How long is the krebs cycle

The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available.

If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. Step 2. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.

There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle.

This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. In particular, protein synthesis primarily uses GTP. Step 6. Step six is a dehydration process that converts succinate into fumarate. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly.

This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion. Step 7. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.

Another molecule of NADH is produced. Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule.

Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms. The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules.

Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic both catabolic and anabolic.

In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A acetyl CoA.

Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle Krebs cycle. The conversion of pyruvate to acetyl CoA is a three-step process.

Breakdown of Pyruvate : Each pyruvate molecule loses a carboxylic group in the form of carbon dioxide. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. Note: carbon dioxide is one carbon attached to two oxygen atoms and is one of the major end products of cellular respiration. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; the lost carbon dioxide is the first of the six carbons from the original glucose molecule to be removed.

This step proceeds twice for every molecule of glucose metabolized remember: there are two pyruvate molecules produced at the end of glycolysis ; thus, two of the six carbons will have been removed at the end of both of these steps. Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA. This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, the citric acid cycle.

Let me draw it a little bit bigger. Let me draw a mitochondria here. So this is a mitochondria. It has an outer membrane. It has an inner membrane. If I have just one inner membrane we call it a crista. If we have many, we call them cristae. This little convoluted inner membrane, let me give it a label. So they are cristae, plural. And then it has two compartments. Because it's divided by these two membranes.

This compartment right here is called the outer compartment. This whole thing right there, that's the outer compartment. And then this inner compartment in here, is called the matrix. Now you have these pyruvates, they're not quite just ready for the Krebs cycle, but I guess-- well that's a good intro into how do you make them ready for the Krebs cycle?

They actually get oxidized. And I'll just focus on one of these pyruvates. We just have to remember that the pyruvate, that this happens twice for every molecule of glucose.

So we have this kind of preparation step for the Krebs Cycle. We call that pyruvate oxidation. And essentially what it does is it cleaves one of these carbons off of the pyruvate.

And so you end up with a 2-carbon compound. You don't have just two carbons, but its backbone of carbons is just two carbons. Called acetyl-CoA. And if these names are confusing, because what is acetyl coenzyme A?

These are very bizarre. You could do a web search on them But I'm just going to use the words right now, because it will keep things simple and we'llget the big picture. So it generates acetyl-CoA, which is this 2-carbon compound.

And this process right here is often given credit-- or the Krebs cycle or the citric acid cycle gets credit for this step. But it's really a preparation step for the Krebs cycle. Now once you have this 2-carbon chain, acetyl-Co-A right here. This long talked-about Krebs cycle. And you'll see in a second why it's called a cycle. Acetyl-CoA, and all of this is catalyzed by enzymes. And enzymes are just proteins that bring together the constituent things that need to react in the right way so that they do react.

So catalyzed by enzymes. This acetyl-CoA merges with some oxaloacetic acid. A very fancy word. But this is a 4-carbon molecule. These two guys are kind of reacted together, or merged together, depending on how you want to view it.

I'll draw it like that. It's all catalyzed by enzymes. And this is important. Some texts will say, is this an enzyme catalyzed reaction? Everything in the Krebs cycle is an enzyme catalyzed reaction.

And they form citrate, or citric acid. Which is the same stuff in your lemonade or your orange juice. And this is a 6-carbon molecule. Which makes sense. You have a 2-carbon and a 4-carbon. You get a 6-carbon molecule. And then the citric acid is then oxidized over a bunch of steps. And this is a huge simplification here. But it's just oxidized over a bunch of steps.

Again, the carbons are cleaved off. Both 2-carbons are cleaved off of it to get back to oxaloacetic acid. And you might be saying, when these carbons are cleaved off, like when this carbon is cleaved off, what happens to it? It becomes CO2. It gets put onto some oxygen and leaves the system. So this is where the oxygen or the carbons, or the carbon dioxide actually gets formed.

And similarly, when these carbons get cleaved off, it forms CO2. And actually, for every molecule of glucose you have six carbons. When you do this whole process once, you are generating three molecules of carbon dioxide. But you're going to do it twice. You're going to have six carbon dioxides produced. Which accounts for all of the carbons. You get rid of three carbons for every turn of this.

Well, two for every turn. But really, for the steps after glycolysis you get rid of three carbons. But you're going to do it for each of the pyruvates. You're going to get rid of all six carbons, which will have to exhale eventually.

But this cycle, it doesn't just generate carbons. So we'll write that here. And this is a huge simplification. I'll show you the detailed picture in a second. We'll do it again. And of course, these are in separate steps. There's intermediate compounds.

I'll show you those in a second. It will produce some ATP. And the whole reason why we even pay attention to these, you might think, hey cellular respiration is all about ATP. The reason why we care is that these are the inputs into the electron transport chain.

These get oxidized, or they lose their hydrogens in the electron transport chain, and that's where the bulk of the ATP is actually produced. And then maybe we'll have another NAD get reduced, or gain in hydrogen. Reduction is gaining an electron. Or gaining a hydrogen whose electron you can hog.

And then we end up back at oxaloacetic acid. And we can perform the whole citric acid cycle over again. So now that we've written it all out, let's account for what we have. So depending on-- let me draw some dividing lines so we know what's what.

So this right here, everything to the left of that line right there is glycolysis. We learned that already. And then most-- especially introductory-- textbooks will give the Krebs cycle credit for this pyruvate oxidation, but that's really a preparatory stage. The Krebs cycle is really formally this part where you start with acetyl-CoA, you merge it with oxaloacetic acid.

And then you go and you form citric acid, which essentially gets oxidized and produces all of these things that will need to either directly produce ATP or will do it indirectly in the electron transport chain. But let's account for everything that we have. Let's account for everything that we have so far. We already accounted for the glycolysis right there. Now, in the citric acid cycle, or in the Krebs cycle, well first we have our pyruvate oxidation. That produced one NADH.

But remember, if we want to say, what are we producing for every glucose? This is what we produced for each of the pyruvates. This NADH was from just this pyruvate. But glycolysis produced two pyruvates. So everything after this, we're going to multiply by two for every molecule of glucose. And then when we look at this side, the formal Krebs cycle, what do we get?

We have, how many NADHs? One, two, three NADHs. So three NADHs times two, because we're going to perform this cycle for each of the pyruvates produced from glycolysis. So that gives us six NADHs.

We have one ATP per turn of the cycle. That's going to happen twice. Once for each pyruvic acid. So we get two ATPs. And then we have one FADH2. But it's good, we're going to do this cycle twice. This is per cycle. So times two. We have two FADHs. So sometimes instead of having this intermediate step, they'll just write four NADHs right here. And you'll do it twice. Once for each puruvate. But the reality is, six from the Krebs cycle two from the preparatory stage.

Now the interesting thing is we can account whether we get to the 38 ATPs promised by cellular respiration. So we have four ATPs.