Back in May 2020 our friends in the science world brought us news that nature had evolved an enzyme
that could chomp through the strong polymer chains in plastics like polyethylene terephthalate or
PET, which is what plastic bottles are made of. It was a very exciting discovery and the video
I made on the subject at the time has had more views than any other video on this channel, so
I guess you guys were pretty keen to hear about it too. Ever since that first enzyme was stumbled
upon at a Kyoto recycling plant in Japan back in 2016 scientists have been working to enhance its
capabilities with the goal of producing a set of super enzymes that could be even more helpful in
our interminable fight against plastic waste. And in March 2022 researchers achieved another
leap forward in that struggle with the development of a variant that can take one of the previously
useless waste by-products of the original reaction and break it down even further into simple
molecules that can then be utilized by bacteria to generate sustainable chemicals and materials,
essentially maximizing the number of valuable products that can be made out of the world's
waste plastic. So it sounds like high time we had ourselves a little progress review!
Hello and welcome to Just Have a Think. The scientists who discovered bacteria feeding
on plastic bottles in that Kyoto recycling dump decided, for reasons best known only to them,
to call it Ideonella Sakaiensis. But we'll forgive them that because it's what they did with it after
they discovered it that's so much more important. When they took a close look under a microscope
they saw that the bacterium used specific enzymes to break down the two components of PET - ethylene
glycol, which is the stuff in the antifreeze in your car radiator, and dimethyl terephthalate,
which to be honest isn't really used in anything at all other than PET itself.
Crucially, the researchers were able to identify the gene in the bacteria's DNA
that was responsible for those enzymes. Then they managed to reproduce the enzyme and demonstrate
that it could do the plastic chomping job on its own, without the rest of the bacterium,
and that meant they could really go to town on it in their laboratories to improve and
strengthen it into a super enzyme that could digest far greater quantities of PET far more
quickly than its naturally occurring cousin. In 2019 the University of Portsmouth here in the UK
established the Centre for Enzyme Innovation, or CEI, and their Professor of Structural Biology,
John McGeehan gave a presentation showing how evolution had solved the PET problem. You can see
the full explanation in my original video which is linked at the top of the screen but essentially
the new enzyme was a mutation of cutinase, which is what bacteria use to break down nature's own
polyester called Cutin. Cutin is what gives the surface of plants a waxy protective coating
and by breaking through that bacteria can get inside the plants to plunder all their goodies.
Cutinase enzymes break those natural polyester bonds using chemical jaws with a gap of just
less than three 10 billionths of a metre, or 0.3 nanometres. What the research team found
was that with only a single amino acid change in its structure this new PETase enzyme had changed
the shape of its jaws to make the gap almost three times wider. And that allowed it to cope
with a man-made aromatic polymer like PET. After a fair amount of genetic manipulation
and an awful lot of trial and error in the lab the Portsmouth team managed to produce a turbocharged
version of PETase with a much faster rate of digestion than the naturally occurring version.
So far so good. Now they had a way of liberating large quantities of ethylene glycol,
which has very good commercial value mainly for use as antifreeze, as I mentioned earlier.
But the enzymatic digestion process also produced a similar quantity of the other PET component,
which is the terephthalate part, also known as TPA, and the molecular configuration of TPA
doesn't really have any uses at all. So the experimentation continued, which brings us to this
latest breakthrough paper published last month in the Proceedings of the National Academy of
Sciences. The paper was co-published by McGeehan's Portsmouth team and a second research team
at Montana State University in the U.S, headed up by Professor Jen Dubois. It is of course packed
full of the obligatory impenetrable scientific lingo that you and I have come to know and love
over the years I've been making these videos. Here's a little sample "terephthelate dioxygenase
and its cognate reductase make up a complex multimetalloenzyme system that dihydroxylates TPA,
activating it for enzymatic decarboxylation to yield proto-catechuic acid". Now if that all makes
perfect sense to you, then thanks for watching, your work here is done. But for the rest of us,
essentially what happened was that during their research the Portsmouth team were able to isolate
and identify another enzyme in the original plastic chomping Ideonella Sakaiensis bacteria.
That's the Terephthalate Dioxygenase or TPADO that I just mentioned in the quote. Using oxygen and
a couple of other balancing reduction agents the TPADO converts the terephthalate molecule
into something called dihydroxycyclohexadine dicarboxylate, which we'll just call DCD,
to save my sanity! The DCD can then be further converted by a zinc-based reductive agent
to produce proto-catechuic acid or PCA. Now that all sounds like a whole bunch of
complicated chemistry to produce a molecule that looks remarkably similar to the one you started
with. But here's the thing - PCA is a naturally occurring acid found in several plant species.
It's a major component of the antioxidants found in green tea for example. And it's been shown to
have anti-inflammatory effects as well, so unlike the TPA molecule, which is more or less useless,
the PCA molecule has all sorts of extremely beneficial applications in medicine and nutrition.
It's even been shown to have the ability to attack certain types of cancer cell, so it may prove even
more useful in the future. Interviewed in this article at the online journal phys.org, Professor
Dubois explained "the Portsmouth team revealed that an enzyme from PET consuming bacteria
recognizes TPA like a hand in a glove. Our group at MSU then demonstrated that this enzyme,
called TPADO, breaks down TPA and pretty much only TPA with amazing efficiency". Professor McGeehan
said "The last few years have seen incredible advances in the engineering of enzymes to
break down PET plastic into its building blocks. This work goes a stage further
and looks at the first enzyme in a cascade that can deconstruct those building blocks
into simpler molecules. These can then be utilized by bacteria to generate sustainable chemicals and
materials essential for making valuable products out of plastic waste". The study was undertaken
as part of the BOTTLE consortium, which is an international collaboration bringing together
researchers from across a wide range of scientific areas to tackle plastic recycling and upcycling.
We humans currently produce more than 400 million tons of plastic waste every year,
most of which eventually ends up in our landfills and in our oceans and food chains. And very recent
research carried out by a team from the University of Hull and Castle Hill hospital here in the UK
has even discovered microplastics in the lung tissue of live human beings for the first time.
13 patients who were due to undergo surgery for various unrelated lung ailments agreed to
allow tiny samples of tissue to be removed from their lungs during surgery and microplastics were
present in 11 of the 13 samples. No-one really knows what the impact of plastics in our food
and in our respiratory systems may be having, but it's probably not unreasonable to assume the
effects are not particularly beneficial, to say the least! So keeping plastics out of landfill
and out of our waterways must surely be one of the top environmental priorities of our time.
Technologies that can either upcycle plastic into higher quality products like those we looked at
in a video a couple of weeks ago or enzymes that can break polymers down into simpler molecules
like the ones we've looked at today will be a vital part of that initiative. Now if
you're involved in these cutting-edge research programs and you can share even more insight
or if you've got some great plastic collection and recycling programs going on in your local area and
you can share some best practice with the rest of us, then why not jump down to the comments section
below and leave your thoughts there. That's it for this week though, as always a huge thank you
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very much for watching, have a great week and remember to just have a think. See you next week