The Goldbeter–Koshland kinetics describe a steady-state solution for a 2-state biological system. In this system, the interconversion between these two states is performed by two enzymes with opposing effect. One example would be a protein Z that exists in a phosphorylated form ZP and in an unphosphorylated form Z; the corresponding kinase Y and phosphatase X interconvert the two forms. In this case we would be interested in the equilibrium concentration of the protein Z (Goldbeter–Koshland kinetics only describe equilibrium properties, thus no dynamics can be modeled). It has many applications in the description of biological systems. The Goldbeter–Koshland kinetics is described by the Goldbeter–Koshland function:
z = [ Z ] [ Z ] 0 = G ( v 1 , v 2 , J 1 , J 2 ) = 2 v 1 J 2 B + B 2 − 4 ( v 2 − v 1 ) v 1 J 2 {\displaystyle {\begin{aligned}z={\frac {[Z]}{[Z]_{0}}}=G(v_{1},v_{2},J_{1},J_{2})&={\frac {2v_{1}J_{2}}{B+{\sqrt {B^{2}-4(v_{2}-v_{1})v_{1}J_{2}}}}}\\\end{aligned}}}
with the constants
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