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Competitive inhibition | Energy and enzymes | Biology | Khan Academy - YouTube
Channel: Khan Academy
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we've already seen that an enzyme helps
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catalyze a reaction so let's say this
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right over here this is our enzyme and
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we have our substrate and it goes and it
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binds to the active site to the active
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site of the enzyme so let's say it binds
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right over there
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so that's site on that on the enzyme we
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call the active site where the substrate
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binds and then the enzyme catalyzes
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reaction maybe it breaks up the
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substrate into two smaller molecules and
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so after the reaction after the reaction
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we the enzyme whoops after the reaction
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the enzyme is unchanged but a reaction
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has been catalyzed we now have we now
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have the substrate being broken up in
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this case at least into two smaller
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molecules maybe I'll draw them that's
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one of them and this is the other one
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right over here so they just came they
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just came from the active site once the
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once the reaction is catalyzed they
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don't they don't have the affinity to
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the active site anymore and they break
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off so this enzyme has just catalyzed
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this reaction what I want to talk about
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in this video is how this might be
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inhibited and and specifically how it
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might be inhibited competitively so
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we're going to talk about competitive
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inhibition so competitive let me write
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it over here competitive competitive
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inhibition in hish case of competitive
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inhibition if there's some if there's
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some molecule that competes for the
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substrate at the active site as we'll
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see this isn't the only form of
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competitive inhibition but this is the
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the one that you will most typically see
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in a textbook so let's do so that's our
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enzyme again and so that's our enzyme
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and we've already seen that this is an
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act this is right over here where I'm
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circling that is the active site active
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active site and if the molecule the
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intended substrate in case you could say
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gets to it we're going to have this
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first scenario up here but in classic
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competitive inhibition or at least the
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version I'm just going to show you right
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now
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you could have another molecule you
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could have another molecule that let's
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say it looks something like this that
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can compete for the active site and if
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it gets and if it gets to the active
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site first so if it gets there first so
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let me show what's going to happen so
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then we have our enzyme we have the
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other molecule not the intended
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substrate binds to the active site first
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well now the intended substrate the one
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for which the enzyme catalyzed reaction
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isn't able to bind and the reaction
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isn't going to happen and you can see
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very clear that they are competing for
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the enzyme in there in this case they're
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competing for the active site now this
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isn't the only form of competitive
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inhibition another form of competitive
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inhibition is allosteric competitive
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inhibition let me write this down so you
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have allosteric all么
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steric competitive competitive
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inhibition in I'm having trouble writing
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inhibition and an allosteric site is a
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site other than the active site but in
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competitive in allosteric competitive
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inhibition or competitive allosteric
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inhibition however you want to say it
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you have a scenario where the competitor
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the competitor doesn't bind to the
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active site but binds to a site that is
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not the active site and now we'll steric
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site we could say so in that one the
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competitor maybe might bind here so
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that's clearly not the active site so
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maybe the competitor looks something
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like that it didn't bind to the active
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site but by binding there the active
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site is no longer it can no longer bind
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to the intended substrate so you have
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the same effect you have the same effect
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right over here where this thing isn't
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going to bind but if this thing binds
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first so let me draw that scenario so if
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the intended substrate binds first then
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the competitor can't bind so in this
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scenario if the substrate is able to get
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to the active site well then the
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competitor can bind so once again
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they're competing so I'll draw the
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competitor
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here so then the competitor whoever gets
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to it first gets the enzyme so in this
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situation the competitor is not going to
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bind so that's true of whether you're
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talking about competitive inhibition
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where they're competing for the active
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site if the competitor gets there first
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the intended substrate isn't going to
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get there and the the reaction isn't
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going to be catalyzed if or if or if the
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intended substrate gets first then the
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competitor is not going to be able to
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get there in fact it could have been
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this situation where because the
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substrate got there first the competitor
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the competitor isn't going to be able to
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bind to the active site we're talk about
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allosteric competitive inhibition we're
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still competing for the enzyme only ones
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going to get it if one gets to the
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enzyme first then the other ones not
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going to be able to get there they are
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competing for the enzyme but the
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competitor the non substrate is just
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acting in an allosteric site by binding
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to an allosteric site it changes the
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conformation of the enzyme so that the
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active site no longer binds to the
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substrate and I want to really emphasize
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this point because when I first learned
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this I said oh it's often sometimes
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confused and even some things you'll
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read on the internet that they'll say
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that this is that this allosteric type
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of inhibition they'll call this
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non-competitive because you're not
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competing for the active site but that
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is actually not the case in
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non-competitive inhibition and I'm going
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to do the whole next video on
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non-competitive inhibition and
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non-competitive inhibition the the
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inhibitor the inhibitor right over here
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can bind regardless of whether the
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substrate has bound or not but when the
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inhibitor does bind it prevents the
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reaction from moving forward it changes
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the the conformation of the protein so
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it no longer catalyzes the reaction so
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non-competitive they both can bind but
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if the inhibitor is there the reaction
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isn't going to proceed in competitive
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inhibition whether we're talking about
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allosteric or non allosteric competitive
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inhibition only only one of the
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substrate or the inhibitor is going to
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be able to bind they're competing for
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the enzyme
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