in Chapter 13 we're going to move on to the citric acid cycle so where we left off when we're thinking about glycolysis is we had glucose and the final product we converted it into pyruvate what we're going to be doing now is taking pyruvate and turning it into acetal COA so this is not part of the citric acid cycle but it is a reaction that needs to occur in order for the citric acid cycle to occur once we have a Seal Co it's going to get fed into the citric acid cycle the citric acid cycle is going to produce carbon dioxide which will be then expired um and then it will generate these electron containing compounds that will then go into electron transport and generate ATP so what we're doing is we're taking aetl COA and we're changing the molecule in order to produce these high energy molecules and co-actors and also release carbon dioxide at the same time so this is a catabolism process we are still going through the process of glucose getting turned into smaller bits of matter okay so let's take another look at this just a little bit more in detail now so first we've got this pyruvate remember that was our end product of glycolysis but we can also have parts of our amino acids or fatty acids that can come into our citric ice acid cycle as well but we're going to deal mostly with pyate so first pyate is going to be turned into acetal koay so acetyl COA is that molecule that's going to go into the citric acid cycle so this is what we consider our stage one so this is going to be taking um our pyate molecule and transforming it into a molecule that can be used in citric acid cycle so in stage two the citric acid cycle is going to oxidize this acetal COA and we're going to go around the cycle we'll produce two equivalents of carbon dioxide an equivalent of ATP and some molecules that will um be able to be turned into electrons for electron transport later on so stage three of this process will be that electron transport and oxidative phosphorilation which we will get to in later videos I just need you to know that it's coming down the pipeline all right so if we're looking at bacterial cells all of this process is going to happen in the cytool because they don't have mitochondria but in Mamon C cells this will occur in the mitochondria and so you've probably heard that the mitochondria is the powerhouse of the cell and that's because uh we're going to have citric acid cycle that occurs there and oxidative um electron transport and oxidative phosphorilation so all of these processes in this picture here are going to be taking place inside the mitochondria all right so here we can see the the mitochondria here we've got these different The Matrix Parts The Matrix is where the citric acid cycle is going to occur all right look looking at this we're going to go through a series of reactions so first we've got condensation so we're going to take our cetal COA and we're going to combine it with oxaloacetate um and we're going to make this new molecule citrate citrate will be dehydrated uh and then rehydrated so essentially we're going to be just changing its form with this enzyme aconitase to make isocitrate isocitrate will then um do a decarbox silation reaction to give us our Alpha ketoglutarate and the cycle is going to continue until we get back to oxyacetate so oxyacetate will then combine with another molecule of acetyl COA and that's how this is going to go into a cycle so the citrate that you get once you combine acetyl COA and oxelo acetate there's the two carbons from acetal COA will become carbon dioxide that is released um and then the rest of the carbons will get turn back into oxaloacetate okay so to insert this acid cycle we need pyruvate to get into our mitochondria and so here we can see we've got this translocase which is going to bring it in with an H+ ion um so that's going to be powering it is this electron gradient and now the pyruvate is in our mitochondria and we can react it with um this enzyme that is going to convert it into acetyl COA so at the same time acetyl this acetyl COA or sorry this COA molecule will become release a carbon dioxide and we will also reduce NAD plus to nadh so just the process of pyruvate becoming acetal COA generates us a carbon dioxide and nadh remember nadh is a reducing agent but it can also be used in our electron transport chain in order to get ATP from it all right it looks like this is a very simple process just take your pyruvate turn it into acetyl COA but it's actually pretty complicated so here's our pyruvate molecule here and what we're going to have is just a series of transfer reactions so our pyu is going to do a transfer reaction with this TP TPP now TPP is going to be um a co- co-actor so it's going to be from your B vitamins as an aside um you're going to notice that a lot of the enzymes that we're going to be dealing with have um B vitamins associated with them and that is because B vitamins are co-actors in most of are glycolysis and citric acid cycle um enzymes so if you're ever looking at an energy drink especially one that says that it has no sugar in it a sugar-free energy drink most energy drinks don't have sugar they don't have fats they don't have proteins and so I hope that you would ask yourself well how are you getting your energy then because all of the energy molecules that are are all the molecules that can be directly converted into energy are carbohydrates fats and lipids but how an energy work drink works is it usually has well first a lot of caffeine and and guanine which is a caffeine derivative but the other thing that it has is it has B vitamins and B vitamins are your co-actors for your metabolism enzymes so they're going to help you do glycolysis and they're going to help you do citric acid cycle so that's why they would be considered energy molecules is because they're used as co-actors in these energy producing processes okay so we have this pyu and we're going to transfer these two carbons on our pyruvate to this TPP so this is going to generate this intermediate it's not really important what this intermediate is but it is attached to this TPP and this carbon uh carboxilate here is going to get released as a carbon dioxide now this group here is going to get transferred to this disulfide uh compound so we've got this uh disulfide Bond on this on this uh five membered ring so it's going to end up opening up that five membered ring and now we've got this acetal group on the sulfur so this is an activated acetal group so we've got a very nice leaving group our co-a is going to come in the sulfur will do um a attack on the carbonal The sulfur here will leave giving us this this diol and our acetyl COA has been generated now that's all we need really to generate the acetyl COA but to get this into a cyclical form so it can work again uh we have a series of um reduction reactions here so we need to oxidize our sulfur and we need to reduce our fad so this fad is going to be reduced making fadh2 this fadh2 can then uh reduce NAD plus to nadh so this is very cool what this means is that we are going to be generating one NAD um this should be nadh here and one fadh2 for every one pyruvate that comes through so when we generate that acetal COA we are going to be getting energy molecules out of it huate dehydrogenase which is the enzyme that we just saw is going to be regulated in exactly the ways that you would expect that it would be regulated so we have these three parts um so again we had this dehydrogenase which was going to transfer the TPP we had this acetal transferase and then we had another dehydrogenase here this is all part of the same complex of pyruvate dehydrogenase so each part is going to be regulated by exactly how you would expect if we've got a lot of nad+ that's going to activate reducing that NAD plus to nadh if you've got a lot of nadh that's going to inhibit it if you have a lot of COA that's going to activate our transferase if we've got a lot of aceto COA that's going to inhibit the acetal transferase so same thing we've seen before we're going to have some positive regulation and some um negative regulation based on concentration of reactants and products okay on to the citric acid cycle I'm going to recommend as we are going through the citric acid cycle that you um take uh this picture and maybe print it out have it near you or have your book open um maybe on your phone just somewhere where you've got this picture so you can see where we are at in the cycle as we're going through because otherwise as I'm talking about the different reactions it's not going to be very clear where we are in the cycle but if you have this in front of you while I'm going through each reaction it will help a little bit keep things straight all right our first reaction we're going to take our acetyl koay and our oxyacetate and we're going to turn it into citrate so here's our acetyl COA our oxelo acetate we're also going to use a water and we're going to uh make the citrate molecule this is the only step in the citric acid cycle that we will increase the size of our molecule so we will be making a carbon carbon Bond so what we can see is that this carbonal group here here is going to get added to this carbon uh the one with a carbonal here and then we will also be taking the water molecule and essentially Distributing the oxygen and hydrogens across our products let's take a little bit closer look at how this mechanism is going to work so we've got our acetyl COA here we'll have some sort of Base so this could be an amino acid side chain like a lysine that's going to remove this hydrogen it's going to make a double bond here and the two electrons in the carbonal are going to come and grab this hydrogen so this is going to make us have this enol intermediate now this enol intermediate is going to be very reactive That Base can then grab that hydrogen back those electrons can fall back down and now the two electrons in our alken can come and attack the carbonal of oxelo acetate so that's why it's going to add to that carbonal to stabilize that that carbonal has this hydrogen it can grab to make this hydrogen here so that gives us this citroil COA intermediate water will come in and do a an attack here on this carbonal which will release our COA group and so now we've got this COA group um that has been released as a product and we've got our citrate molecule and of course we'll have some protonation de protonation St steps based on PKA and pH all right our next step is the aconitase reaction we're going to take the citrate molecule and we are going to turn it into this D isos Cate so what is interesting about this reaction is that we're going to take this molecule which is a chyal there is not a chyro carbon in this molecule so this carbon here is SP2 hybridized SP2 hybridized SP2 hybridized so they can't be chyal because they're SP2 hybridized uh these three carbons are all sp3 hybridized but this carbon and this carbon have two hydrogens on them remember you need four different groups to be chyro and then this carbon gives the illusion that it could be a chyro carbon but it's not because up here we've got a ch2 with a carboxilate and here we've got a ch2 with a carboxilate so it still has two of the same group on it so this is an ayal molecule we're going to first do this dehydration reaction where we form the alken and then this alken is going to be hydrated selectively so essentially what we're doing is just switching the location of this o and this hydrogen so there's this o and this hydrogen so we're doing a dehydration and then a hydration reaction but when we do that we are going to be generating two chyo carbons so now this carbon and this K carbon are both chyro and you can see how this occurs in the enzyme if you imagine that um the O and the carboxilate group um so that's this o and this carboxilate group have to go into specific locations then we've got our ch2 Co minus from each of these so that's this group here and this group here are also going to go into specific locations so that when you do the reduction reaction followed by the oxidation reaction the O is always going to be adding from a particular direction and that's what's going to give this molecule the the um chyo carbons is the specificity that's going to be conferred by the enzyme itself so very cool reaction there where we're uh stereos selectively generating this um these chyo carbons now we're going to take that isocitrate so again we've moved that o from this carbon to this carbon and we're going to do a dehydrogenase reaction so we're going to be taking this um this alcohol and turning it into this Ketone we call this a beta uh carbonal here so this would be our Alpha and our beta so we're removing two hydrogens because we are going to be oxidizing this carbon that means something else has to be reduced and that is a major theme in the citric acid cycle and really all of metabolism is that if something is oxidized something else has to be reduced so since this carbon here is being oxidized we're making a double bond between it reducing the number of carbon hydrogen bonds this nad+ can be reduced and it's reduced to this nadh and now we've got this carboxilate it will be removed as carbon dioxide and and that will become this alphao glutarate which has the ch2 group here so again we're just decarboxylating here this one is um also an oxidation reduction reaction but uh what is oxidized is the carbon here it's being oxidized to carbon dioxide and then the carbon itself is being reduced as we add a hydrogen to it next we're going to take that Alpha ketoglutarate um and we are going to turn it into sucal COA so that COA that we've generated earlier we can now use it again um what we're going to do in this case is is we're just going to be replacing this carboxilate here so we're going to be replacing it with the acetyl COA so we'll do a backside attack here attack this carbonal and um have carbon dioxide leave again we have another oxidation reduction reaction the carbon in this carboxilate is going to get oxidized to carbon dioxide and so we're able to take our nad+ and reduce it to nadh so again another oxidation reduction reaction now we have activated this carbonal we have this very excellent leaving group here with this co-a group our next reaction is the suin COA done with sexin COA synthetase we're going to take that sein COA and turn it into succinate so what we're going to do is we're just going to be oxidizing this carbon uh transferring this acetal COA group but in the process of transferring the suin COA group we're able to uh remove a phosphate and get go from GDP and inorganic phosphate to GTP and remember that GTP is equivalent to ATP so this would be an energy producing step all right so here's what it looks like in the active site of the enzyme so we've got our suin COA um we've got our COA group here on the carbonal our inorganic phosphate is going to come and attack this carbonal and our COA group is going to leave now we've got this histadine side chain in the protein that can come and attack this phos phorus with this oxygen as the leaving group now we've got this imidazol group with a phosphate on it and our succinates and we can transfer this phosphate that's on the histadine from GDP from from this histadine to GDP to give us GTP and regenerate our histadine that it does not have the the um phosphate group on it giving us back our active enzyme which can find another molecule of sual COA so we have Su succinate uh which is this molecule here we can see we've got two ch2 groups and then two carboxilate groups so what we've done here is we've put this kind of in a um a configuration so we can see that if we remove these hydrogens and they're in an anti-configuration from each other if we remove these two hydrogens and make a double bond across them that is going to give us this molecule Fumarate where our carboxilate and our hydrogens are trans to each other so because we are removing these hydrogens this would be a reduction uh or sorry an oxidation reaction um so we're going to be reducing something else and what we're going to reduce is this molecule fad it'll be reduced to fadh2 um and then this fadh2 that will these two hydrogens will get transferred to this co co-enzyme Q which will then be reduced to co-enzyme qh2 uh don't really worry too much about that we will talk about this when we get to electron transport but the important part is that this fadh2 molecule is something that we can take into our electron transport chain and generate energy from and in fact this enzyme itself is part of our electron transport chain so it is complex two of our electron transport chain that means nothing to you now but when we get into electron transport it will mean something and it's so much a part of our electron transport chain that this particular enzyme is embedded in the membrane of our Matrix it is not a cytosolic enzyme so it is actually a membrane bound protein because that is where electron transport uh chain takes place but now we have Fumarate generated so this Fumarate with our trans carboxilate and trans hydrogens uh will uh will add hydrogen across sorry water across our double bond to make this molecule malate so this is the hydration reaction of our alken so now we have this L malate that we have formed all right we're going to take the malate and nad+ we're going to be oxidizing this alcohol to a ketone because we're doing an oxidation reaction of course we have to do a reduction reaction we've done a lot of oxidation reduction reactions and that's intentional we're generating on purpose a lot of nadh and fadh2s because we'll be able to use them to generate ATP later on so we've got this secondary alcohol we're reducing it to the Ketone and that will help us sorry oxidizing it to the Ketone that will allow us to take our nad+ and reduce it to nadh so that gets us back to where we started which is oxaloacetate so we've gone through this entire cycle we're back to oxaloacetate it can now bind another molecule of aceto COA and go through this cycle again here we have the summary of all of our reactions of the citric ail CC acid cycle and the enzymes that are going to catalyze them uh so we're starting from acetal COA and oxyacetate making citrate citrate is going to become isocitrate in the aconitase enzyme isocitrate goes to Alpha glutarate in the isocitrate dehydrogenase enzyme we're going to take that and turn it into seino Co in our Alpha ketoglutarate dehydrogenase complex we're going to take the cinal COA and turn it into suxin in our sual COA synthetase enzyme then we'll take the seate turn it into Fumarate again in seate dehydrogenase Fumarate will become malate in our fumarase enzyme and the malate becomes oxaloacetate and malate dehydrogenase so it's good to know what each enzyme does in in the citric acid cycle in addition going through one cycle from one turn so one molecule of acetal COA going through is going to generate for us an nadh here an nadh here an ATP P equivalent here an fadh2 here and an nadh here so every one turn of our citric acid cycle will generate us three nadh's one fadh2 and one ATP or GTP equivalent now remember for every molecule of glucose though we will go through the citric acid cycle twice because every molecule of glucose will give us two pyruvates those pyruvates will become two acetal coas so we will go through a citric acid cycle twice for every glucose molecule okay so if we're looking at this in terms of energy equivalents um if we start with glucose and we are going from glucose to pyruvate that's uh our glycolysis reaction that's going to give us two atps and two nadhs that was just from glycolysis so every nadh is going to give us 2 and 1 12 ATP molecules so two nadh's will give us the equivalent of 5 atps going from pyruvate to acetyl COA we've got two molecules this time we will generate two nadhs which will again give us five ATP equivalents the acetal COA can then go through the citric acid cycle one turn of that citric acid cycle is going to give us six nadh's to this is fadh uh 2 here remember it can get transferred to that um coenzyme qh2 um the 6 nadh's at 2 and 1/2 ATP each will give us a total of 15 atps the fadh2s will give us one and a half atps so not as energy rich as the nadh is so those two will give us a total of three atps and then also that GTP is the equivalent of 2 ATP so going from one molecule of glucose through glycolysis and then through the citric acid cycle we will net a total of 32 ATP molecules so we'll get four just from going through the cycles and then we'll get 28 more by taking these molecules here and putting them in oxida phosphorilation so that is a lot of energy that we get for every one molecule of glucose of course all of this is going to be regulated and I'm not going to spend a lot of time on this I just want you to see that we've got a lot of Regulation that occurs so first p rate to acetyl coate is very highly regulated we've got some up regulation that occurs and some down regulation that occurs so think about these molecules why would pyruvate activate this this enzyme well of course if we've got a lot of pyruvate we need to turn it into acetyl COA um why would insulin increase the activity of this think about that so if we've got insulin that is telling us we've got high blood glucose levels so we want to be metabolizing that glucose so we already talked about how insulin is going to help turn the glucose into glycogen this helps us turn glucose into ATP if we've got that insulin signal of course if we've got a lot of ATP then we want to inhibit this process if we've got a lot of nadh we don't need to generate more nadh all right as we're going through we can see that we do have a lot of regulation in on these initial steps so turning acetyl COA into citrate isocitrate into Alpha keto glut glutarate and then that Alpha ketoglutarate into cinal COA there is a lot of Regulation that occurs in our citric acid cycle in addition to the regulation the citric acid cycle can provide intermediates for many other biosynthetic processes so here this circle here is indicating our citric acid cycle here's aetoo which comes in it's generated from pyruvate so please keep in mind that just because something goes into the citric acid cycle doesn't even mean that it's going to complete the citric acid cycle so the citrate that we generate can be used um in fatty acids synthesis and to make steriles our Alpha ketoglutarate can be transaminated to make glutamate which can make other amino acids or our purine bases suino Co is used to make heem and chlorophyll oxelo acetate can be used to make aspartate so some amino acids and your purines and your peridin which are used in your RNA and your DNA so these molecules can get removed from the citric acid cycle and so we also will have mechanisms that we can replenish the oxaloacetate and so that's why we see this pyruvate has an arrow down to oxyacetate is because so many molecules can get removed from the citric acid cycle if they all get removed then your acetal COA doesn't have anything to react with so we regenerate the oxaloacetate so that it can combined with acetyl koay and continue our citric acid cycle process