IV. Tolerance Adaptations
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.
b. adaptation to pressure
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.