
An engineered PET depolymerase to break down and recycle plastic bottles - nkjoep
https://www.nature.com/articles/s41586-020-2149-4
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wdwvt1
The exciting thing about this research is that the authors substantially
exceeded the normal bar for publishing a scientific article (even one in
Nature). They demonstrated industrial feasibility of their proposed enzymatic
process by depolymerizing waste PET then repolymerizing it into a useful item
(water bottle) that met industry standards for strength/ductility and
coloration. I work in microbiology, and seeing research that connects basic
biochemistry to actual industrial implementation (at the lab scale) is really
impressive. There are pieces of their process that would need to be refined
(and the cost of oil would have to rise) for this to be cost competitive with
just manufacturing from virgin PET units, but it's a strong foundation. It's
much closer to realization than most "researchers discover enzyme that does X
and will revolutionize industry Y".

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snovv_crash
It doesn't need to be cheaper than virgin PET, it just needs to be cheaper
than virgin PET plus the cost of landfill/"recycling". With more countries now
refusing to be dumping grounds the cost of getting rid of old bottles stands
to rise considerably.

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loeg
You may be overestimating the cost of landfills.

~~~
stuartd
Landfill costs (understandably) vary between countries. Got a lot of empty
space? Cheap. Crowded? Not cheap at all.

~~~
londons_explore
Even in crowded countries, landfill land area is still a tiny percentage.

Its more environmental laws and how strict regulations are about seepage.

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maxander
I don't have access to the article right now either, but I would like to note
to the "this enzyme is going to eat all our stuff!" commenters that, in
general, most enzymes don't work very effectively in ambient conditions. The
abstract doesn't go into detail into how they acheived the "90% in 10h"
metric, but I would assume this was in a precisely controlled solution,
incubated at an optimal temperature, with fresh enzyme that hadn't had a
chance to start denaturing. You aren't going to be able to spray this on the
plastic siding of someone's house and watch it wither away (even if home
siding was PET and not, IIRC, vinyl or something.)

I _guess_ I can't rule out at that some recombinant bacterium expressing this
enzyme might escape the lab and start causing PET objects to get moldy or
develop a patina if left out too long [0]. But that would require that this
enzyme somehow benefits the bacterium enough that natural selection wouldn't
favor dropping the gene for it.

[0] Hrm, there's a thought - what if your refrigerator needed to be
refrigerated?

~~~
perl4ever
This all sounds sensible and reasonable, but just _at this exact moment_ ,
it's hard to feel _totally_ confident that something bioengineered can't
_possibly_ create a plague inadvertently. And I'm assuming that covid-19 _isn
't_ in fact an example.

It made an impression on me, something I read on HN not too long ago,
comparing an epidemic to a nuclear reaction - it's really hard to create a
critical mass, and when it's dispersed that's it; the really _acute_ problems
are local and temporary, despite peoples' fears of invisible radiation and
contamination. But once a global epidemic gets going, essentially the whole
world may have reached critical mass and has to be diluted. Maybe obvious when
stated, but presenting the comparison in the context of the fears people have
of nuclear weapons/power is the point.

The probability is low, but the stakes in the near future of biotech seem
higher than anything else people can mess with.

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econcon
It's interesting, I've been helping plastic situation by buying shredded
plastic scrap and turning it into filament for 3d printers, you need to mix it
with virgin plastic (30-70 or 40-60)

Here's the project details: [https://medium.com/endless-filament/make-your-
filament-at-ho...](https://medium.com/endless-filament/make-your-filament-at-
home-for-cheap-6c908bb09922)

Communities can setup their own machines and create filament from scrap
plastic, this is good enough for 3d printing where your life doesn't depend on
it.

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HarryHirsch
The exciting thing is that there is real progress in increasing temperature
stability of enzymes. That's the point of the article, engineering an enzyme
that is still active above the glass transition temperature of the substrate.
I remember a paper where they tried random mutations all over some enzyme, it
was just mindless make-work for an army of graduate students. This one is
different.

~~~
dnautics
I can't read the article due to paywall. How did they achieve this
thermostability? Supercharging the surface?

~~~
HarryHirsch
Site-saturation mutagenesis in the active site followed by introduction of
disulphide bridges.

~~~
dnautics
Man. I would have guessed that adding disulfide bridges would decrease
catalytic conversiom rates by making the protein too rigid.

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makapuf
For a layman like me, what does this enzyme decompose the plastic into?

~~~
jkqwzsoo
PET is formed via the condensation of ethylene glycol (ethane-1,2-diol, aka
EG) and terephthalic acid (1,4-benezenedicarbyoxylic acid, aka BDC). In the
process, a proton (H, a hydrogen) is lost from the carboxylic acid (COOH ->
COO) and a hydroxyl is lost from the alcohol (R-OH -> R). The H and OH combine
to form a water (hence "condensation") and leave, and an ester bond (RCOO-R')
is formed. Amide bonds (RCONR'), the backbone proteins and materials like
Kevlar, spider silk, and Nylon, are similar, but less flexible and harder to
break.

This condensation reaction (and others) is reversible. If you heat PET in
boiling water, you can begin to depolymerize the polymer into its constituent
monomers by having a water molecule consumed in the reaction (RCOOR' \+ H2O ->
RCOOH + HO-R').

However, PET is a rigid plastic that acts as a barrier material (that's why
it's used for bottles and packaging), and has a high glass transition
temperature (the temperature at which polymer chain mobility becomes broadly
possible -- the polymer is in equilibrium, while it is kinetically trapped
below the Tg). This inhibits the breakdown of PET, particularly at low
temperature.

Hence, the sentence from the abstract:

> With a high ratio of aromatic terephthalate units—which reduce chain
> mobility—PET is a polyester that is extremely difficult to hydrolyse.

Aromatic rings are just that -- rings. They don't flex a lot and can't undergo
a lot of thermal motion, can't change conformation, etc.

> Here we describe an improved PET hydrolase that ultimately achieves, over 10
> hours, a minimum of 90 per cent PET depolymerization into monomers, with a
> productivity of 16.7 grams of terephthalate per litre per hour (200 grams
> per kilogram of PET suspension, with an enzyme concentration of 3 milligrams
> per gram of PET).

> We also show that biologically recycled PET exhibiting the same properties
> as petrochemical PET can be produced from enzymatically depolymerized PET
> waste, before being processed into bottles, thereby contributing towards the
> concept of a circular PET economy.

So, they depolymerize the PET, an subsequently repolymerize the terephthalic
acids to produce new PET. I haven't clicked through to find if they recycle
the EG as well, though.

~~~
airstrike
Yours was a wonderful summary – I've actually favorited this comment. Thank
you.

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Gatsky
See the end of the article, patents were taken out starting 2017, and there is
a company [https://carbios.fr/en/profile/](https://carbios.fr/en/profile/)

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DavidPiper
What does this mean for PET already in landfills?

Can we trawl through our landfills for old PET and bring it back to
(perpetual) life with this process?

~~~
mediaman
It would not make economic sense. You'd spend far more money and CO2 output
moving mountains of garbage and trying to sort than would ever be recovered by
recycling some scraps of plastic. The supply chain of incoming recyclable
material is very important to make any recycling process work well.

But yes, it would be possible.

~~~
tandr
Would spraying garbage or landfill with bacteria that could produce this
enzyme work? At least at slowly reducing volumes of plastic in landfills into
liquid-y form?

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stenlix
Don't have access to the full article so I've only read the abstract, but
what's the feasibility of deployment here?

~~~
lowdose
[https://sci-hub.tw/downloads-ii/2020-04-09/c8/10.1038@s41586...](https://sci-
hub.tw/downloads-ii/2020-04-09/c8/10.1038@s41586-020-2149-4.pdf)

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JoeAltmaier
Its got to be cost-effective. That's the gold standard. Is it cheaper than not
breaking the plastic down? E.g. other solutions - burning, burying...

Not a moral argument. Just a practical one - the free market isn't going to
use this unless it has a short payback period.

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kumarski
There's complicated stuff for figuring out if it's useful:

\- Energy in vs energy out

\- The price fluctuation of the underlying feedstocks

\- Scalability - (chemistry is 1000x harder to scale things than comp sci.)

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EGreg
Can’t plastic already be melted down and reshaped?

Now the question is how to collect plastic in the wild

~~~
barney54
Yes, but for plastics like PET that downgrades the plastic. This process
breaks it into monomers so the recycling can be truly circular.

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battery_cowboy
> An engineered PET depolymerase to break down and recycle plastic

Wrong title, and it's not for all plastics. We all complain here about
journalism taking the wrong conclusion and I'm just pointing out the factually
wrong message in the title.

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maxander
"An enzyme decomposes 90% of plastic in 10h" is more succinct and readable
than the real article title, and it's not wrong. A depolymerase is a kind of
enzyme [0].

[0]
[https://en.wiktionary.org/wiki/depolymerase](https://en.wiktionary.org/wiki/depolymerase)

~~~
battery_cowboy
I didn't have any trouble understanding it, but I guess we'll just dumb it
down into the factually wrong message...

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baq
i don't want this anywhere near my home please

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anonuser123456
Because?

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greglindahl
I have a large number of plastic things inside my home that I hope do not
become decomposed.

Garbage, sure, decompose away.

~~~
the8472
Enzymes are proteins. They're applied to powderized plastic in a bio-reactor.
It's unlikely that they'd be able significantly attack bulk plastics before
they themselves get broken down.

~~~
lostmsu
Wait until some organism eats them and learns to reproduce.

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jhoechtl
Sounds like a weapon

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avalys
That’s alarming. Sounds like an effective way to destroy every modern
manufactured good.

~~~
ladberg
It only destroys the goods if you let them soak in the solution for hours. How
does that make it dangerous? If someone wanted to just destroy goods, fire is
way more effective.

