[bioch]

I understand noncompetitive inhibitor binds to an allosteric site on an enzyme and so is a type of allosteric regulation. However if this is the case why do noncompetitive inhibitors show Michaelis Menten Kinetics and not sigmoidmal kinetics? Because allosteric enzymes show sigmoidal kinetics due to having more than one binding site.

Also I would be grateful if someone could sum up the difference between non competitive inhibition and allosteric inhibition. Is it because noncompetitive inhibitors completely prevent a reaction from happening whilst allosteric inhibitors reduce the enzyme's affinity for its substrate?

[Babcock_Hall]

Noncompetitive inhibitors usually bind at the active site of enzymes that have two or more substrates.  NC (sometimes called mixed) inhibitors sometimes resemble the first substrate in an ordered sequential mechanism.  When the second substrate is varied in concentration and the first substrate is fixed, the inhibition pattern turns out to be noncompetitive.  Allosteric inhibitors may bind near to or far away from the active site.  When the substrate binds cooperatively, the kinetics are sigmoidal, regardless of whether or not an inhibitor is present. 

[lansuminc]

Actually, they are the same.
Non-competitive inhibitors are substances that combine with the enzyme at a region that is away from the active site to change the structure of the enzyme.

The effect, where a chemical reaction involving one region of an enzyme molecule alters the shape and properties of a second region, is known as an ALLOSTERIC effect.

Allosteric effectors are substances present in cells that reversibly bind with an enzymes away from the active site, yet cause a change in the structure of the active site (it is the same as non-competitive inhibitors)

Allosteric inhibitors slows down the reaction and provide a way of controlling enzymic activity in metabolism. (vice-versa for allosteric activators)

[Babcock_Hall]

When an enzyme uses a sequential (as opposed to a ping ping) kinetic mechanism, the active site must accommodate two substrates that may have entirely different structures.  Sequential mechanisms come in two flavors, random and ordered.  Voet and Voet (Biochemistry 3rd ed.) state that when an Ordered Bi Bi mechanism is followed the product P gives mixed inhibition when either substrate (A or B) is varied.  Q gives competitive versus B and mixed versus A.  I am taking mixed inhibition and noncompetitive inhibitions to be synonyms.

[kriggy]

I dont think they are the same. I mean, non-competetive means that Vmax is lowered. But it doesnt say how. Allosteric means that it binds to other than active site but it doesnt say what is the efect of the inhibitor binding here.