Enzyme Kinetics - what's the real reason for Vmax changes?

[LIC2015]

Competitive inhibitors don't change Vmax, while uncompetitive/noncompetitive inhibitors do.

Why? They all do the same basic thing - prevent substrate from being processed by the enzyme. But the way they do it is different. However, for the purposes of my question, please address the following:

- Why does adding substrate to a rxn help overcome the effects of competitive inhibitors? Once the inhibitor binds, doesn't it remain bound to the active site? If not, why does it detach? What's the point of being an inhibitor if it doesn't permanently inhibit the active site?

- Is the reason for competitive inhibitors detaching from an enzyme due to increased substrate concentration answered by looking at binding affinity?

For example: there are a million enzymes and some substrate particles in a rxn mixture. I then add competitive inhibitors, which bind to some of the enzymes. Will these enzymes be permanently bound by the inhibitors, such that they will no longer be functional for the remainder of the rxn?

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[laczlacylaci]

Competitive inhibitors don't change Vmax, while uncompetitive/noncompetitive inhibitors do.

Why? They all do the same basic thing - prevent substrate from being processed by the enzyme. But the way they do it is different. However, for the purposes of my question, please address the following:

- Why does adding substrate to a rxn help overcome the effects of competitive inhibitors? Once the inhibitor binds, doesn't it remain bound to the active site? If not, why does it detach? What's the point of being an inhibitor if it doesn't permanently inhibit the active site?

- Is the reason for competitive inhibitors detaching from an enzyme due to increased substrate concentration answered by looking at binding affinity?

For example: there are a million enzymes and some substrate particles in a rxn mixture. I then add competitive inhibitors, which bind to some of the enzymes. Will these enzymes be permanently bound by the inhibitors, such that they will no longer be functional for the remainder of the rxn?

I believe it is due to the site of binding. Competitive inhibitors only bind to the active site, where the others bind to the allosteric site.

1) So competitive inhibitors bind to the active site preventing the substrate from binding. Think about the addition of inhibitors:
a) in the system there are no inhibitors=normal ES formation
b) we add a little bit of inhibitors=still normal ES formation, but just less
c)we add a ton of inhibitors, more than substrate, still normal ES formation, just very rare.
d)we now add in more substrate>inhibitors, it goes back to normal ES formation, or scenario a
It does not always not bind or stay binded. Covalent interactions=permanently bound, other interactions are able to let go. Let's just say that you were taking drug A to get high, if the mechanism was the drug A was an inhibitor, then that would not be ideal for your body on whatever it was trying to inhibit.

2) It depends.

 

[theonlytycrane]

Biggest clarification that needs to be made is that the enzyme kinetics model you're talking about deals with non-covalent inhibitors, meaning that the binding is reversible and transient.

Covalent and irreversible binding of an inhibitor would render an enzyme completely useless.

 

[aldol16]

- Why does adding substrate to a rxn help overcome the effects of competitive inhibitors? Once the inhibitor binds, doesn't it remain bound to the active site? If not, why does it detach? What's the point of being an inhibitor if it doesn't permanently inhibit the active site?

If an inhibitor permanently inhibits the active site, then it's bound covalently and has trashed the enzyme. Covalent inhibitors exist, but in general, we want inhibitors to be reversible because then we can modulate the activity of the enzyme. Put another way, if the inhibitor wasn't reversible, that enzyme would not be able to carry out its natural function - until your body makes more of the enzyme.

So the inhibitor will "detach" or be outcompeted for the active site at high substrate concentrations. Think about it this way. Competitive inhibitors mimick the substrate in binding to the active site. But the active site will still have a high affinity for the substrate. So whether substrate or inhibitor wins out depends on how much of each you have around.

- Is the reason for competitive inhibitors detaching from an enzyme due to increased substrate concentration answered by looking at binding affinity?

You can look at it in terms of binding affinity but it's more of a probabilistic argument. Binding of a competitive inhibitor is reversible. So it's an equilibrium. The more substrate there is, the more the equilibrium shifts towards unbound inhibitor because the enzyme is more likely to encounter a substrate molecule than an inhibitor molecule.

For example: there are a million enzymes and some substrate particles in a rxn mixture. I then add competitive inhibitors, which bind to some of the enzymes. Will these enzymes be permanently bound by the inhibitors, such that they will no longer be functional for the remainder of the rxn?

No. The enzymes will be reversibly bound by the competitive inhibitors. Biochemists just define competitive inhibitors as reversible inhibitors. There is a whole other class of irreversible inhibitors but these inhibitors run into the problems listed above. One is unable to modulate its effects downstream. So say you want to inhibit your COX/prostaglandin synthase enzymes when you get whacked by a hockey stick. Well, if your inhibitor binds to those enzymes irreversibly, those enzymes are now trashed and basically junk. You can't reverse the effect. You have to wait for your body to make new COX/prostaglandin synthase enzymes to have a normal inflammatory response again. That's not a problem for enzymes that are constantly being made and degraded but for enzymes that take more time to make/are not made as frequently, that's problematic.

When we design drug molecules, one of the key things we look for is reversibility. That is, is the effect reversible? If it's not, then you run into safety issues down the road - when the FDA looks at this stuff, they want to know that you 1) measured a real effect and 2) can reverse the effect without causing irreparable harm to the system.

 

[LIC2015]

If an inhibitor permanently inhibits the active site, then it's bound covalently and has trashed the enzyme. Covalent inhibitors exist, but in general, we want inhibitors to be reversible because then we can modulate the activity of the enzyme. Put another way, if the inhibitor wasn't reversible, that enzyme would not be able to carry out its natural function - until your body makes more of the enzyme.

So the inhibitor will "detach" or be outcompeted for the active site at high substrate concentrations. Think about it this way. Competitive inhibitors mimick the substrate in binding to the active site. But the active site will still have a high affinity for the substrate. So whether substrate or inhibitor wins out depends on how much of each you have around.



You can look at it in terms of binding affinity but it's more of a probabilistic argument. Binding of a competitive inhibitor is reversible. So it's an equilibrium. The more substrate there is, the more the equilibrium shifts towards unbound inhibitor because the enzyme is more likely to encounter a substrate molecule than an inhibitor molecule.



No. The enzymes will be reversibly bound by the competitive inhibitors. Biochemists just define competitive inhibitors as reversible inhibitors. There is a whole other class of irreversible inhibitors but these inhibitors run into the problems listed above. One is unable to modulate its effects downstream. So say you want to inhibit your COX/prostaglandin synthase enzymes when you get whacked by a hockey stick. Well, if your inhibitor binds to those enzymes irreversibly, those enzymes are now trashed and basically junk. You can't reverse the effect. You have to wait for your body to make new COX/prostaglandin synthase enzymes to have a normal inflammatory response again. That's not a problem for enzymes that are constantly being made and degraded but for enzymes that take more time to make/are not made as frequently, that's problematic.

When we design drug molecules, one of the key things we look for is reversibility. That is, is the effect reversible? If it's not, then you run into safety issues down the road - when the FDA looks at this stuff, they want to know that you 1) measured a real effect and 2) can reverse the effect without causing irreparable harm to the system.

Thanks, always appreciate your help!