Biology Reference
In-Depth Information
4.2.4
Specificity is not an experimental concept
Finally, there is an epistemological problem related to using stereo-
specificity as a concept. Proteins cannot be specific quite simply
because the concept is not relevant for describing experimental real-
ity (Kupiec, 1999). It automatically imposes an arbitrary order on
the way we look at the natural world, even if this order does not
actually exist. It is a qualitative notion, in fact, whereas in practice
we analyse molecular interactions with quantitative parameters.
Specificity follows the 'all or nothing' rule and according to the way
of thinking it imposes, two molecules either are or are not specific
to each other. Reality does not however comply with this
Aristotelian logic and its ordered way of dividing up the world in a
discontinuous manner. A molecular interaction is measured by the
equilibrium constants for the complex that the molecules form, no
interaction being absolutely stable. What is measured is the longer
or shorter average life span of the complex between two dissociation
events. The greater the affinity, the more stable the complex will be
and the longer its average life. A given molecule can always inter-
act with many partners, with stronger or weaker variable affinities.
The experimenter is obliged, owing to this continuous, quantitative
character of molecular affinities, to set a threshold below which he
will consider the interaction as non-specific, but that does not mean
that weak interactions do not exist or that they do not occur in the
organism.
This approach is subjective, and leads to a bias in our appreci-
ation of reality and to a contradiction. Nothing gives us leave to
declare
a priori
that a weak interaction has no biological effect. It
may even be that a weak interaction repeated often would have
more biological effects than a strong interaction that occurs rarely.
Even if weak interactions do not have direct physiological conse-
quences, the simple fact that they occur means that they come into
competition with strong interactions and affect their kinetics. They
therefore also contribute to determining the state of a biological
system. Despite this, we always operate arbitrary selection, which
leaves weak interactions to one side.
