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On the contrary, by definition, all the considered microorganisms have the
phosphopyruvate hydratase enzyme that in turn makes part of SCC. Nevertheless
the inter-organisms difference in their carbon source utilizations (metabolic
network distances) is mirrored by differences in the aminoacid sequence of a
shared enzyme, metabolically 'very distant' from the reactant subset.
This result forces us to consider the metabolism of an organism as a strongly
unitary system in which apparently 'distant' portions of the system, nevertheless
share a 'family air' we are still far to rationalize with the current biological
theories. Going down to a more usual biological 'reasoning' we can state that the
attaining of a strong species specificity (much higher than the one attained by
nucleic acid comparisons) and the stability of classifications indicates that pure
topological wiring allows for a meaningful picture of the studied organisms.
The striking similarity between phosphopyruvate hydratase sequence space
and the general metabolic network space is consistent with the recently
discovered relevance of the so called non-hub-connectors described in the
previous paragraph: this enzyme represents the major non-hub-connector of the
studied system and the sequence/network mapping is in line with the crucial role
these network elements are supposed to play.
In their 'cartographic' representation of the metabolic networks of twelve
organisms, Guimera and colleagues in define different 'roles' for the nodes
involved in a metabolic network based on its within-module degree and its
participation coefficient, which define how the node is positioned in its own
module with respect to the other modules.
The authors assess the plausible hypothesis that nodes with different roles are
under different evolutionary constraints and pressure. In fact, they found that
nodes called 'non-hub connectors', that is nodes with many links to other
modules, are well conserved in the networks analyzed. This means that nodes
connecting different modules, when deleted, have a larger impact on the global
structure of fluxes in the network than nodes with many connections within a
module. This result is in line with the above discussed 'essentiality-from-
topology' problem: the crucial role played by the enzymes at the borders of
metabolic modules is probably at the basis of their co-evolution with the
metabolic wiring pattern.
Many applications can be imagined for this kind of comparative network
studies, extending from the correlation between metabolic network shapes and
the pattern of sensitivity to specific antibiotics to large scale environmental
studies of ecological communities so as to correlate metabolic similarities to
trophic networks. Clearly, the proposed metrics for comparing different networks
is oversimplified not taking into account flux consideration as well as other
