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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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