Ultrasensitivity

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In molecular biology, ultrasensitivity describes an output response that is more sensitive to stimulus change than the hyperbolic Michaelis-Menten response. Ultrasensitivity is one of the biochemical switches in the cell cycle and has been implicated in a number of important cellular events, including exiting G2 cell cycle arrests in Xenopus laevis oocytes, a stage to which the cell or organism would not want to return.^[1]

Ultrasensitivity is a cellular system which triggers entry into a different cellular state.^[2] Ultrasensitivity gives a small response to first input signal, but an increase in the input signal produces higher and higher levels of output. This acts to filter out noise, as small stimuli and threshold concentrations of the stimulus (input signal) is necessary for the trigger which allows the system to get activated quickly.^[3] Ultrasensitive responses are represented by sigmoidal graphs, which resemble cooperativity. The quantification of ultrasensitivity is often performed approximately by the Hill equation:

${\textstyle {\ce {Response}}={Stimulus^{n} \over ({\ce {EC50}}^{n}+Stimulus^{n})}}$

Where Hill's coefficient (n) may represent quantitative measure of ultrasensitive response.^[4]