Sigmoidal binding curves structure

The importance of the changes in quaternary structure in determining the sigmoidal curve is illustrated nicely by studies of the isolated catalytic trimer, freed by p-hydroxymercuribenzoate treatment. The catalytic subunit shows Michaelis-Menten kinetics with kinetic parameters that are indistinguishable from those deduced for the R state. Thus, the term tense is apt in the T state, the regulatory dimers hold the two catalytic trimers sufficiently close to one another that key loops on their surfaces collide and interfere with conformational adjustments necessary for high-affinity substrate binding and catalysis. 

The quaternary structure of HbA allows it to bind oxygen with positive cooperativity giving a sigmoidal oxygen dissociation curve. 

In the case of the oligomeric enzymes the kinetic cooperativity is more complex. The interactions between the subunits can influence the rate (speed) of the transition or even alter the three-dimensional structure of the subunits themselves. Weakly coupled subunits generate no sigmoidal substrate saturation curve and in this instance the kinetic cooperativity can be greater or smaller than the corresponding substrate binding cooperativity. This is the case for V2, as it can be seen from the values of h exf(13)-... 

Diporphinatodiiron 5fe(Mi = Mj = Fe(II) in 5) showed the same reduced affinity to CO, whereas it bound two CO molecules due to the diiron structure The CO-binding equilibrium curve for the 5h-imidazole complex appears sigmoidal, while the curves for the 5a-imidazole complex and the imidazole complex of diporphinato-iron 5c (M, — Fe(II), Mj = 2 H in 5) are hyperbolic The cooperative parameter in) was estimated for the CO-binding to 56 to be 3.4 which meant a strong coopera-... 

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