Panel Discussion, continued...
Dr. Somero, an actin chain will usually degrade at those temperatures. What do they use for the molecules themselves? Do they use the normal molecule and some kind of change to bonding or do they have different molecules, different proteins?
George Somero: People have begun doing some really nice molecular studies and I think what's most intriguing is that they find that you don't have to tinker with the proteins very much. You can change a few amino acid residues and that's enough to increase thermal stability. When you look at the proteins, it isn't as if there's some quantum jump to make these proteins qualitatively different. They're quantitatively different, a few additional hydrogen bonds, maybe an ionic bridge that isn't present in a more misaphilic protein. So, in terms of proteins, even though they're stable in boiling water, it hasn't taken a huge amount of amino acid substitution to change them.
Now, what's interesting is you look at these proteins sort of working happily at 90 degrees C or 100° C in vitro, take them up to maybe 150° C, and that's where you see break down of covalent bonds creating the protein. So, some of the amino acid side chains, in fact, some of the types of peptide bonds that hold amino acids together, are probably getting to the edge of their inherent stability. That's one of the reasons people have pretty much concluded that we're not going to find organisms living much above 110 ° C because you can add additional hydrogen bonds or increase ionic bonding proteins and stabilize up to that point without doing major differences. You can still use the same basic 20 amino acid building blocks but when you get up to 150 or 200° C - and again, it depends on a matter of temperature times time to get the intensity of the stress - but proteins are not going to be very happy at 150, 200° C because that's where their covalent bonds start breaking down.
The membranes of a lot of these ultrathermaphilic archae have interesting properties. Normally, a membrane is a bi-layer, where you have one layer of lipid molecules, proteins, and another layer. In some cases, it still looks like a bi-layer but your lipids extend all the way through the membrane. The thought is that these organisms have to really toughen up their membranes. When you look at temperature biology, you find that the most consistent type of adaptation to temperature is actually in membrane systems because they're made of lipid molecules, to a large extent and everybody knows as you go from peanut oil to olive oil to butter back and forth, your lipids have very different properties. If you want them at very high temperatures, you have to have some very highly saturated or very viscous type of lipid. These ultrathermaphilic organisms have really played that card strongly and made very tough membranes. That seems to be, in a way, the most unique type of adaptation they have. There's also some evidence that the base composition of the DNA is slanted to give the kinetic information a higher thermal stability. But they're basically using some of the same principals that you can find in less heat tolerant organisms. They simply amplify from a more normal organism.
George Matsumoto: I'd add to that and say they're not that rare. You're not only going to find them at places with hydrothermal activity like vents or hot springs, but you can also find them in your hot water heaters where you do have the hot water. So, it's not something that only occurs in very specific and isolated areas, it's something that most of us probably have in our houses.
George Somero: They're big in compost bins. They're big in oil drilling sites where you find conditions of very high temperature and often very anoxic conditions. That's one of the reasons people think of the hydrothermal vents or maybe deep parts in the earth as maybe where life started because there was lots of energy. It was warm. The chemical reactions can grow quickly and there was no ultraviolet light to break up chemical bonds. You find these Archae or Archaebacteria, whichever name you choose to use, in the most extreme environments from our perspective, but those might have been just the right place to be when life started. So we're very biased in what we call, I hate to use the word hostile or stressfull environment, that's very anthropomorphic. If you were to bring a tube worm up and put him at this table, that tube worm is going to say, gosh, what a hostile environment, nothing can live here. It's all relative.