The citric acid cycle intermediates are used to make many things, and amongst these are porphyrins. These must be replaced by the anaplerotic reactions that form oxaloacetate. Now, there are other things that the intermediates can make. For example, succinyl CoA, which makes porphyrins, can also be used to make hemes. Alpha-ketoglutarate is used to make amino acids. Oxaloacetate is also used to make oxalo acids and is used to make pyrimidine, so nucleic acids ultimately. It's important to know that the anaplerotic reactions are going to replace oxaloacetate, right? They are forming oxaloacetate to replace whatever intermediates need replacing.
Now, the isocitrate dehydrogenase reaction, I have drawn below. Here it is. I'll jump out of the image to give you a better look at it. So, that's what's going on, right? Basically, what's happening is we're losing CO2. That's the CO2 here. We are forming NADH from NAD+. Remember, this reaction is not like the pyruvate dehydrogenase reaction. Oh, and also we are forming a double bond between this oxygen and this carbon. See that over here.