Metabolic network
= Metabolic network
A <metabolic network> is a connected system of <chemical reactions> that transform the <metabolites> of a <biological cell> or <organism>. One reaction's product can be another reaction's <substrate>, so the activity of one <enzyme> cannot by itself determine flow through the whole system.
A minimal example has input $u$ into metabolite $A$, conversion $A\to B$ at rate $v$, and removal of $B$ at rate $w$:
$$
\frac{dA}{dt}=u-v,\qquad \frac{dB}{dt}=v-w.
$$
At a <steady state>, $u=v=w$. Doubling the maximum capacity of the middle <enzyme> does not double throughput if the input remains fixed. In a larger <metabolic network>, the same bookkeeping is expressed as $\dot{\mathbf{x}}=S\mathbf{v}$, with <stoichiometric matrix> $S$ and <metabolic flux> vector $\mathbf{v}$.
A genome-scale reconstruction for <Escherichia coli> combined annotated <genes>, biochemical literature and strain information to specify possible reactions. The network constrains which conversions are possible; predicting actual rates still needs environmental constraints and modelling assumptions.<The Escherichia coli MG1655 in silico metabolic genotype: its definition, characteristics, and capabilities by J. S. Edwards and B. O. Palsson>[2000, abstract: reconstruction and metabolic capabilities]{ref}