On the Structural Biology of Photosynthetic Light Reactions
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00:00
Chemische ForschungNobeliumChemische ReaktionLichtreaktionVorlesung/Konferenz
00:31
NobeliumLichtreaktionChemische ReaktionSyntheseölMedikalisierungLichtreaktionVorlesung/Konferenz
00:57
NobeliumMedikalisierungProteineLichtreaktionCholesterinKörpertemperaturVorlesung/Konferenz
01:52
LichtreaktionSauerstoffversorgungEukaryontische ZelleNobeliumVakuoleThylakoidKohlendioxidfixierungMolekülKohlenstoff-14KohlenhydratchemieSystemische Therapie <Pharmakologie>LichtreaktionDeprotonierungProtonenpumpePhotosystemProteineChemieanlageWerkzeugstahlSpektroskopieCobaltoxideChemische ReaktionPlasmamembranOrganische ChemieElektronische ZigaretteElektron <Legierung>ZunderbeständigkeitKörpertemperaturEmissionsspektrumMolekülBiologische OxidationWasserKohlenhydratchemieCytochromeKohlendioxidSingle electron transferKohlendioxidfixierungPhotosystem IIV-Typ-ATPasenEukaryontische ZelleFrischfleischLactitolCubanProlinMultiproteinkomplexTransportChemischer ProzessQuerprofilAtomsondeKohlenstofffaserStockfisch
09:42
BiskalcitratumNobeliumNucleolusChemische ReaktionLichtreaktionStereoselektive SyntheseElektron <Legierung>StoffwechselwegElektronentransferHydrophobe WechselwirkungVerzweigte VerbindungenSonnenschutzmittelKohlenhydratchemieKohlendioxidfixierungKohlenstoff-14PlasmamembranMolekülKohlenstofffaserCytochromePhotosystemLichtreaktionSystemische Therapie <Pharmakologie>ProteineFunktionelle GruppeElektronentransferUntereinheitElektron <Legierung>SekundärstrukturMetalloenzymHelicität <Chemie>ElektronenakzeptorChemische ReaktionBenzochinonePolypeptideStoffwechselwegKrankengeschichteBeta-FaltblattTiermodellMultiproteinkomplexOrdnungszahlIdiotypEisenWasserstoffPeriodateSyntheseölChromerzBranntweinFließinjektionsanalyseMemory-EffektKohleCobaltoxideSonnenschutzmittelPentapeptideBlauschimmelkäseGangart <Erzlagerstätte>FarbenindustrieStickstoffatomDeprotonierungLinkerInlandeisRöntgendiffraktometrie
19:39
MetalloenzymCobaltoxideKawasaki-KrankheitChlorideLipideIonenkanalOligomereMultiproteinkomplexNobeliumHydrophobe WechselwirkungSonnenschutzmittelCarotineChemische ReaktionUntereinheitSystemische Therapie <Pharmakologie>Chemische ReaktionFunktionelle GruppeVollernterMetalloenzymUntereinheitInselMultiproteinkomplexDigoxigeninPosttranslationale ÄnderungBindegewebeMenschenversuchKrankengeschichteCobaltoxideTeststreifenQuerprofilProteineMeeresspiegelOrganische ChemieMeerAdvanced glycosylation end productsMolekülDiamantähnlicher KohlenstoffParasitismusLichtreaktionMetallWasserElektron <Legierung>ChemieanlageHydrophobe WechselwirkungSonnenschutzmittelÜberlebenStrahlenbelastungZellfusionBlauschimmelkäseHelicität <Chemie>PhotosystemErdrutschLegierenPhotosystem IIWursthüllePhäophytineStrahlenschadenAtomclusterBenzochinone
29:19
NobeliumCobaltoxideComputeranimation
Transkript: Englisch(automatisch erzeugt)
00:17
Good morning. Thank you for the introduction.
00:21
If you allow me, I want to talk about a hobby that I developed in the 1980s. And that hobby is to study and follow the development of specially structural discoveries on photosynthetic light reactions.
00:45
And I did that even though I was working for the last almost 30 years in this institution here. It's the University of Texas Southwestern Medical Center. And at the medical center, if you talk about photosynthesis, you may be met with a polite interest, but not very sort of intense interactions.
01:13
The people there are well known for their studies on cholesterol, homeostasis, on related medical discoveries and cheap proteins and so on.
01:33
But I was able to keep this interest alive and I want to tell you about or give you a review of what happened in these last 30, 35 years.
01:48
So, of course, photosynthesis is the basis of life. It relies on the presence of our star here, the sun, and the earth and the
02:03
temperature difference between these two, which leads to a very, very different spectrum of electromagnetic radiation. The photons coming from the sun are, of course, much, much higher energy than those coming from the earth.
02:23
And this difference is used by photosynthetic organisms to capture energy and use the resources on earth, especially water and carbon, to produce oxygen and carbohydrates.
02:50
And these products then are used in cell respiration combined again to produce carbon dioxide and water.
03:02
And the difference between dark reactions and light reactions is that these reactions that use water to produce oxygen are absolutely dependent on light, whereas those that fix carbon dioxide can be run, in principle, at least, without the presence of light.
03:34
So, if you look in a textbook, you can find pictures like this.
03:40
This is the organelle, a so-called chloroplast, where photosynthesis happens. And you see here a wall, and then inside a system of membranes that contain the relevant proteins, and I will come to that in a second, that perform the reactions,
04:09
especially the light reactions. And this chloroplast is believed to be a so-called cyanobacterium that was incorporated by plants into their leaves.
04:30
Cyanobacteria are independent organisms of this organization, and they live mostly in the sea.
04:42
And to this, they produce about half of the oxygen produced in photosynthesis. And again, from a textbook, this is a sort of simplified scheme of the reactions that happen in a chloroplast.
05:04
There are two photosystems, photosystem II, photosystem I. Both absorb light, this one, maximum absorption at 680 nanometers, this one at 700 nanometers.
05:22
And the light absorption leads to lifting an electron to high energy. So for electrons, here is, as you can imagine, a scale here that is low to high energy.
05:41
So electrons are lifted to high energy. They are processed through a proton pump, lose some of their energy, deliver to the photosystem I, under the influence of light, lift it to high energy, and add it to NADPH. And this is one of the inputs into the carbon fixation cycle, and here are the dark reactions.
06:10
The electrons come from water. Water is split in a very interesting set of reactions, and oxygen is released to the atmosphere.
06:25
So we have three products, oxygen, carbohydrates, and the energy from this proton gradient. We have two light absorbing systems, and we have input water and carbon dioxide.
06:44
Now, the early pioneers of photosynthesis working in the second half or in the 20th century, let's say, they didn't have the tools to dissect and investigate such a complicated system,
07:08
and they were looking for more simpler organisms that still perform key reactions in photosynthesis. And what they found were photosynthetic purple bacteria.
07:26
And these purple bacteria consist of something that's similar to a photosystem II and a proton pump, but this photosystem cannot, in purple bacteria, cannot split water,
07:41
and therefore they just recycle the electrons into the photosystem. So they perform a cyclic electron transport, and the output is the proton gradient. And this system in the bacteria has a lot fewer different proteins.
08:06
It's much simpler, and it was the preferred object of study by, as I said, the pioneers. And they did things like time-resolved laser spectroscopy and similar things,
08:30
but in the end they needed structural information, especially this protein here,
08:42
to fully understand what happens after the absorption of light. And here's a picture of such a bacterium, now called Blustochlorus viridis, and you can see it has stacks of membrane inside.
09:03
It's about two micrometers in diameter. That's stacks of membranes inside. And these are filled with photosynthetic molecules, especially reaction centers,
09:25
light-harvesting molecules, and the proton pump called Cytochrome VC1 complex, and of course the ATPase that uses the proton gradient, et cetera.
09:41
Okay, so the problem was proceeding from this to structural information on the molecules in this membrane was that, as Hartmut pointed out in Monday,
10:02
there was a widespread belief that one cannot crystallize proteins that exist in the membrane. But luckily he did not believe that, and put his efforts and his skills on this task,
10:23
and lo and behold, they could grow crystals of the photosynthetic reaction center from this bacterium. And this was my point in his personal history where I had to be in the right place at the right time.
10:44
And so I was lucky to become a member of the team that worked together with Hartmut on these crystals, and after a while we could determine the structure of the reaction center.
11:03
Here it is. As usual, on the left side here, the proteins are represented as cartoons just following the course of the polypeptide chain, indicating the presence of secondary structure
11:30
like helices or beta sheets by either spirals or arrows. And that allows one to get some idea of the complexity of the structure, see through it.
11:49
The proteins are carrying prosthetic groups, non-protein groups, that are shown as stick models.
12:04
On the right here I have a picture showing all the non-hydrogen atoms. As usual in protein crystallography, we don't see hydrogens, they're too light, too few electrons,
12:24
and resolution is not good enough, so they are not depicted here, but all the other atoms. The carbons are colored according to the subunit they belong to.
12:41
These are subunits L, M, H, and cytochrome. And the nitrogens, oxygens, are colored blue and red. I see a lot of them here. Sulfurs, one of them visible here, is yellow.
13:03
And a short analysis of that structure shows that this part here, this central part, has a very hydrophobic surface, and that indicated that this is sitting inside the membrane, so these and these are sticking out.
13:23
And this was in accord with the expectation that most of the membrane-spanning proteins consist of helical structures. Of course, for most people thinking about the function of these proteins, the cofactors, the prosthetic groups,
13:50
these were the most important. And here in the membrane-spanning part, there are four bacterial chlorophylls in green,
14:01
two bacterial pheophytins, two quinones, an iron, a carotenoid, and four hemes in the cytochrome subunit. In total, 14 cofactors. The hemes are of some interest, but not central to the function of the protein,
14:27
because there are even some bacteria that don't have this subunit altogether. They can function without the hemes. But this here is essential, and I've shown that here in a larger version.
14:50
One can see that the arrangement of these cofactors is symmetric, so you can imagine a twofold symmetry axis running vertical in the picture,
15:04
and relating this to that, this to that, and there are two chlorophylls close together on the symmetry axis, and they run through the iron. Also, the quinones are related.
15:22
But this was one of the biggest surprises that the structure provided to people who were working in photosynthesis research, because it was already known at the time, or suspected,
15:42
that there is a well-defined pathway of electrons transferred through this structure. So imagine this structure being hit by absorbing a photon, bringing this pair of chlorophylls to an excited state,
16:03
and then this is followed by electron transfer steps, three picoseconds to the next chlorophyll, 0.7 picoseconds to the pheophyte, 200 picoseconds to the first quinone.
16:21
Then, from the cytochrome, there is the electron hole in this pair of chlorophylls. It's filled again from one of the hemes. And finally, the electron is transferred across the symmetry axis to the second quinone.
16:41
And this has to be repeated twice to reduce this quinone two times at two protons from outside the membrane, here from the lower side, coming in here.
17:03
And then the quinone will diffuse out of the structure and will be replaced by another one to serve as electron acceptor again. So, to this day, it is not entirely clear how the reaction center,
17:30
the composition of the protein, actually causes this one-way electron transfer only.
17:41
And you can still see papers in which people try to introduce mutations that would bring the electrons to being transferred this way. Anyway, so these cofactors are bound to the proteins L and M,
18:07
the brown and the blue subunit. And if you look from this side, you see, and remove the connecting segments of the helices,
18:22
we see the symmetry of the structure expressed in the cofactors and also in the helices. I want to point your attention to the fact that the helices appear in the order 1, 2, 3, 5, 4,
18:43
as they are in the sequence. So these two are actually interchanged and the same here. And after we had published the structure and refined it,
19:03
or during our sort of work on the final part of the structure, other people started to think about the real thing, the chloroplast proteins, and especially a group in Berlin with Wolfram Sanger and Horst Wirth and Peter Fromme
19:29
and others were quite successful in studying for the system 2 and for the system 1. And here are some papers on photosystem 2 structures.
19:46
This is the first one from the Berlin group. And I will briefly mention results of this study, cyanobacterial photosystem 2, that was solved at 1.9 angstrom resolution
20:04
by a group from Osaka City University. And this is the overview of the structure. It's a much, much bigger protein complex than the reaction center,
20:23
20 protein subunits alone, 35 chlorophylls. It includes light-harvesting proteins, but it also has in its center a brown and a blue subunit, and I will show that in a minute. But the central cofactors look very much like the one
20:47
in the bacterial reaction center, chlorophylls, 4 chlorophylls, 2 pheophytins, 2 quinones, and there are some others that make a connection to the light-harvesting proteins
21:02
and of course a complex of metal ions and oxygen that are the oxygen-evolving complex. This was the highlight of this structure for many people. It for the first time really reliably defined the structure of this complex
21:23
and Dr. Lubitz's group, for example, has done research on how this thing manages to extract 4 electrons from 2 water molecules
21:42
and then preserves all the intermediate state and release molecular oxygen. And here's the central part of the photosystem II, with the brown and the blue subunit, and if we compare this to the bacterial one,
22:04
you can see the similarities, and especially if we look along the twofold axis, there are these five helices and they are again in the order 1, 2, 3, 5, 4, and the same here.
22:22
Here's the twofold axis. So briefly about photosystem I, there was again the Berlin group who published the first structure and also a group from Tel Aviv University, and here is the result of the Berlin group.
22:49
Here the brown and the blue subunits are a lot larger, and that is because they are a fusion of light-harvesting proteins and central units,
23:02
and you can see here they extend far away from the twofold axis that exists. The arrangement of the cofactors is similar, but the two pheophytins are actually chlorophylls in this case,
23:25
and in this case there is no distinction between the quinones because the electrons are ending up on iron-sulfur clusters and then transferred on to reduce the NADP.
23:48
And if we look at the helices surrounding the central cofactors, we again find five helices, and looking down the twofold axis of symmetry,
24:04
we see them in the order 7, 8, 9, 11, 10. Again, the last two interchanged, and it starts at 7 because the six helices before
24:21
are now part of the same protein subunit and part of the light-harvesting complex. And so in summary, the structures of the purple bacterial reaction center, the photosystem II and photosystem I,
24:43
looking down the twofold symmetry axis look pretty similar to my taste, and especially this order of helices must indicate, or is a strong indication, a strong support for the idea
25:01
that they all come from the same ancestor. And Hartmut already showed this slide. It is a history of Earth's atmospheric oxygen content,
25:22
and there is a standard model indicated by this red-brown curve here, and there are possible modifications that are mentioned by these authors. And the standard model would say that for,
25:45
let's say, it's now believed that life started, or the earliest geological findings of signs of life on Earth are about 3.8 billion years old.
26:04
But for a long time, the oxygen content of the atmosphere was very low, like less than 10 to the minus 5 of the present atmospheric value.
26:20
And then there was a so-called great oxygenation event, which rose by about four orders of magnitude. And then it should have stayed again constant almost for 1.8 billion years or so,
26:41
and then rose again by a factor of 10 to 20 to the present day's value. And this paper is discussing possible modifications to the scheme, but the main idea is the same, that oxygenic photosynthesis slowly developed over time
27:07
and then somehow succeeded, but not to the present-day level. And maybe this part has to do with, the second rise has to do with the colonization of the land area.
27:26
Up to that, about this time, everything happened in the ocean. And if I think about this picture, I have the feeling that the oxygenic photosynthetic organisms
27:44
may have done not exactly a favor to themselves to rise the oxygen to this level because this enabled the development of parasitic organisms.
28:01
That lived off the oxygenic photosynthetic organisms. And this includes us. And whether this is good for the plant world and the photosynthetic world is an open question.
28:22
So I leave with that impression. So remember, human history is about less than the thickness of this line here. And we should not really be proud of digging out
28:48
the remains of the photosynthetic organisms and burn them in order to heat our homes or drive our cars.
29:05
We should think about how to directly utilize solar energy and leave much less damage on the planet. Thank you very much.
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