IV. Tolerance Adaptations
a. role of enzymes
Tolerance adaptations to pressure can be witnessed
in a variety of different physiological and chemical systems. I want to
illustrate some of the types of adaptations in this category that occur
in protein systems. Now, as all of you know, proteins, along with nucleic
acids, are the key macromolecular bases of life. All of the metabolic processes
that go on in cells are driven by catalytic proteins, named enzymes. Enzymes
basically lie at the root of all the metabolic transformations that go on
in an organism. Therefore, if an enzyme turns out to be a pressure-sensitive
entity, enzymes may be a very interesting study system to use in examining
tolerance adaptations. Now, why should proteins be sensitive to temperature
and to pressure? Well, one of the things that biochemists have learned as
they've gone into the details of molecular structure is that, during their
function, proteins undergo changes in shape or in their conformation which
are associated with changes in volume. If you see any of the computerized
versions of how an enzyme works, you will see that an enzyme flexes as it
performs its functions. Every time it goes through a catalytic cycle, converting
substrate to product, the enzyme goes through substantial changes in its
three dimensional configuration.
When you do the molecular modeling, one thing you find is that these changes
in conformation or shape have a concomitant change in volume. Well, volume
changes basically are the Achilles heel as far as pressure sensitivities
are concerned. If a system occurs with an expansion in volume, pressure
will tend to make it more difficult for that process to go on. As it turns
out, a lot of enzymatic reactions do occur with an increase in volume. Thus
the enzymatic reaction is inhibited by pressure. The simplest analogy to
use in telling a student this story is to say, "Try blowing up a balloon
underwater." It's difficult because you're trying to expand the balloon
against the compressing force of the liquid. In an analogous way, when a
protein is undergoing it's conformational changes that go on with function,
it's like a balloon trying to blow itself up underwater. The fact of the
matter is, as you begin to survey different types of enzymatic reactions
from all different types of metabolic pathways in an organism, you'll find
that most of them are inhibited by pressure.
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