Maxwell's demon for calcium binding to calmodulin?

In the recent paper (2011) Faas and co-workers claimed that calmodulin could directly detect Ca 2+ signals by acting as extremely fast calcium buffer. However the results and their interpretation raise serious doubts about this conclusion. The major concerns are:-The on-rate constant for calcium binding by calmodulin (CaM) has not been directly measured and was instead found via Markovian fitting of the complex scheme that includes many steps. Having obtained via fitting an extremely fast time-constant for calcium-CaM binding, the authors intended to substantiate this by comparing the estimate with diffusion limit. As shown by Einstein and Smoluchowsky the latter determines the maximally possible rate constant of reaction between two particles from the frequency of their collisions and limits the on-rate constant from above. The authors argue that the diffusion limit for Ca-CaM binding may correspond to the rate constant 5 that corroborates the results of fitting. The estimate is however much higher than ever reported for calcium binding to other molecules as in the case of ATP considered below.