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An elasticity coefficient quantifies the sensitivity of a reaction rate with
respect to a change of concentration or a kinetic parameter, while control
coefficients refer to the change of steady state flux and concentration
distributions due to a change of individual reactions rate. Elasticity coefficients
are local properties regarding individual reactions and can be calculated for any
given state while control coefficients are global properties related to the new
steady state reached after a perturbation.
MCA was used in to model the human erythrocyte and in the specialized
literature, there are plenty of applications.
8.4. Conclusion
It is important to realize how powerful the network approach can be and, at the
same time, to understand that the search for 'the definitive network' of any
organism is devoid of any sense: the actual shape of metabolism (as well as of
gene regulation or protein interaction pattern) changes in time and with respect to
the environmental conditions.
The possibility to design (and even to profitably study) topological Charts-of-
Metabolism must not make scientists forget those charts report 'what in principle
could happen' and not what necessarily happens in any situation. The same kind
of relation holds between the entire genome sequence and the effective activation
of certain genes. Nevertheless, the purely topological approach for metabolic
networks has proved to be fruitful and rich of biological consequences.
This takes us to the basic question: how and when the network approach gives
the scientist some doubt free advantages with respect to other methods?
We think the best criterion is embedded in the idea of what a network is: a
bunch of nuclei (where matter as well as activities are highly concentrated)
linked to each other by arcs (streets, mechanical junctions, power cables) passing
through a much less dense environment.
In this view, the highways connecting different big cities constitute a network,
because there is a sharp difference between the activity density (crowding,
population density, energy expenditure) of the nodes (cities) with respect to the
country along which the highway passes by. On the other hand, the intermingled
texture of internal city streets is much less naturally formalized as a network,
with the nodes/arcs distinction much less clear. This is the reason why the
network paradigm was very useful to study metabolism where organic molecules
are immediately evident 'crucial points' of the entire 'game', while it is much
less natural to consider different RNAs transcription levels (like in microarray
experiments) as the nodes of the differential gene expression.
