Allosteric activity in enzymes.

[shredder666]

I've always wondered how enzymes could tell the difference between too much substrate and too little substrate to initiate allosteric activities, my teacher tells me that its kinda like collision theory in chemical bonding. That would make sense because if there's too much product, then some of it has got to bind with the allosteric site in the last enzyme. But then what if there's like 1 product in 100 gizzilion substrates that gets binded into the allosteric site? Does that mean even in "normal" enzyme activity, some enzymes are in the "work faster" mode and some enzymes are in the "work at normal rate" mode?

 

[Ygggdrasil]

Binding between an enzyme and an allosteric effector (or any other molecule that binds to the enzyme) is governed by the dissociation constant for the interaction (K_d, which has units of concentration). Mathematically:

$$f = \frac{<i>}{K_d + <i>}</i></i>$$

where f is the fraction of enzymes bound by the effector and is the concentration of free effector in solution. When the concentration of effector is much larger than the dissociation constant, nearly all of the enzymes are bound to an effector molecule. When the concentration of effector is much smaller than the dissociation constant, only a very small fraction of the enzyme is bound by the effector.

In most cases, the enzyme has evolved so that it's K_d for its allosteric effectors is near the physiological concentrations of these acceptors. In this case, as you correctly state, there is some amount of enzyme that is bound by the effector and some amount that is unbound. Therefore, there are two populations of enzymes that work at different rates. The overall rate of the reaction, however, is governed by the amount of enzyme in the relative proportion of enzyme in the high-activity state versus the low activity state.