title: Ultrasensitivity part I: Michaelian responses and zero-order ultrasensitivity.
abstract: Quantitative studies of signal transduction systems have shown that ultrasensitive responses - switch-like, sigmoidal input/output relationships - are commonplace in cell signaling. Ultrasensitivity is important for various complex signaling systems, including signaling cascades, bistable switches, and oscillators. In this first installment of a series on ultrasensitivity we survey the occurrence of ultrasensitive responses in signaling systems. We review why the simplest mass action systems exhibit Michaelian responses, and then move on to zero-order ultrasensitivity, a phenomenon that occurs when signaling proteins are operating near saturation. We also discuss the physiological relevance of zero-order ultrasensitivity to cellular regulation.
authors.affiliation: Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA 94305-5174, USA. Electronic address: james.ferrell@stanford.edu.
authors.forename: James E
authors.initials: JE
authors.lastname: Ferrell
authors.affiliation: Department of Chemical and Systems Biology, Stanford University School of Medicine, Stanford, CA 94305-5174, USA.
authors.forename: Sang Hoon
authors.initials: SH
authors.lastname: Ha
mesh_terms: D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism; D011485:Protein Binding; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D006454:Hemoglobins; D010100:Oxygen