Allosteric binding is where the enzyme can be regulated through having ligands bind onto somewhere that is not the active site. This will then induce a conformational change on the active site, hence preventing binding and catalysis.
Often after a long pathway, the final product can act as the modulator and bind at the allosteric site of the first enzyme.
My question is why does the curve now change to look sigmoidal instead of hyperbolic?
I get that allosteric binding should affect the shape - but this is not what I expected. What I do not understand is why does the velocity decrease so sharply at low concentrations of substrate?
Allosteric binding should mean that at small amounts of substrate, there should be very little amounts of reaction. Thus, there are small amounts of product and hence very little allosteric binding. Thus the reaction should go at full speed, looking just like the diagram on the left. It is only at higher concentrations of substrate that the modulating effect would kick in.
Where is my reasoning wrong?