  A cell has the ability to convert an environmental change into the expression
of genetic information through a chain of intracellular signal transduction
reactions. Here, we aimed to develop a method for quantifying this signal
transduction. We showed that the channel capacities of individual steps in a
given general model cascade were equivalent in an independent manner, and were
given by the entropy production rate. Signal transduction was transmitted by
fluctuation of the entropy production rate and quantified transduction was
estimated by the work done in individual steps. If the individual step
representing the modification to demodification of the signal molecules is
considered to be a Szilard engine, the maximal work done is equivalent to the
chemical potential change of the messenger that is consumed during the
modification reaction. Our method was applicable to calculate the channel
capacity of the MAPK cascade. In conclusion, our method is suitable for
quantitative analyses of signal transduction.
