hello organic chemistry students in this video we're going to talk about the KB cycle it's also known as the tri carboxilic acid cycle among several other names that are commonly used but the most predominant name that people accept is the KB cycle now it's not saying those other names are wrong but this is the most commonly used one now we just finished talking about the electron transport chain which took nadh and donated it to complex one fadh2 donated um either a hydride or two individual electrons to complex 2 keep in mind that the nadh donates a hydride the complex one and this all is occurring on The Matrix side of the ETS system which is on the inner membrane correct right so as we're donating electrons from The Matrix side into the complex system on the inner membrane it's all on The Matrix side so where does the KB cycle occur it occurs in the Matrix of the mitochondria and this makes sense if the ETS is utilizing nadh and fadh2 to power the etss it's better for it to have a quick access of these reduced forms of the electron carriers and that's why having the CB cycle in The Matrix is a perfect balance of this now just like when we talked about with the ETS oxygen is required for the electron transport chain to run the electron transport chain running is required for the kreb cycle to work in its cyclic form of taking electron carriers in its reduced form and allowing them to do their thing basically forming ATP so the etss has to work in order for the kreb cycle to produce electron carriers that can be used for ATP production why am I stressing this parts of the kreb cycle can work without the ETS system but it's not for energy production let me stress that it is not for energy production so in terms of energy production which is what we care about the ETS is required for the kreb cycle to be operating so now let's go ahead and look at what the kreb cycle looks like oops oops sorry just moving everything else around but the image I'm trying to move wow there it is okay here is the kreb cycle and we're going to learn each step of this of the system if you notice right now there's nothing shown over these arrows and that's what we're going to be talking about next now a blank copy of this it has been put on our module page on our canvas site highly recommend that you print this out and fill in all the steps that we're going to be talking about with each reaction we're going to be talking about the first reaction second 3D fourth fifth 6th 7th and 8th now when we look at the kreb cycle we're going to start with oolo acate binding with acetal COA what is all that we're going to get into it in just a couple of minutes in a couple of moments I should say and that's going to form citrate citrate is going to have the o on this carbon and a hydrogen right here ripped off basically removing water and then it's going to flip the molecule and put the same o and hydrogen back on if you notice it took the O from this this tertiary carbon right here and it put it on this secondary carbon what a waste of steps doesn't it seem like that but we're going to see why that's important then next we're going to somehow lose this carboxilic acid that's going to be crazy and then we're going to come through lose another carboxilic acid add on a COA lose the COA to form a dicarboxylic acid species then do an oxidation of this carboncarbon single bond to a carbon carbon double bond add water across it oh wait we know how to do that and then we're going to take this alcohol and oxidize it to a ketone to return oxal acetates very very important stuff so let's go ahead and get right into each individual step of the KB cycle all right so the very first reaction in the kreb cycle is when acetal COA is going to be bound to an enzyme now each step of this kreb cycle requires a specific and unique enzyme it's not the same one doing at all we're not going to get into the names of them whatsoever this enzyme right here is going to take a cetal COA and bind to it in one pocket of it one one part that has a high tertiary structure that recognizes acetyl COA and the other side it's going to bind oxaloacetate its tertiary structure recognizes that molecule so they're both being held in let's say the left and right hand of this enzyme they're going to be brought in close proximity of each other and allow a reaction to occur let's go ahead and dissect this so in this enzyme there is a base present within the aceto COA binding pocket when we look at this aceto COA now before I get too much further in you're looking at COA um this s COA here here and throughout the KB cycle we see it here and there what is it is it just a sulfur with a Cobalt and an a which doesn't have an abbreviation no it's part of a much bigger system so let me go ahead and scroll down this is what acetyl COA is look at that here's the sulfur that we're mentioning here is the COA P portion of it so the COA is this right here COA for this big part and here is the SH now this is just used as an ACL transfer reagent within our body to allow for the transport of systems we don't we don't have to worry about the structure of this I just want to show you what it looks like it is huge it has a nitrogenous base component right here a carbohydrate derivative notice this phosphate right here and these two phosphates so for a um a nucle type of system then we have this part in here for recognition into different enzymatic processes and then the sulfur that can add into carbon yields pretty nicely all right let's go ahead and go right back up forget about this big structure and talk about the reactions that are happening all right so here in acetyl COA if it's bound in an enzyme let's go ahead and just have this enzyme right here here's the binding pocket of it who cares how big it is there is a base present in this binding pocket it is bound to the enzyme and this base is going to pull one of these protons on the carbon directly attached to the carbonal HM does this sound familiar wow pulling a proton on a carbon directly attached to a carbonal that's class three carbon yal chemistry that's why we were learning it it's the first step of the kreb cycle so the base will pull this proton allowing electrons to Res through the carbonel and what we end up forming here is our enolate species now keep in mind the enzyme has aceto COA now deprotonated and oxaloacetate in close orientation to one another so what it's going to do with the O um oilo oxaloacetate excuse me let me go ahead and just write the carboxilic acid here is the ketone ch2 carboxilic acid it's going to bring it into close proximity so we can donate electrons down and that carbon will attack the carbon yel breaking open the double bond wow this is direct class 3 carbon yel chemistry and this looks like a aldol like reaction so right here we're doing aldol chemistry in the kreb cycle class 3 carbon Neal chemistry and then when it's all said and done the COA portion gets hydr off and we get the CO2 right here so here's the acetyl COA here's the oxaloacetate forming citrate now notice I called this carbon 1 2 3 and four let's go ahead and number them over here where is carbon 1 carbon 1 is this right here Carbon 2 is now an alcohol carbon 3 and carbon 4 carbons five and six are the components that came in from acetal COA why are we mentioning this why do we care about this when met metabolic Cycles are first discovered or were first discovered we have most of them down we had to figure out how they work so we'd put radio labels in at different positions of the substrates and track them in a living system we'd see where it would end up and we can start figuring out how the reactions occur in these um these metabolic pathways so now citrate is formed then citrate is this is the one I was telling you about we're going to remove water flip the m molecule and put the water back on so if you look at the numbering of citrate up above here is our carbon one here is 2 3 and four five and six so we took the alcohol off of carbon 2 and a hydrogen off of carbon 3 swapped them and put them back on the molecule this step right here is done by an enzyme called aconitase I absolutely love this enzyme it's so cool it seems like it's the most ridiculous enzyme ever why would you pull water off flip the molecule and put water back on seems totally crazy and when they first discovered this scientists thought they made a mistake so they kept researching it more and more and more thinking nope we're making a mistake making a mistake and guess what no mistake was made this is exactly is what is happening in the second step of the kreb cycle water being removed flipped putting water back on Wow Let's go ahead and look at step three now so when we're looking at this here is our carbon 1 again here is two here's three here's four five and six in this we're going to have NAD and there should be a plus sign there please forgive me and we're going to produce n DH wow NAD takes electrons in the form of a hydride off of a the substrate right what in this molecule could be oxidized can carboxilic acids be oxidized any further no can ch2s be oxidized easily no what about alcohols ooh this secondary alcohol could be oxidized to a ketone right there wow so previously when we had um citrate that was a tertiary alcohol which was nonoxidizable here we oxidized the secondary alcohol we get an nadh out which can go to the electron transport chain and give us 38p but what we also form here is is something interesting we form CO2 gas is liberated so here is carbon 4 here is carbon 3 this is Carbon 2 5 and six we just lost carbon 1 as CO2 how on Earth the this happen so in the first step the alcohol is going to be oxidized to a ketone that is what the NAD is doing it is removing the electrons as a hydride forming an nadh oxidizing the alcohol to a carbon yal now where when we're doing this organically we kept removing the electrons from the oxygen in this system right here we are actually going to have the hydride and if we think about remember remember the structure of the NAD just going to show the active portion of it there it is right there this carbon hydrogen bond breaks we lose a hydride into the nad+ and then these electrons donate down so we lose the hydride on the carbon of the alcohol this is reverse of the oxidation we did organically with chromium reagents Sr and PCC which is a chromium reagent we always pull the electrons through the oxygen there here it is a direct hydride removal from the carbon we're not pulling the hydride off the oxygen and that should make sense because oxygen is more electronegative than carbon we can't pull the electrons in that way so when everything is all said and done well from this first step of NAD going to nadh we have now formed this molecule we have carbon one here com up here we have carbon uh 2 3 and four is carbon 1 in a beta position to this carbon y it is so I have a beta carbon yield to a carboxilic acid now at physiological pH these carboxilic acids are in their deprotonated form that's why I drew this way a lot of people do show them in their neutral State both are perfectly acceptable but keep in mind the pka of carboxilic acids are relatively low so they'll be in their deprotonated state so if we remember about our decarbox reactions we first lost the proton oh it's gone could this negative charge donate down break open the double bond donate electrons back down and break our carbon carbon Sigma Bond yes it can in doing this this is the decarbox that we talked about in the class 3 carbonal chemistry so if I went ahead and showed those arrows we donate electrons oop wrong Arrow we donate electrons down break open the double bond donate it down and resonate electrons through the carbon Y which forms a carbon carbon double bond with an O on it that tzes to give us Alpha keto glutarates Wow first step or the third step of the KB cycle the first step that's actually producing nadh so let's go ahead and pretend that the KB cycle stopped right here it doesn't we have so much more fun to go through let's say it stopped how many ATP did we directly produce at this step of the creb cycle now right now some of you are thinking three but notice I said directly the kreb cycle didn't form any ATP did it no the electron transport chain did so there is no ATP formed directly from the kreb cycle now you might be thinking oh that's just at this step right here no that's for the whole cycle no ATP is formed directly from the kreb cycle the electron transport chain is required to do that oxidative phosphorilation so now the fourth step we're going to take this carboxilic acid carbon 4 and we're going to lose it oh my gosh look at that there's carbon 4 gone it left the molecule it's out of there so now we have carbon 3 2 5 and six so we can still track them and we put another COA tag on this molecule in this process we're oxidizing the system because as we lose this CO2 right here we have a ketone or it then becomes an aldhy it gets oxidized to a carboxylic acid NAD harnesses the hydride to become nadh and we lose a carbon dioxide molecule and that is what how we form sual COA the nadh because it's in The Matrix can then go to the electron transport chain and form three more ATP so as of this point right here how many atps has the KB cycle produced directly once again the answer is still none it's formed two nadhs and two co2's and if we combine it with the electron transport chain we have now formed six atps wonderful now we lost a carbon dioxide here we lost a carbon dioxide here two carbons when we think back to that acetal COA we don't care about the COA part that's pretty big how many carbons were in there two so when we started off the kreb cycle we started with oxaloacetate that has four carbons we then add aceto that has two carbons that we care about to form citrate six carbons we then do that crazy reaction forming isocitrate and then isocitrate is oxidized and decarbox to form Alpha ketoglutarate Li liberating one CO2 we're starting to get carbon balance back in order for the KB cycle to stop we have to get back to oxaloacetate four carbons long and then Alpha ketoglutarate to sexanal KOA we liberate another um CO2 while forming an nadh and if you notice whenever we form CO2 we're also forming nadh and we have more electron carriers and we've returned our second carbon back to the environment and we're now down to four carbons but sual COA is not Alpha is not um oxaloacetate so more Transformations are required and more electron carriers will be formed step five sual Co um is now going to go through oh and let me go a and pause let me pause this here all right pardon that little pause right there I just wanted to get the carbon numbers back on that molecule 3 2 five and six we've lost carbons one and four to carbon dioxide sual COA is going to be transformed into suin what happens here we're losing the COA when we lose the COA we're actually forming an energy surrogate and here a GDP and an inorganic phosphate will be form will be joined together to form a GTP it sounds like ATP but it's not g is guanine a is Adine this is not ATP and in this process we oxidize the aldhy that we get when we clip off the COA to a carboxilic acid and that's where these electrons come from and now we have formed succinate go ahead and tell me where is carbon 3 where's carbon 6 uh-oh I could say this is carbon 3 I could say this is carbon 6 and this is 2 and five but someone else might say that was 6 5 2 and 3 both of them are correct we've lost our ability ility to track the carbons at this stage and this is important for studying metabolism every step up until the formation of su8 we could track and find the reaction now we're scrambling it throughout the rest of this reaction and that's going to be very important for us to keep in mind so once we form succinate we have scrambled the carbon numbering and we can say if I was to ask you what carbon number is this from the start of the C cycle you would say it's either Carbon 2 or carbon 5 Carbon 2 from aolo acetate or carbon 5 from acetyl COA we don't know which one it is it could be either or because we now have a symmetric molecule or dare I say it a miso molecule let's go ahead and look at step six step six is going to take succinate and we're going to form a carbon carbon double bond right here this carbon carbon double bond species will have two carboxilate or car carboxilic acids if you want to show them in this trans orientation why trans it's more stable now in this we're going to take an fad and form fadh2 how many atps do we get from fadh2 we get two with the electron transport chain how many do we get for one nadh we get three why on Earth is our body using an fad here instead of another NAD and getting more bang for its Buck the oxidation of alcohols is easy an nad+ is a mild oxidizing reagent it can do small oxidations nothing too strenuous but here we are oxidizing the carbon uh C2 to C5 single bond to a double bond this is Harsh when you think about F A when we show how to it accepts electrons we're not destroying aromaticity there are we no we're we're rearranging carbon nitrogen double bonds to allow electrons to flow through it nad+ destroys aromaticity so we need to have a high energy hydride such as the oxidation of an alcohol here this is even higher in energy but it requires something more stable and robust to pull the hydrides off of it the hydride and the proton and that's why fad is used sadly I wish it could be nadh for more energy but we have an fad pulling the hydride to form fadh2 so my big question is right now is what carbon number is this right here once again it could be three or five no idea how about this carbon it could be six or three we're still unknown it scrambled everything when we formed suat all right we're nearing the completion of the CB cycle now so now we have our F now if we remember the electron transport chain weren't we pumping protons from The Matrix to the inner membrane space and then those protons came flooding back through the ATP a pump forming ATP yes we did so is the Matrix acidic yes it is now it's not super acidic it's not a pH of one by any means but it's acidic where we have free H+ so could a carbon carbon double bond pick up a proton it could doesn't this kind of sound like our alkane chemistry I'm sorry alken chemist chemistry that we talked about which seems like a long time ago and form a carbocation we can't could water then attack that carbocation and lose a proton we could and what do we just form right there actually we won't do the proton I'm going to actually show the proton transfer so we'll get rid of that right there so the water then adds onto it here CO2 minus CO2 minus and we have the hydronium ion we will then lose one of those protons and we've now formed malates so my question is this could be carbon 3 or 5 this could be carbon 3 or 5 is the alcohol on carbon 5 three or a um a proportion of both a scrambling of both is the proper term and the answer is a scrambling of both remember both of them are secondary carbons on that double bond so we can form either secondary carbon cadine and have water attack said carbo cadine so we have no selectivity because we have the same exact types of carotin but now we're at malet we are so close to forming or reforming our oxaloacetate now here malate will then be oxidized as secondary alcohol to this Ketone and we're now back to oxaloacetate back at the beginning is this carbon right here carbon 3 or five once again we don't know it's a scrambling isn't it it could be five or three we've returned to oxaloacetate we formed another nadh that can go to the electron transport chain and form three more ATP and we're going to talk about the counts here in a second but the important thing is we've now tracked the carbons through this pathway we've tracked where electron reduced electron carriers are formed as well as good old carbon dioxide and this is what we want to make sure that you write down on the blank c copy of the KB cycle sheet that's attached in this module so let's go ahead and look at the overall summary here in summary what comes in acetal Co we don't care about the co- part we care about the acetal groups two carbons as a whole so if we're kind of keeping track of this here are two carbons one oxaloacetate for four carbons comes in we're going to use three NAD pluses one fad one GDP and a in IC phosphate what comes out of the kreb cycle let me say that again what is produced directly from the kreb cycle we produce two co2's we produce one oxaloacetate our carbon balance is in check six came in six came out three nadh's one fadh2 and one GTP the three nadh's can go to the electron transport chain and form 9 ATP the 1 fad H2 will go to the etss and form two fad or or form two um oh that's supposed to be atps oh my gosh why did I write nadh like that let's get rid of that that was my bad we form 9 ATP and we form two atps and the one GTP gets converted into ATP in a step outside the kreb cycle so once again there's no ATP form directly in the krebb cycle all the ATP is formed outside of the kreb cycle the only thing the kreb cycle forms is electron carriers and carbon dioxide that's it so one acetyl COA activating the KB cycle for a full turn gives us 12 atps wow so if we have three acetyl coas are present in The Matrix under aerobic conditions how many ATP do we form if one acetyl COA gives us 12 we have three of them now we have 36 ATP let's make this a little bit harder let me go ahead and Shrink this down if I can there it is so what if we have the same set of conditions up above but complex one or actually say complex 2 is inhibited so comp complex 2 is inhibited which means all the fadh2s that we form or the ATP from the fadh2s don't get formed so we're having nine atps per acetyl COA we have three of them we have 27 a TP kind of notice how we can play around with the Inhibitors again as well with the electron transport chain so what if complex one was inhibited we'd only form a gr total of six ATP from the atps from the fadh2 of complex 2 what if we uncoupled the atpa pump from the electron transport chain we'd form no ATP we still have the electron transport chain running but there's no coupling to the atpa pump so no ATP being formed whatsoever all right now some people at that point right there might argue but what about the ATP from the GDP going to ATP that conversion right there actually let me get rid of that line this this does not require the electron transport chain and this is where some people would say if the atpa pump was uncoupled you would still form 3 ATP and that's a true statement it absolutely is I'm not going to expect you to know that very small technicality here if you remember it wonderful but I'm not going to ask you questions about oh if it's uncoupled how many ATP no that's too technical we just want to focus in on what's the big picture how does the kreb cycle produce electron carriers and where do those electron carriers go we now know it produces the electron carriers right here for the ETS to form ATP so going in reverse the electron transport chain forms ATP but we needed nadh and fadh2 to do this job where did those come from o the KB cycle wonderful now the KB cycle started with oxaloacetate didn't it sure did and the joining of one ACL Co where did the acetyl co come from aha that will be a topic of another video so right here just remember the KB cycle forms three nadh's one fadh2 and one GTP as well as two co2's and one new oxaloacetate when one acetyl COA joins with one oxaloacetate in the kreb metabolism highly encourage that you go back and watch this video again fill out your blank copy of the kreb cycle sheet so you can have all the wonderful story on one piece of paper and if you have any questions please feel free to email me or come to office hours or discussion section I'll be happy to help you whatever I can I hope each of you are doing well and I look forward to seeing you all soon