Non-competitive Reversible Inhibition – Enzyme Inhibitors

by Kevin Ahern, PhD

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    00:01 Another type of inhibition that is important for us to understand is that of non-competitive inhibition.

    00:05 It's fundamentally different from a competitive inhibition and we can see it depicted on the screen here.

    00:10 On the left again we have the enzyme with its normal substrate which catalyzes a reaction.

    00:15 However, the enzyme has a site on it that if it's properly targeted by an inhibitor the inhibitor can bind to it and keep the enzyme from functioning properly with the substrate in the active site. That is shown in the image on the right.

    00:30 Now when this happens, the non-competitive inhibitor has a fundamentally different way of interacting with the enzyme then what we saw before.

    00:39 They effected by binding at a different location and by binding at a different location they do not compete okay? Now this changes the parameters of the velocity that we have been studying considerably.

    00:52 And because the inhibitor doesn't compete with its substrate and the substrate can't out way it by doing a reaction with not a lot more substrate.

    01:04 It means that in every reaction that we do what happens is that we are inhibiting a fixed amount of an enzyme.

    01:12 It doesn't matter how much enzyme that we add, there is always the same amount of enzyme inhibited.

    01:17 In the first reactions, the competitively inhibited reactions, we saw that as we added more substrate the substrate out-competed the inhibitor and it was as if the inhibitor disappeared.

    01:29 So the quantity of enzyme being an inhibited was changing, the more substrate we added the more normal enzyme we had.

    01:37 With a non-competitive inhibitor, we don't have that.

    01:39 It doesn't matter how much substrate we have; because, they are not competing for the same site. The non-competitive inhibitors always going to knock-out the same amount of enzyme in every tube irrespective of how much substrate is added to it.

    01:55 That means that we have changed the amount of enzyme.

    01:58 And if we change the amount of enzyme, we have already talked about the limitations of an enzyme in studying of the Vmax.

    02:05 Remember the factory analogy? In the factory analogy I said, that if we added an extra factory would double the amount of product.

    02:12 What if the factory only worked half a day? If the factory only worked half a day, it would make half the amount of product.

    02:20 We have changed the numbers of workers.

    02:22 So, what if we use enough inhibitor that we only have half the amount of enzyme? Well we will change Vmax accordingly.

    02:28 So when we have a non-competitive inhibitor we are changing the amount of enzyme and in changing the amount of enzyme, we changed the value of Vmax.

    02:38 So Vmax decreases for a non-competitive inhibitor.

    02:42 That wasn't the case for a competitive inhibitor, right? Now we can only measure Km for an active enzyme.

    02:48 And not surprisingly if we change the amount of enzyme Km, the affinity of the enzyme for the substrate doesn't change; because, the enzyme is still the enzyme when it's active and we are only studying an active enzyme. So the Km value doesn't change for non-competitive inhibition.

    03:06 In a Lineweaver Burk plot we see something different than we saw with the competitive inhibition, but consist in it what I just told you.

    03:14 In green again, we see the linear the linear plot showing, of course, the uninhibited reaction.

    03:21 In blue we see the non-competitively inhibited reaction and we notice that the two lines cross at -1/Km.

    03:29 Well this is consistent with what we learned in the last plot which is that the Km value doesn't change. They should cross at that point.

    03:35 However we see the blue line has a higher slope than does the green line, meaning that the crossing of the y-axis is at a higher point.

    03:47 Now they may seem kinda intuitive that if we decrease the Vmax we actually are raising the value of that line. But remember we are doing a reciprocal.

    03:56 So by decreasing Vmax, 1/Vmax actually increases.

    About the Lecture

    The lecture Non-competitive Reversible Inhibition – Enzyme Inhibitors by Kevin Ahern, PhD is from the course Enzymes and Enzyme Kinetics.

    Included Quiz Questions

    1. Alanine
    2. Pyruvate
    3. Fructose-1,6-bisphosphate
    4. cAMP
    5. Methionine
    1. The km value remains unaffected in the presence of inhibitor, but the Vmax decreases
    2. The km value increases and Vmax decreases
    3. Both km value and Vmax increase
    4. Both km and Vmax remain unaffected
    5. The Km value decreases and Vmax increases

    Author of lecture Non-competitive Reversible Inhibition – Enzyme Inhibitors

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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