The metabolism of an organism makes up a very well defined
network of reactions catalyzed by enzymes. These highly
complex networks has presumably evolved from a simple
primordial metabolism, from where they have diversified and
specialized under the constraints of an underlying biochemistry.
But how diverse are these networks? How much do they have in
common, what can an ensemble of metabolic networks tell us
about there common past, to what extent does a common core
exist, and how does the underlying biochemical constraints
influence the network evolution?
We here address these questions by studying the overlap of
the metabolic-reaction networks of 134 bacterial species. We
introduce the concept of organism degree (OD), the number of
organism in which the reaction is present. Network analysis
shows that common reactions are found in the center of the
network, and that the average OD decreases as we move to the
periphery. Also, nodes of the same OD are more likely to be
connected to each other compared to a random OD relabeling.
Our results lend additional support to the importance of
horizontal gene transfer during metabolic evolution, and
suggest that the biochemical constraints can help both to
diversify and narrow down metabolic evolution.