Binding of Multiple Substrates – Enzyme Classification

by Kevin Ahern, PhD

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    00:01 Enzymes as I noted at the beginning can bind reactions in different ways and I talked about one substrate going to one product or two substrates going to one product.

    00:11 Or the case I am going to describe here, two substrates going to two products.

    00:14 Now when we think of two substrates that can bind to an enzyme we realize that there is different ways that they could bind.

    00:21 For example if we have the reaction A plus B goes to C plus D.

    00:26 We could imagine that maybe A would have to bind first and then B. Or maybe B binds first and then A.

    00:33 But what we find is that for some enzymes it really doesn't matter which one binds first.

    00:39 This is called random binding as it shown in first example that I have on the screen.

    00:44 Random binding means it doesn't matter.

    00:47 Now some enzymes bind substrates randomly as I am showing you here. But a lot of enzymes do what's called ordered binding.

    00:54 That means that either A must bind first or B must bind first.

    00:59 Now that model or that mechanism is significant and the reason it's significant is it's probably the best illustration that I can give you for the Koshland Induced Fit model.

    01:11 Because what order binding tells us is that if one of these must bind first before the other one does. That means then that the binding of first one is actually changing the shape of the binding side for the second one.

    01:25 Because, if the second one tries to bind first, the change hasn't already happened and that's why the second one can't bind first. So ordered binding reinforces the Koshland Induced Fit model.

    01:37 Now that might seem to cover all the territory but there is actually a third model that enzymes use to catalyze reactions and this one is kinda interesting and it has a fun name.

    01:46 We called it the Ping-Pong mechanism and it's also called double displacement reaction.

    01:51 But the point is the same, the Ping-Pong mechanism is an enzyme that actually exists in two covalently different states.

    02:02 It means that the enzyme is actually physically binding to something and causing a change. We will see this happen in the next slide.

    02:09 Now this illustrates a reaction of A plus C going to B plus D, and we are seeing it split into two reactions.

    02:21 Alright. In this reaction what's happening is A is starting out with an oxygen on it.

    02:27 And in the reaction of A to B the oxygen is being replaced by an amine.

    02:34 So, we see this happening and where is the amine coming from? The amine is coming from the enzyme.

    02:40 So the enzyme is carrying the amine and it's carrying it to A.

    02:45 So when A interacts with the enzyme the enzyme swaps the amine that it is carrying for the oxygen that's on A.

    02:55 So on the right side of the top equation we see that A has become B; because, it now has an amine and the enzyme has grabbed the oxygen. It no longer has an amine. So I have colored it with green so you can see that.

    03:08 In the second part of the reaction, C which has an amine is interacting with the enzyme that now has an oxygen.

    03:18 And when that happens they trade places C becomes D, where D has a double bond to the oxygen, and the enzyme has become linked to an amine.

    03:30 Alright? So the enzyme has returned to its original state.

    03:34 So by this Ping-Pong mechanism the enzyme is continually going from amine to oxygen, to amine to oxygen, and depending upon which states it is in, it determines which of the substrate it binds and swaps with.

    03:51 Now this type of reaction that I just described to you is a common reaction that is used by enzymes called transaminases.

    03:57 Transaminases are enzymes that do just what I have described.

    04:00 They swap oxygens for amines and this is a very important reaction in the metabolism of amino acids; because, amino acids get their amines, in some cases, by the reaction that you see on the top.

    04:16 They start out with a double bonded oxygen and they become an amine.

    04:19 A really good example of this is the molecule alpha ketoglutarate in the citric acid cycle.

    04:25 Alpha ketoglutarate can become glutamic acid if the oxygen on it is swapped for an amine.

    04:32 In this way the cell can make an amino acid that it might need; because of this mechanism.

    04:39 On the other hand we might have the situation where glutamic acid or even another amino acid is needed for energy.

    04:47 People that go on low carb diets for example, don't have a lot of carbohydrates.

    04:52 But they are not starving to death; because, they are eating plenty of protein. Proteins providing amino acids.

    04:57 And amino acids provide energy as a result of what I am showing you here.

    05:02 The lower left reaction has an amino acid that has the amine replaced by a double bonded oxygen. So imagine if amino acid on the lower left side is actually aspartic acid.

    05:15 Aspartic acid can be converted by swapping its amine with an oxygen into oxaloacetate.

    05:23 And oxaloacetate can be oxidized in the citric acid cycle.

    05:26 So this transaminase reaction is important to both for making amino acids and also for metabolizing amino acids for energy.

    About the Lecture

    The lecture Binding of Multiple Substrates – Enzyme Classification by Kevin Ahern, PhD is from the course Enzymes and Enzyme Kinetics.

    Included Quiz Questions

    1. Ordered binding requires one substrate to bind first
    2. The ping-pong mechanism refers to the enzyme flipping between T and R states
    3. Tandom binding allows the enzyme to rapidly switch between two different states
    4. Random binding is best explained by the Induced fit model
    1. Transaminases
    2. Ligases
    3. Lyases
    4. Hydrolases
    5. Peroxidases

    Author of lecture Binding of Multiple Substrates – Enzyme Classification

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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