What is the role of the binding site in enzyme-inhibitor competition?

[Dr.Brain]

I had a small doubt.Enzymes have an actyive site to which the substrate attaches itself and this is the place where all the chemical changes takes place and the active site brings changes in shape of the incoming substrate.

So when we talk about the inhibitor interfering with the enzyme-substrate complex , there is a sort of competition between the substrate and the inhibitor.

My first question is that , does the inhibition-competition takes place at only active site or both sites (active and binding) , and my second question is that I know that the incoming substrate will attach to the active site and this is the place where all the chemical changes take place , so what role does binding site play for the incoming substrate?

BJ

 

[DocToxyn]

I had a small doubt.Enzymes have an actyive site to which the substrate attaches itself and this is the place where all the chemical changes takes place and the active site brings changes in shape of the incoming substrate.

So when we talk about the inhibitor interfering with the enzyme-substrate complex , there is a sort of competition between the substrate and the inhibitor.

In terms of inhibitor/antagonist interactions, there are three generally accepted mechanisms of competition and they are all defined in how they alter the function of the enzyme in question. To begin, the kinetics of the enzyme are measured by looking at the Vmax, how fast the reation can occur, and the Km, the amount of substrate required to run the reaction at half the Vmax. Competitive inhibition alters the Km and not the Vmax, typically by binding of the inhibitor at the active site of the enzyme. Non-competitive interaction is genrally defined as an interaction between the inhibitor and the enzyme which doesn't involve the active site and is distinguished by a shift in the Vmax only. When both Vmax and Km change, it is considered an un-competitive interaction. These types of interactions are best viewed and compared using equations and plots such as the Michaelis-Menton equation and Lineweaver-Burke plots, you can find these in any chemistry text or on the web. Such calculations allow one to determine the Ki, or inhibitory constant, of the inhibitor.

My first question is that , does the inhibition-competition takes place at only active site or both sites (active and binding) , and my second question is that I know that the incoming substrate will attach to the active site and this is the place where all the chemical changes take place , so what role does binding site play for the incoming substrate?

BJ

I think I answered your question with the non-competitive interaction above, but to elaborate, an inhibitor may bind at a region of the enzyme quite distant from the active site and still alter the enzymes function by number of ways. It could inhibit specific folding that the enzyme must do to open an active site or it could block the binding of some necessary cofactor(s) that would then slow down or stop the reaction. I'm sure there are others as well. As far as binding site vs active site, I would say that its the binding site that holds onto the substrate and the active site where the chemical modification takes place. These sites, on a whole protein scale, may not be mutually exclusive, however they may involve separate amino acids.

 

[Dr.Brain]

So when you say that the inhibition-compeition doesnot affect the Vmax but affects the Km , does that mean that 'when inhibitor interferes in enzyme-substrate complex formation , there is a need of more amount of substrates to overcome the inhibitors and soes succeed in the competition'?

BJ

 

[DocToxyn]

It depends, if the enzyme/inhibitor interaction is irreversible, then the only thing that can can push it in the favorable direction would be production of more enzyme which would (hopefully) soak up enough of the competitor to eventually allow enzyme function to return to normal. If the inhibitor binds and then releases, then having an excess of substrate might then tip the balance toward the substrate rather than the inhibitor, but the inhibitor concentration must also factor in. In the case on non-competitive interaction you would have better success with adding more substrate.