  Cooperative interactions among sensory receptors provide a general mechanism
to increase the sensitivity of signal transduction. In particular, bacterial
chemotaxis receptors interact cooperatively to produce an ultrasensitive
response to chemoeffector concentrations. However, cooperativity between
receptors in large macromolecular complexes is necessarily based on local
interactions and consequently is fundamentally connected to slowing of receptor
conformational dynamics, which increases intrinsic noise. Therefore, it is not
clear whether or under what conditions cooperativity actually increases the
precision of the concentration measurement. We explictly calculate the
signal-to-noise ratio (SNR) for sensing a concentration change using a simple,
Ising-type model of receptor-receptor interactions, generalized via scaling
arguments, and find that the optimal SNR is always achieved by independent
receptors.
