when we saw the image of the reaction occurring we saw these various states that you see on the screen the enzyme plus the substrate bound together to make the es complex which converted upon the change in the enzyme to the es star complex which created the EP or the enzyme product complex which ultimately resulted in the release of the enzyme in the product now I come back to this because we are going to need to consider some things about the kinetic parameters that is the Meccan of the speed of parameters of the reactions that we're going to study now this rate of formation of product is really what we're interested in when we talk about how fast an enzyme can make a reaction occur this is the guts of what we're after we want to know how fast is the enzyme able to do this well if to do this we need to make some simple assumptions and so we assume in the simple case that the enzyme substrate complex proceeds directly to enzyme plus product okay so we've simplified this more complicated equation above to a simpler equation below and this is done to help us better understand what's going on in the price and the overall mechanism now these constants that are here won't really enter into our consideration but the cake at that you see in the enzyme going to be plus P will in fact be an important consideration for us as we talk about the kinetic parameters the k-kat as we shall see is the rate with which product is forming now let's consider what's happening inside of a couple of different scenarios of a reaction we can imagine that we have enzymes for example shown in yellow and we have substrates as little red colored balls that are there we could have a situation first of all where we have a reaction going on in a condition of low substrate and if we have a little amount of substrate in a solution we could imagine that there's very few enzymes that are going to be bound to substrate because the chances of encountering a substrate are reduced in the middle of course we have an intermediate state where we have a little bit higher concentration of substrate than we did before and so we can see here that there are more enzyme molecules bound to and engaged in the process of making the product and the third scenario we could imagine is high substrate and when we have a situation of high substrate we notice here that every enzyme is bound to a substrate and that's important because it high substrate concentrations we have enzymes that are what we call saturated with substrate meaning that once it is bound to substrate made a product and released it almost instantaneously it grabs another substrate it's not sitting around and waiting for things now so enzymes interestingly have some kinetic considerations which is of course what we're interested in studying here but we see now for the first time a projection of the way that the enzyme is working so I need to explain some things on the graph that you see first of all we're plotting on this graph a reaction the reaction is plotting the velocity of the reaction on the y axis versus the substrate concentration that's used in the reaction on the x axis now you notice the V has a little zero beneath that and the zero beneath that I'll explain later but it's called the initial velocity for our purposes the velocity of a reaction is measured as the concentration of product made divided by time the concentration of product made for time when we measure concentration in molar millimolar micro molar etc so that would be some molarity per time that is how velocity is measured the substrate concentration varies because to generate a curve like this I do not one reaction but I do a series of reactions so let me set that up we could imagine for example that I'm setting up a series of 20 reactions 20 different test tubes I want to measure the velocity in each one of those test tubes and what I do is I take into that test tube I place the buffer that holds the substrate I place the substrate and I place the enzyme now when I'm doing an experiment I want to have one variable because one variable is the only thing I can really manipulate and measure the effect of that the variable I have here is substrate concentration I use the same amount of enzyme in every tube all twenty tubes have the same amount of enzyme they all have the same amount of buffer and they have varying amounts of substrate starting from very small amounts to very high amounts I take and I let each one react for an exact same time and then I measure the amount of product so by doing that I can see the effect of measuring of changing substrate on the velocity and then I plot it so what you see on the screen is the some of those plots that is each each point on that lot on that dot came from a series of reactions that I did and each one of those individual reactions had a specific substrate concentration and a specific velocity that was reached well not surprising as we look at this what do we see well on the far left were at low substrate concentration what's the velocity it's very low and that's what I showed on the original image low substrate concentration enzyme is sitting there waiting for substrate there's not going to be much velocity when I get to a high substrate concentration such as they see on the right side of the screen I've got a high velocity make sense okay low substrate low velocity high substrate high velocity I want you to remember that now I'm showing another plot here to illustrate a principle of a reaction on that y-axis I have the concentration of product we could think of that again as velocity but on the x-axis now I'm plotting the time of reaction so I could take one of the tubes that I used in the previous one and for example look at how fast the product is being accumulated and what happens to that product over time well we can see on this plot that over the early range of the reaction there's a linear relationship between the production of product and time okay but after a while what happens is that that curve flattens off now what that means is that the longer that we let a reaction go it doesn't stay linear forever and the reason it doesn't stay linear forever because remember enzymes catalyze reversible reactions so the more we let product accumulate the more likely product will start being converted back into substrate well that's not what we're interested in studying we want to study how fast the enzyme makes product so if we're going to study an enzymatic reaction we have to study what's called initial velocity we don't want to wait too long in order to study the concentration of product because if we wait too long we're actually starting to study the reverse reaction and that's not what we're after so that's why we use vo or the initial velocity in our measurements you just completed your first video of the world's best medical exam preparation lector EO brings the knowledge of worldwide leading medical experts and teaching Award 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