Which of the following represents a reaction of an isomerase? Alright. So if we take a look at the first one, what's occurring here is that we have this hydrogen getting replaced by this phosphate group. So we're removing a phosphate group from ATP in order to do this. We're transferring a phosphate group. So this is a kinase. Kinase doesn't belong to the class called isomerases, so this would not work.
For the next one, it may look like we've done something to our starting material to create this other one, but we really haven't. If we were to number this in order to name it, we'd start off from this end: 1, 2, 3, 4, 5. Here, this would be 2-pentanone. And again, we've discussed naming ketones in earlier chapters. So it's going to be important that you are able to identify that these are the same molecule. Here would be 1, 2, 3, 4, 5. This is also 2-pentanone. Nothing has been done to this molecule; it's just been flipped and turned around to make it look like something has occurred. So this would be nothing.
For the next one, we have lost hydrogens here in order to make a pi bond here. We can say that this is a form of oxidation, we've lost hydrogen so we could say maybe an oxidase got involved here, something that helped us to lose hydrogen. And we know that that is not an isomerase. We didn't make an isomer by moving things around. So this would not work.
Now, by process of elimination, d is the answer. But why? Remember, an isomerase is making an isomer. An isomer has the same formula but different connections. And if we were to look at these 2 and add up all the carbons, all the hydrogens, all the oxygens, they both would have the same exact formula, but they look different. That's because they're isomers of each other. So this one was a bit tricky, but these are our answer. K? It may have been hard to see, but again, isomers, same molecular formula, different connections in this case. And because of that, because we have isomers, that means an isomerase had to have been used. Alright. So again, our answer would be d.