  The connection between optimal biological function and energy use, measured
for example by the rate of metabolite consumption, is a current topic of
interest in the systems biology literature which has been explored in several
different contexts. In [J. P. Barton and E. D. Sontag, Biophys. J. 104, 6
(2013)], we related the metabolic cost of enzymatic futile cycles with their
capacity to act as insulators which facilitate modular interconnections in
biochemical networks. There we analyzed a simple model system in which a signal
molecule regulates the transcription of one or more target proteins by
interacting with their promoters. In this note, we consider the case of a
protein with an active and an inactive form, and whose activation is controlled
by the signal molecule. As in the original case, higher rates of energy
consumption are required for better insulator performance.
