Michaelis-Menten Kinetics – Enzyme Catalysis

by Kevin Ahern, PhD

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    00:01 Now this is kinda complicated so I wanna step you through it. But these are considerations for doing reactions and what are called Michaelis-Menten Kinetics.

    00:08 We can see here that on the y-axis again, we have concentration and on the x-axis we have time.

    00:14 And before we saw simply the accumulation of product as shown in the orange icon here, at the very beginning of a reaction what are the circumstances. Well, we have four different things to think about that can be measured. We have the concentration of substrate.

    00:33 We have the concentration of the enzyme. We have the concentration of the enzyme substrate complex, and ultimately we are gonna have concentration of product which is what we are interested in studying, okay? At the beginning, the concentration of product is low, as you can see.

    00:47 And that's not surprising, because the reaction hasn't had a chance to get started.

    00:53 The concentration of ES is low because there hasn't been an opportunity for the substrate to really encounter the enzyme very much.

    00:59 The concentration of the free enzyme, that is the enzyme not bound to substrate is relatively high. You see it's coming down from the y-axis.

    01:07 And finally the concentration of substrate is high because none of the substrate has reacted.

    01:13 So at the time 0, we have these circumstances going on. And these circumstances turn out not to be ideal for us to measure the enzymatic reaction. Now as the reaction proceeds we see changes to these entities.

    01:26 We see first of all that the concentration of substrate by the end reaction is low and it's following during the entire process.

    01:33 The concentration of the ES substrate which started out at 0 is going higher and we will see that it will eventually sort of level off.

    01:42 We also see that the concentration of the free enzyme which started out to relatively high position is falling and it too will sort a level off in time. And finally we see of course that the concentration of product is going to start at the low and go to high by the very end.

    01:55 Well, I show you this graph not to complicate the picture too much hopefully, but rather to demonstrate what we tried to do in studying enzymatic reactions.

    02:07 In the very initial phase, I hope I have made the case for you, that we are in a set of conditions called pre-steady state. Now I will explain what's steady state means in a minute, but we have a circumstance where the reaction hasn't had a change to get started.

    02:19 The enzyme isn't doing its thing and everything is there changing pretty rapidly.

    02:25 The change in substrate, the change in enzyme, the change in enzyme substrate complex and the change in product.

    02:31 This is gonna give us a lot of variability in a reaction. Now I said we want to study initial velocity, but we wanna be careful if we do it too soon we may not get what we are after here.

    02:42 So it's important to think about really studying a reaction at a place where these things have sort of leveled off.

    02:49 Now in a conditions of steady state, what's actually happening is that these other quantities that were varying fairly rapidly in the very initial phases of the reaction will start to even out, and that's very important for our consideration.

    03:03 So we can see for example that, in that early state the concentration of free enzyme and ES complex are changing.

    03:10 The concentration of E is falling very rapidly and the concentration of ES is rising very rapidly.

    03:16 However, under steady state conditions, as we can see here, they have started to flatten out.

    03:21 They are not exactly linear but they are much closer to linear then they were in that pre-steady state condition.

    03:27 That turns out to be important for us; because, what we are interested in studying is the conversion of enzyme substrate complex into product. And so if we have a relatively constant concentration of enzyme substrate complex, then that decay or that folly into product that has actually happened is happening at a relatively constant rate.

    03:49 That's the place we wanna be and that's why it's important for us to be studying these reactions under steady state conditions.

    03:53 Steady state conditions, of course, again, meaning that these quantities are not varying significantly.

    03:59 Now, we can see now the overall plot of what's happening on here. The steady state conditions are where we make our measurements and we see that this relatively linear portion of the plot for the concentration of free enzyme and concentration of ES complex is happening under the conditions as we measure enzymatic reactions.

    04:20 So in this presentation, we have seen some details now about how enzymes interact with substrate, how enzymes can manipulate energy to make things happen the way they want, how at the electron level, enzymes can accomplish what they do. And we have also learned something about the way that we study the kinetics or the speeds of reactions. In another presentation, we will put these together or use this information to study some specifics in enzyme kinetics.

    About the Lecture

    The lecture Michaelis-Menten Kinetics – Enzyme Catalysis by Kevin Ahern, PhD is from the course Enzymes and Enzyme Kinetics.

    Included Quiz Questions

    1. The velocity of a reaction is related to the concentration of substrate used
    2. There is an inverse relationship between substrate concentration and reaction velocity
    3. The velocity of a reaction is unrelated to enzyme concentration
    1. In relatively steady state reaction conditions
    2. At high product concentration conditions
    3. At high enzyme concentration conditions
    4. At low product concentration conditions
    5. At low substrate concentration conditions

    Author of lecture Michaelis-Menten Kinetics – Enzyme Catalysis

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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