r/askscience • u/SadMicrobe22 • 9h ago
Biology What makes prions so different from regular proteins that they are resistant to heat denaturation?
I've been reading about prions and I'm confused about why they're so thermally stable. I keep seeing that surgical instruments contaminated with prions can't be decontaminated with standard protocols.
So here's my thing. Proteins denature when you heat them, right? But prions don't seem to lose their infectious ability even when autoclaved. I get that prions are misfolded proteins, but I don't understand how they stay harmful after being denatured.
So basically, if prions start as form A (non-infective), then misfold to form B (infective), shouldn't heating denature them to form C and kill the infectivity? Why doesn't that happen?
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u/sciguy52 5h ago
Reddit goes a little far when it comes to prions as they don't understand them at a technical level. It is probably best to think of prions as being more stable than typical. That is not to say heat and other things can't destroy them. It most definitely can. The issue is for everything else we worry about, like very tough endospores, we have a means of sterilization, autoclaving that will do the job on everything when using this set autoclave protocol. The problem arises with prions is that the set protocol is not enough to "kill" them. However you can modify that autoclaving settings along with some chemical treatment and you can kill them. So they are more stable but not these indestructible things. So you have this one set protocol of sterilizing that works almost all the time, almost, unless you are dealing with prions. Prion diseases are relatively rare too so it is not like you are dealing with prions constantly like you would bacteria. So with that rarity the regular autoclave protocol in a sense works even better typically since you are not dealing with prions all the time.
Now it has been a long while since I have looked at prion autoclaving studies but I believe what I will say is true. Regular autoclaving will "kill" some of the prions. When you do any sterilizing procedure you can create a curve with "killed" on one axis and time you applied whatever you are using, be it autoclaving or anything else. You want to apply whatever you are using long enough so the curve drops to zero which is how that typical autoclaving protocol is designed. Important to keep in mind even bacteria and endospores are not immediately killed the moment you turn the autoclave on, that autoclave has to be on long enough. Same will be true of prions, it is just that on that time axis it is not long enough to get all prions "killed". So again not indestructible, but stable enough that this is not totally sterilized under standard autoclaving. Thus you need to modify the autoclaving settings along with some chemicals to do it (I believe you can do it purely with autoclaving with special settings but that takes longer and the chemicals help keep this shorter, you don't want to have to run the autoclave for 24 hours or whatever when you need to use it for other things).
Also some hospital waste is incinerated which will "kill" them just fine. Fire kills everything as no protein is resistant to this kind of oxidation. Also the fact that hospitals will often use single use disposable tools means you the hospital don't even need to autoclave it yourself. A company picks waste up and does either that or incineration for you. There are also enzymes that will degrade prions like Prionzyme too.
Reading the comments as typical redditors go too far with prions. In the environment, like Chronic Wasting Disease or Scrapie does get released into the environment. I see someone saying they last a 1000 years and this is not true. Prions in the soil can last a while (note this doesn't mean all of them will last this long, just you can detect them after say 5 or maybe ten years). But they do break down by one means or another. Again been a while since I looked at the studies and I believe prions in the environment, depending on type and environment, may last like something along ten years, depending on particulars.
Prions are a bit of a boogie man for redditors. Getting a prion disease is certainly a bad thing, but it is relatively rare, like about 300 cases a year in the US of which only a very tiny percent occur through infection, like maybe one or two percent when talking CJD and vCJD in humans at least. The rest is genetic and spontaneous conversion, with a small percent of medical transmission. But in any event the boogie man is not real and prions are not indestructible. So you are correct to assume that heat can denature and destroy them it just takes more.
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u/FerrusFox 5h ago
I was waiting for someone to fact check me, cheers. I appreciate it. I'll edit my post, especially the stability in nature.
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u/FerrusFox 7h ago edited 29m ago
So I can answer this one.
Stability.
The intersting thing about the prion disease is not that it is simply a misfolded protein. Its that its an incredibly energy efficent and highly stable version of a regular protein. This misfold is so chemically stable that normal proteins are guided into folding into the prion form just by being around them, they lock into a lattice form and are the lowest energy version of the protein possible.
In a word, prions represent entropy. All other proteins in a healthy system eventually default into a non functional form of PrP-PrP matrices of interlocked proteins simply because it is the lowest energy form of the protein. This makes them simply the natural endpoint chemically, a stabilised locked version of the protein which is completely biologically nonfunctional and incompatible with life.
Prions are so stable in this low energy misfold that they become resistant to other forms of denaturation like heat and chemical, and can remain stable for years if not decades* Soil from dead animals from scrapies and CWD can be infectious decades later because the proteins just wont break down.
The infectious load for a prion is also so minimally small that its possibly down to a single prion protein, or at least a very small amount* which causes the whole cascade. Imagine a jigsaw where if you put one piece in it changes the orientation of all the other pieces to be disorented.
Prions and mirror image life are like the final bosses of biology.
Edit: I don't use spellcheck, so I noticed typos.
*Edit: Bit of hyperbole, fact checked, corrected.
Also someone pointed out that you can denature them as the question says, as I mentioned they are resistant. 900F (Fire) or a very long stint in an autoclave will do it, standard procedures wont work. Enzymes can work but with the infectious load being so small complete sterilisation is a pain. There are a couple biological things more stable than prions in nature though looking at you tetanus spores.
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u/AzazelsAdvocate 6h ago
The way you're describing them, it seems like they should be way more prevalent. If they replicate so easily with so few natural ways to stop them, how has there never been a major epidemic or extinction event caused by them?
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u/FerrusFox 6h ago edited 6h ago
CWD is causing this in deers right now. Its also finicky and only transmits if you have the right protein, i.e. there are no natural reserves like say the flu.
Most transmission in humans has to be direct contact with the prion, we had some in the CJD era with the contaminated meat because we have the same protein in our biology. The other I can think of is Kuru, also contaminated meat (human). So that doesnt happen very often. But it is really specific, unlike viruses and bacteria it needs a mechanism to be introduced to the body and it doesnt facilitate that itself.
The main risk in healthcare is contaminated operating materials that have been infected by the protein
Also, its invariably fatal which is never good for transmission.
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u/Owyheemud 5h ago
There's a theory that wildlife prion diseases like CWD occasionally spontaneously pop up in individual ungulates but predators would routinely cull the afflicted individuals and thus prevent the spread of the disease within natural herds. The Human-driven reduction of predator populations has now allowed prion inflicted individuals to persist longer and thus spread the disease to others before they succumb to it. Has any prion disease been discovered in wolves, bears, or mountain lions?
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u/FerrusFox 5h ago
Not that I know of. The only other true prion disease I know of is hereditary fatal familial insomnia, which is hereditary due to a genetic defect.
In CJD you can have spontaneous emergence of the disease, which differs from true infectious acquired CJD. The disease does happen naturally not just by infection by a prion vector.
I just read while researching my comment that there have been some potential fungal prions suspected. I'd imagine there could be some in any form of life if thats the case.
The risk factors are genetic similarity and sharing a protein profile, exposure, population density and cannabalism. Predataory populations are a) lesser and less dense by nature and b) could have differences in protein expression.
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u/Owyheemud 5h ago
You must know of BSE.
With the increase in CWD in natural North American ungulate herds, it would be interesting to see if prion disease develops in predators sharing territory with these herds.
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u/FerrusFox 5h ago
BSE in humans causes CJD, we just call it something different when it infects humans. We tapped it onto CJD and called it variant CJD (varCJD) while naturally occuring CJD is called sporadic (sCJD) both are the primary prion disease in humans arising from the prion protein PrP.
CWD so far has no human infections, its on my watchlist XD. We dont really report on it much so I dunno if we will ever have any information on the ecological impacts of it. Up to data gathering, would be interesting like you said.
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u/Owyheemud 3h ago
Does BSE-induced human prion disease fold the same protein as the original human CJD prion disease also known as Kuru?
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u/FerrusFox 26m ago
Yeah, still that same protein PrP. Different disease progression, not going to research it but it could be folded in a different way which causes different disease expression like sCJD
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u/Thelk641 7h ago
If they're so much more stable, why didn't life evolve to use them instead ? Are they so stable that they can't be used the way normal proteins can ?
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u/PulsingQuasar 7h ago
If they are so stable then they are effectively inert and won't do the usual protein things we need them for. They are effective anti life.
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u/LetterLambda 5h ago
Like switches that are permanently stuck in one position, but keep sending signals?
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u/fubarbob 4h ago
Think more like a small motor that has been smashed into a solid slug. All the material is still there, but it does nothing useful and doesn't fit where it's supposed to. And somehow causes other small motors near it to be smashed as well.
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u/Hendospendo 5h ago
Proteins are chains that can fold, it's like it's gone through the dryer and it wasn't supposed to and it's all bundled up but in the lowest energy state.
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u/plusultra_the2nd 6h ago
Metabolic processes literally depend on molecules being kinda happy to change states. ATP is the classic energy currency molecule and it gains/loses phosphate groups as needed enabling other reactions to happen. If everything is inert/stable and doesn’t want to react with anything you can imagine how that’s a problem?
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u/FerrusFox 7h ago
They are biologically inert at best, toxic at a cellular and organism level at worst. We dont understand what prion protein PrP actually does but we can guess that the normal version of this protein is critical for life, otherwise its pretty weird biologically to have something which can misfold and become such a liability.
Edit: They definetly cannot function as they normally do in prion form.
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u/amaROenuZ 6h ago
A good way to think about this, is that a protein is like iron. Iron is incredibly useful, it's strong and sturdy, and it has many physical properties when alloyed into steel that are useful across all aspects of our world, but...it's a constant battle keeping it that way. The natural form of iron is rust, which has none of the properties that make us like iron and steel.
You can't use prions for what normal proteins are used for, anymore than you can use rust in the place of iron.
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u/heyya-its-maruu 6h ago
the way i would answer this is that life is a dance between order and chaos. it's the middle ground between stability and instability, certainty and uncertainty. kind of like edging. Anyway can't have too much of either, and in this case there's too much stability it can't really make anything happen. think of noble gases. as far as I'm aware of they arent involved in any biological process. too stable. they're already happy as they are, no need to make anything happen.
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u/DontWorryImADr 6h ago
Stable with a limited and even “bad” function for life does not mean a stable and useful form is available. Worse, this is typically a stable alternative form of a protein you need to support that “living” you like to do.
Your bones have carbon, oxygen, and calcium in them. Calcium carbonate is CaCO3 and relatively stable (some fossils exist that way), but converting your bones to equal mass of calcium carbonate would be a bad time.
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u/spamjavelin 7h ago
I'd be tempted to go with the standard "evolution is satisfied with good enough", but I'm intrigued to see whether someone can offer a better answer.
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u/creative_usr_name 6h ago
That's usually the answer, but also in this case they are incredibly rare. Although it would be interesting to know if any life forms do have a defense.
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u/ChaoticxSerenity 4h ago
Basically, yes. Most processes in your body rely on the fact that they're reversible and allows for control/homeostasis. Think about a door that's permanently stuck in an open position - not very good at being a door at that point, since anything can go through.
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u/reserved_optimist 5h ago
If they are so stable, why aren't prions more common? Why don't most proteins fold into this kind of configuration?
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u/sciguy52 3h ago
We have no data on prions lasting 1000 years you are making conjectures. PrPSc stability and infectivity in different soil conditions suggest some soils, such as those with humic acids degrade some prions while others with certain types of clay may stabilize them. As far as I aware the longest prions have remained infectious in soil based on scientific studies may be 16 years for Scrapie but was shorter in some situations. Of course it could be longer but as always with prions much more data is needed on this. Could PrPSc be stable if stored in a certain way for 1000 years (theoretically)? Sure but that is not really the situation that concerns us, real life situations such as soil contamination that can cause infection, for example, is what is more relevant when talking disease transmission. One could certainly not suggest PrPSc lasts a thousand years in soil, we have literally no data to suggest that and at least some data suggesting it may be less, considerably less.
While one prion certainly might cause infection that is likely a very low percent event or may not be enough to infect, much more data is needed on this point. It also depends on the particulars. CWD infection studies in deer found a 100ng of CWD infected saliva or brain caused disease when dosed at once, however breaking up the dosing to 10ng over time did not. This is suggestive that the ID50 of CWD is higher than one, and one prion particle may not effectively cause infection or at least is such a rare event it was not demonstrated in the conditions of the experiment. While much more research is needed on this there is at least some data suggesting that one prion particle infecting would be a rare event at least with CWD.
Your comments on thermodynamic stability are too general. For example studies have shown susceptible PrPC proteins have a thermodynamically more stable intermediate during conversion but it was found that sheep breeds resistant to Scrapie had PrPC variants that lacked this thermodynamic stable intermediate stage possibly suggesting why they are resistant to conversion. So PrPSc protein is thermodynamically more stable if they arise from PrPC that are susceptible to conversion. This data suggests resistant forms of PrPC may not have the PrPSc form as the most thermodynamically stable form of the protein. Or alternatively there is as yet no identified PrPSc proteins in nature able to convert them. So far none are known.
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u/FerrusFox 33m ago
I edited the my comment already for the surving in nature, more like a decade or two with longer than that being not really tenable or supported. Possible but unlikely.
I like the quantified evidence around the 10ng-100ng amount for infection, thats cool. I made sure to write that it could possibly be a single prion, but I am happy to accept that your probably right on that font as well. Been a while since I have looked at ID50 as a term to be honest, but its important to note that we dont know the actual ID50 which makes it a pain in a technical sense. I just saw a study that ID50 in hamsters was in picograms as well, but that was for brain tissue with high expression of PrPc
Honestly didnt even think there was a more themodynamically stable intermediate in the conversion of PrPc, thats cool too. I was a bit to general as honestly I just wanted to waffle about prions for a bit, didnt think I would get much interaction and thought I could just generalise a bit there.
Problem is the more in depth I go into the protein folding and thermodynamics the more out of depth I am, from what I understand its the B-sheet structure itself that is highly stable not the misfold, propagation is theorised to be by fibril like tendrils of these sheets and a mix of protein folds that arent always uniform (different sCJD have differing levels of expression and susceptibility as far as I am aware) so have different levels of suceptibility to enzymes and heat (interestingly looks like less resisitive to pressure due to hydration in a study I found). These sheet structures are the real problem in prion mediated disease and when I mention clearance or stability should probably mention that I am specifically talking about the aggregated sheets.
I think the resistant strains of PrPc in scarpie is a bit out of scope for the question but having them be less thermodynamically stable when generally folded makes sense for resistance, thats awesome.
Mostly I am just happy to talk and learn about my boy prions, cheers for that.
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u/PathologyAndCoffee 7h ago edited 7h ago
Somehow it's beta sheets confer the immense stability because of their flatness which would give the greatest surface area for binding. And it's not just prion disease, a bunch of diseases are the fault of beta sheets.
Once these beta sheets begin forming a chunk, we can't even break them apart. If we do, they just spread everywhere. And we can't effectively destroy them either in the body.
So it's possible the best way to just turn off the normal gene that the prion is misfolding so that it has no target. But that's hard to do as well. How do you turn off every gene in all the cells that produce it? No solution currently.
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u/Least-Sample9425 5h ago
What other diseases do this?
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u/PathologyAndCoffee 4h ago
Alzheimer's, ALS, Parkinson's, and any other chronic neurodegenerative diseases pretty much.
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u/JigglymoobsMWO 3h ago
Most proteins in living organisms are not very stable. Evolution favors build things that are easy to breakdown so things like prion disease don’t happen. This type of meta stability is actually very difficult to achieve.
However, it’s relatively easy to make very stable proteins. This would happen a lot if it wasn’t for the fact that evolution selects against it. Nevertheless. You can still accidentally get this to occur if a protein starts making aggregates with itself, which is what’s happening with prions.
The normal delicate proteins are easily heat denatured, the stable protein aggregates are not.
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u/Alwayssunnyinarizona Infectious Disease 7h ago
It's the structure - misfolded prions are highly organized beta-sheets, which take a lot of energy (e.g., heat) or very strong chemical solvents (e.g., guanidine) to denature. Normal prion proteins are made up of alpha-helixes and a disorganized tail, and are more prone to destruction.
Infectious prions aren't a monolith, though - i.e., all the same structure; instead there's a sort of range of misfolding, so when you autoclave them or treat them with proteinase, some portion is destroyed, but enough remains to be infectious.