MCAT Tutoring Sample: Enzyme Inhibitors

MCAT Tutoring Sample: Enzyme Inhibitors



all right so let's go ahead and continue our discussion on enzyme kinetics so now we're going to be looking at the different types of enzyme inhibitors and you haven't recall how many types of enzyme inhibitors there are I think before so people often say fuller but as we're gonna see it's actually only three types of enzyme inhibitors and we'll see what the fourth one really is soon so let's draw for the easiest one the competitive inhibitor so by its name it means it competes directly with the substrate to get to the enzyme so if you add the inhibitor to the solution it can bind to the enzyme to form this enzyme inhibitor complex which the problem with this of course is that it can't react and that's not what we want because what we want to happen is the enzyme to bind substrate to form to the enzyme substrate complex and then react to form our enzyme and the product so in this case you haven't recalled the competitive inhibitor where on the enzyme does it bind I think the active site that's correct so if they're competing directly to bind to the enzyme that means the substrate inhibitor have to bind to the same site and the substrate we know binds to the site of catalytic activity also called the active site so if you think about what we have here when you add the inhibitor to the solution is there now easier or harder for the enzyme to bind to the substrate probably harder mmm-hmm harder to bind to the substrate so would you say that the enzyme now has a greater or lower affinity from the substrate lower affinity for the substrate that's correct in one important relation to keep in mind for the MCAT 4km is to recall that km is inversely related to the affinity of the enzyme for the substrate so based on what we just discussed then does that mean the km is going to increase or decrease with a competitive inhibitor it's going to increase that's correct the decrease in affinity and the next thing we can talk about is V Max so v-max is the michaelis-menten saturation curve which you know on a diagram looks something like this where as you increase the substrate concentration the reaction velocity increases but you can also see that it has this maximum value where it starts to top off so that's what we call the v-max the maximum reaction velocity and we also recall by definition that the km is the substrate concentration necessary to reach one half of that maximum reaction velocity so when we look at the v-max are basically just asking ourselves an extremely high substrate concentrations what happens to the reaction velocity so here when we add the inhibitor the competitive inhibitor what do you think happens to the v-max probably decreasing mmm-hmm so we often think that decreases but one thing we want to keep in mind is just think well if I increase the substrate concentration a whole bunch how does that affect these reactions so from what you recall from general chemistry if I increase the concentration of the substrate by less chatelier's principle is this reaction going to shift to the left or shift to the right to the right that's correct and if I do that I'm gonna farm a whole bunch of this enzyme substrate complex and I'm going to deplete my supply of enzyme so if I no longer have any enzyme left is my inhibitor able to bind to anything yeah no so in that case is our inhibitor still affecting the reaction rate nope no so in this case the v-max actually stays the same in another way this is often explained is that since they're competing for the same site if you've got 10 substrate molecules 10 inhibitor molecules then it's like 50/50 which when the enzyme binds to but if you've got a billion substrates and only 10 inhibitors then you're really never going to have the enzyme get to the inhibitor because it's just covered with substrates okay mm-hmm all right so I do want to draw what this curves looks like with an inhibitor that the stall deadline was without an inhibitor so we can look at let's say in a dashed line what it does look like within a tempter and what we're gonna see is that with inhibitor it looks like this so you can see that it has the same v-max but if you want to look at the km with inhibitor we can see indeed the value is increased as we discussed does that make sense so far yeah okay so now let's take a look at the next type of inhibitor which is the uncompetitive inhibitor now the competitive inhibitor competes directly with the substrate to bind to the enzyme by uncompetitive that means there is no competition here and as it turns out the inhibitor doesn't even bind to the enzyme itself it binds to the enzyme substrate complex but it creates a similar issue that once you form this enzyme substrate complex you can't react so in this case the inhibitor is it binding to the active site nope and it certainly can't if it can't bind to the enzyme directly so where does the enzyme bind then something else that's not the active site I think allosteric site yes you're absolutely correct Alastor exciting allosteric site is very loosely defined as literally anywhere on the enzyme except the active site so that's absolutely correct so in this case if you look at the reaction and when we add then Hilter are we affecting the ability of the enzyme to bind to the substrate no no so what do you think is happening to the km the affinity of the enzyme for the substrate probably the foam so that's what it appears like but it's a little bit more complicated in once again we can just explain this with a bit of basic chemistry that when I add the inhibitor to the solution what happens to the concentration of the enzyme substrate complex ah does it decrease yes it decreases because it's being used up to form this enzyme substrate inhibitor complex look at this reaction and the concentration of product is decreasing does this reaction shift to the left or does it shift to the right if this is the es decreases it'll shift to the right now that's correct and does that imply that you have more binding or less binding more buy they more binding and does not suggests an increase or decrease in the affinity more binding would be a increase in the affinity but it's increasing damn yes your absolute wreck so our cam here actually decreases and you understand why mm-hmm okay so the way for you to think about it is what the inhibitor is doing is it is locking the enzyme onto the substrate okay so now we want to do the same thing as before and go ahead and consider the v-max which we know from this diagram is just looking at extremely high substrate concentrations and how the v-max is affected so in this case what do you think happens to the v-max um I don't know does it get bigger so in this case if we look at the inhibitor I mean hitters in general do you think they're gonna make the reaction go faster or slower probably slower so you could be thinking slower but we also saw in the last case that it might potentially stay the same so if we want to consider what's going to happen once again just look at this reaction if you dump a whole bunch of substrate we know that's gonna give you a whole bunch of enzyme substrate complex and by for me a bunch of enzyme substrate complex is that somehow overcoming or inhibitor I don't think so no because the enzyme substrate complex is exactly what the inhibitor wants to bind to so the inhibitor is still able to function at high substrate concentrations and if anything's actually able to function even better at high substrate concentrations so in this situation there's no way for us to overcome then Hitler with high substrate concentrations so the v-max does decrease and if we want to go ahead and draw this out on the graph we'd end up with a graph that looks something like this where you can see that v-max is decreased and if you look for the km you'll see that they came with inhibitor is lower with inhibitor than what's out okay mm-hmm all right so now we've discussed two different types of inhibitors the competitive inhibitor can only bind to the enzyme and the uncompetitive inhibitor it can only bind to the enzyme substrate complex so our next inhibitor is the mixed inhibitor and if you had to guess what do you think this inhibitor can bind to complex absolutely hence the idea of mixed so when it binds to either you form either the enzyme inhibitor complex or the enzyme substrate inhibitor complex and in both cases you have an issue because I can't react so the next question then we're on the enzyme do you think this inhibitor binds an allosteric site so you're absolutely correct it's the allosteric site and it's actually a little tricky because the law students would think oh if you can bind the enzyme or Lance I'm substrate complex why can't you bind to both the active site and the allosteric site and that's not how mix inhibitors work they bind to one site and we know that that site can't be the active site because the inhibitor can bind to that enzyme substrate complex where the active site has already been occupied so it does indeed have to bind to the allosteric site okay so now let's think about how this affects the km but before we actually discuss this I do want to mention we've discussed two different inhibitors now that can bind to the allosteric site the uncompetitive inhibitor and the mixed inhibitor but then uncompetitive inhibitor can only bind to the enzyme substrate complex whereas the mixed inhibitor can bind to both the enzyme and the enzyme substrate complex so do you have any idea how we can explain this difference they don't so the way to think about it is when an enzyme binds to a substrate there are two models you've learned about in biology to explain how the Poynting works one model we know is incorrect and one model we know is correct or a base or current prevailing model and you happen to recall what those models are I think it's I think it's the lock and key model maybe like the inducible fit yes correct so locking key model is the one that is wrong which basically say something enzyme and substrate you know just perfectly match each other whereas the induced fit model states that when the enzyme binds to its substrate there is a conformational change that essentially makes them binding even more even more tight so the idea is that when you have that induced fit it can change the shape of the enzyme and in the case of the uncompetitive inhibitor the allosteric site was not available to binding before that induced fit has occurred so the induced fit effectively made that allosteric site available to binding what the mixed inhibitor there is a change in conformation but it just wasn't so significant that the allosteric site became available only after binding or wasn't available afterwards it was available in both cases however that being said just because the inhibitor can bind you both the enzyme and the enzyme substrate complex it doesn't mean that the inhibitor has to have equal affinity for both the enzyme and the enzyme substrate complex so we can actually have two situations so one situation is if the inhibitor has a greater affinity for the enzyme well is that more like competitive inhibition or is that more like uncompetitive inhibition that's correct in that case what do you think was going to happen to the cam-cam would get bigger because the affinity has gone down mm-hmm that's correct so it's more like competitive so cam goes up whereas they've then hip trans greater affinity for the enzyme substrate complex it's more like uncompetitive inhibition so what would happen to the cam would decreed that's correct so well we can see with mixed inhibitors is is actually more complicated because your inhibitor could have different affinities from enzyme an enzyme substrate complex so when it comes down to determining what happens to the cam it's actually a question mark we don't know and you'd have to be given more information about whatever specific enzyme you're looking at to know what happens to the cake okay but that being said we can talk about the Michaelis Menten saturation curve which we know this is what it looks like without inhibitor but for the V Max when you add the mixed inhibitor remember v-max is determined at extremely high substrate concentrations can a ton of substrate overcome this inhibitor I don't know if you add a lot of substrate you have a lot of enzyme substrate complex which would help the inhibitor that binds to the enzyme substrate complex right but can you completely overcome the inhibitor um no because some of it still binds to the enzyme mm-hmm well so not to the enzyme but if you had a bunch of substrate you've got a bunch of enzyme substrate complex but the temperature can still bind it can in still slow down the reaction right yeah so v-max does decrease for sure the cam is variable but this for sure decreases so we can't actually draw a graph because once again depends on the cam but at least we can say that v-max is lower so does that make sense in terms of the three types of inhibitors that there are yes mm-hmm you can see there's really only three types of inhibitors because you either bind to the enzyme or the enzyme substrate complex or both right just looking at the reaction there's really no other way for you to put together another type of inhibition so then we do have that other type of inhibition non-competitive inhibition so what exactly is this and he can probably tell from that we'll have labeled this it's not a type of inhibition it's a subtype of mixed inhibition so remember we said that in mixing ambition you can have an inhibitor that has a greater affinity with for the enzyme or the enzyme substrate complex non-competitive inhibitors are mixed inhibitors with equal affinities for the enzyme and the enzyme substrate complex so in this case what do you think is going to happen to the cam exactly because the increase from the enzyme and decrease from the enzyme substrate complex effectively cancel out so this stays the same and since we've discussed that the inhibitor binds to an allosteric site for mixed still allosteric site and the v-max also has to decrease because once again it is mixed so looking at the graph we can draw the one with no inhibitors which we know we're going to have a certain km and what's interesting is that when we draw the graph for with an inhibitor you can actually see that with inhibitor we end up with the same km so this doesn't affect the affinity of the enzyme for the substrate but it does decrease the reaction rate okay all right so do you have any questions about the differences between the different inhibitors now I think I get it Thanks ok so let's go ahead and spend the last five minutes having you teach this back to me so on this slide right here I have a little table so do you want to go ahead and walk through this table for me and kind of fill in you know what you think are these terms and I of course can go ahead and write this in for you and just to see how well you understood everything we've just discussed okay try my best here so we discussed what a competitive inhibitor is one that actually binds to the acid site of an enzyme and so because of that it the enzymes affinity for the substrate goes down which means your km increases because they're inversely proportional and your V Max I think stays the same just because if you add a lot a lot of substrate it can overcome the effects of the inhibitor mmm-hmm we got there the uncompetitive inhibitor binds to an allosteric site because it's binding to the enzyme substrate complex I think that the km goes down here and then the B max also decreases in this case mm-hmm any recall at km decreases because the uncompetitive inhibitor is locking the enzyme on to the substrate okay so the affinity is actually increasing which is my your km decreases yes a mix inhibitor also binds the allosteric site so it binds either the enzyme or the enzyme substrate complex we talked about the km for this one is variable just because it depends on the affinity that the inhibitor has the Emacs definitely decreases in this case and then non-competitive was the weird fourth one that also binds to the allosteric site because of the type of mixed inhibitor and we talked about that the km would stay the same just because you've got an equal to Z for the enzyme and the enzyme substrate complex and this case the v-max also goes down mm-hmm great job you're absolutely correct and you can see here in the next slide I basically had all the answers and everything you described was absolutely correct so very nicely

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