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IV. Tolerance Adaptations

b. adaptation to pressure


When one compares the same type of enzyme in species from different depths one finds that there has been a very high degree of tolerance adaptation to pressure. These are data that were gathered in our laboratory by Dr. Allen Gibbs on an enzyme that's involved in the pumping of ions at fish gills. It's a plot that would apply to a lot of different types of enzymatic reactions. If we look at shallow living fishes, we find that imposing higher pressures and here I've shown you the pressure at a given depth in the ocean, 1 kilometer or 2 kilometers, 3 kilometers and so on, in your assay system decreases the activity of the enzyme. Inhibition occurs in the deep sea fishes as well, but to a much lesser extent than in shallow-living species. So the enzymes in high pressure adapted organisms exhibit mechanisms that we still don't fully understand which allow them to carry out their reactions with a reduced pressure sensitivity, i.e., a reduced volume change. And, as one moves from one type of enzymatic system to another, one finds that shallow-living and deep-living species show this characteristic difference. So there has been a tremendous amount of molecular evolution at the protein and gene levels that allows deep-sea organisms to have enzymatic reactions that are relatively insensitive to pressure.


Protein Structure

The same considerations that apply in terms of the catalytic functions of proteins apply in general to their structural stability. It may be that adaptation to pressure would entail the evolution of tougher protein structures. What we find, in fact, is that proteins of deep-living fishes are often unusually resistant to denaturation.

The analogy I like to show here is my famous styrofoam experiment. When I was down at Scripps we took a styrofoam cup down on the submersible Alvin. This is an actual experiment. Your standard-sized coffee cup was taken down to 1200 meters and you can see what happens. It's a compressible system. But, hopefully, none of your students will believe this one (Mt. Everest). My point is that proteins have a certain compressibility. We hope to have a deep-sea protein that wouldn't undergo this sort of transition from this form, into this form, as you raise pressure.

Tough proteins are often found in the most high temperature-adapted organisms. I will come back to this point when I mention the work that's being done on proteins from deep-sea hydrothermal vent species living at high temperature.


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