enzymes that work in metabolic cycles, etc.

Synthetic life does not have most of those restrictions, so its goal is defined by those who create them and it probably isn't survival in a natural environment. So we can create a big body (at least by mass) species with the metabolic rate of a hummingbird, say as big as an elephant for example. From the perspective of natural survival, it would be insane just by size and accessibility and the extent of the area/territory which is needed to feed such a creature. But for us, it is a lesser problem as we can probably attach some energy converter and supply that creature with the energy it needs.

For our 5000kg elephant hummingbird it will be 390kw per organism. Quite a lot actually, it can be directly compared with some lower-tech versions of combustion engines. Inefficiencies aside, injecting the energy to our elephant as waste heat from that process can be kept external, but still, we hit some physical limitations which synthetic organisms are subject to, which we have to solve through the design of such an organism - but in the shape of an elephant, it will be quite hard to dissipate 390kw. This again depends on the technology available - we still can solve that by an external embedded cooling system, but again such things will obviously be subject to physics.

  • Just a side note about how much 390kw is in nature. 1 tonne of dry grass/straw contains about 3900kwh of energy. Coincidentally, that number is quite close to what we need. So our elephant hummingbird needs to digest at least 100kg of dry grass per hour and about 2.4 tons of it per day. Again, coincidentally that amount of grass grows on 100x100m area (yields are 1-3t per hectare). Dino descendant is that u?

    By itself, it looks doable, but they have to procreate, and that occupies some common area, causes competition, etc. Again possible, I guess in dino days things were good enough to allow that, but we all know what happened - the environment was shaken and they didn't make it.

    As grass is not necessarily the best food source there is - efficiency-wise and factoring in speed of digestion - actual numbers including those factors will bring a few times higher consumption rates and therefore area necessary. Since grass grows for 3-4 months, a season - the total area needed to feed one organism will be a few square km per creature.

  • Energy dissipation depends on the surface area, so a flat body plan can dissipate 390kw better - idk probably a centipede?

  • Connected google overlord just to check and they have answered:

    How much do elephants eat and drink per day?

    Adult elephants can eat between 200-600 pounds of food a day. As herbivores, elephants consume grasses, tree foliage, bark, twigs, and other vegetation daily. Elephants can also drink up to 50 gallons of water a day about as much as a standard bathtub holds.

    Hence, I have to conclude that I didn't shoot high enough with our elephant hummingbird, they still do not put nature to shame.

Summing Up

In general, limits of what's already available in nature are higher than what we observe typically, and those limits encompass low and high extremes. Biological systems use the building blocks available to adjust themselves to the conditions they survive in.

Synthetics are freer from evolutionary restrictions, so we can have performances higher than anything that is typical in nature, combining the best from all different species. For example, humans do not have the best muscle tissues, there are creatures that have more robust muscles as tensile strength goes (and that can mean more strength in a smaller package). However, a trained human is a top predator because they can exhaust any creature (land ones I guess, so again it mammal vs mammal) by not stopping in pursuit of them over days. Sure, it's more from our hunter-gatherer times, not many tribes of today can do that(my guess), but there are some which can still do that(seen about them).

So the whole biological system/library is our lego blocks, we are more restricted by our knowledge and how much we have read in that library. There is still a long way to go, but in the future, we can imagine it not being as big of a limitation as it is today. Hopefully, we won't burn it down like The Great Library of Alexandria before we do read it, lol.

But also, physics trumps all the systems, including biological ones, so do not forget physics is king.

Things I forgot.

With metabolic festivities, I totally forgot about the respiration problem, and more like blood capacities specifically. But respiration limitations is probably out of the scope of the question, but they also have some science behind it and works studies on that subject, and after all it still connected to ATP. It's more archeological as of today's, biological differences environments in previous eras. So it may have the sense to dig in that direction, however, it may be less applicable to synths as we may address this and other problems in ways not accessible to biological systems, which I forgot to mention as well, did I? hm, I forgot ...

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  • $\begingroup$ it is surprising how a better spelling plugin is capable to bring more spelling errors - that's progress ... I hope caught them, but if not anyone is free to edit $\endgroup$
    – MolbOrg
    Feb 18, 2021 at 20:06
  • $\begingroup$ Thanks, @Baron_vonCernogratz for the edit, it looks way much better now $\endgroup$
    – MolbOrg
    Mar 14, 2021 at 14:15

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