all right engineer in this video we're going to continue on and we're going to talk about the transition step or even sometimes they call it the preparation phase so if you remember we left off with forming pyruvate from glycolysis right so from glycolysis we made pyruvate and pyruvate was a three carbon molecule right it was a three carbon molecule that we formed from the end product of glycolysis and if you remember I said so again it's three carbons if you remember I told you that the only way that pyruvate can get taken into the mitochondria right is if there is oxygen because and remember nadh that was generated within glycolysis has to go and drop those hydride ions onto the electron transport chain if not and there's no Oxygen it drops those hydrides off on pyruvate and converts them into lactic acid and this pathway won't occur but if there is oxygen so again when is this part occurring this is occurring in the presence of oxygen in other words it has to be under aerobic conditions so the only way that pyu can get taken into the mitochondria and start getting converted into this next molecule is if there is oxygen present now when pyruvate is transported from the cytool into the mitochondria there's a specific enzyme right here that's going to be controlling this we're going to talk about this in more detail in the biochemistry video but specifically look what happens here pyruvate is three carbons right look what happens at the end of this reaction it goes from three carbons to two carbons another thing happens let me show it like this look at this you see that right there I added something onto this two carbon molecule this two carbon molecule has this thing on it called a CO enzyme a so what did I do I took pyruvate which is my three carbon molecule so this is my Pi Ru of8 and guess what I convert it into I convert it into a c COA acetal COA now acetal is two carbons pyruvate is three carbons so that means I had to lose a carbon I do and whenever you lose carbons you know when you know what it's called Whenever there is a removal of a carbon they call that decarbox okay so decarbox silation is removing a carbon off of this substrate in the form of CO2 so that's one part of this enzyme okay this is going to be working I'm going to mention the enzyme in a second another thing that the enzyme does is obviously it's adding on this coenzyme a it's adding on this Co enzyme a this is getting added into this reaction you know what else is happening I'm taking n a positive and I'm converting this into nadh so this guy has must has a specific hydride ions that I'm going to rip off of him in order to convert him into acetyl COA and for him to be able to gain hydride ions so now here's another thing remember I told you that at the end of glycolysis I form pyruvate right but how many of the pyruvates two I'm actually making two of these pyruvates so I'm actually getting two pyruvate molecules so if I'm getting two of these pyruvate molecules how many aceto am I making two if I'm making two acetyl coas how many co2's am I actually generating I'm technically one from the first pyate and then another from the second pyruvate so that is a total of two co2's how many coas am I adding into this process then I must be adding in two coas because I'm going to get two acetyl coas and how many NAD positives are being converted into nadh's well if there's two pyruvates one for one pyruvate and then second for the other pyu so this is two of these this whole process is regulated by a specific enzyme and this enzyme we'll talk about this in great detail in Biochemistry this enzyme is called a pyruvate D hydrogenase enzyme and this enzyme is super super interesting but he is the enzyme that is catalyzing this reaction here he is the one who is catalyzing this reaction now whenever this happens remember that this is a this is a irreversible step in other words this step can't go backwards I can't go from a cocoate to pyruvate so remember that pdh is not going to be a reversible enzyme it's irreversible it only works in uni directional Not bidirectional Only One Direction so again as a result what did I get out of this transition state I went from I brought the pyruvate into the actual mitochondria and the presence of oxygen aerobic conditions in other words the nadh had to drop those hydrides onto the electron transport chain if that happens pyu gets brought into the mitochondria acted on by the pyu dehydrogenase and then what happens this enzyme will pull off two CO2 molecules add in two coas generate two nadhs and form two acetyl coas which will then go down into the crab cycle all right in the next video we're going to go over the mechanism of this process in more detail and the regulation all right see you then