In this video, I will show you the best way to remember the Krebs cycle or the citric acid cycle. Many students find it extremely difficult to remember the Krebs cycle as it contains so many intermediates, enzymes, chemical reactions and carbon dioxide flowing around. Guess what, there is a very easy way to remember it and retain it for years. I will first tell you how you can remember the names of all the intermediates of Krebs cycle.
Second, we will discuss an easy way to remember the carbon counts and energy changes. And lastly, we will look at how to remember all the enzymes and what they do. Before beginning, make sure to watch our video on detailed explanation of Krebs cycle as this will help you to utilize this video in a much better way.
So this is the Krebs cycle, not a very pleasant sight right? Let's simplify the Krebs cycle to the bare bones of this cycle and the first step to memorizing the Krebs cycle is to know about the key molecules. Now there are 8 key molecules that make up the Krebs cycle. In these, citrate is the first molecule. Now here is a simple mnemonic to memorize the 8 key molecules.
Can I keep selling substances for money, officer? In this mnemonic, C stands for citrate, I stands for isocitrate. Similarly, we have alpha-ketoglutarate, succinyl-CoA, succinate.
Fumarate, and then we have malate and oxaloacetate. Since this mnemonic consists of two S alphabets consecutively, just remember succinate is always late, so succinate always shows up after succinyl CoA. Next it is important to learn about the carbon count changes and the energy changes in the grep cycle. Remembering the carbon counts is very simple.
We will start this with a very simple concept. When we eat food, it is converted into pyruvate and stored. When we need energy, this pyruvate is converted into acetyl CoA molecule. This acetyl CoA actually is the starting point of Krebs cycle. Acetyl CoA is a 2 carbon compound and it combines with oxaloacetate which is the 8th molecule of Krebs cycle and it is a 4 carbon compound.
This combination produces citrate which is a 6 carbon compound. Now the krebs cycle goal is to take this acetyl CoA and break down all the energy storing bonds in acetyl CoA and to release this energy in a form that we can use. The best way to memorize the carbon count is to think about the process starting with the 6 carbon citrate and the carbons just dropping off from there until we get the 4 carbon oxaloacetate again. So acetyl CoA has 2 molecules and oxaloacetate has 4 molecules of carbon. They combine 4 plus 2 to form the 6 carbon molecule citrate.
In the next molecule, no carbon change occurs, just molecular rearrangement occurs, so 6 carbons again. In the next reaction, we remove 1 carbon again to produce the 5 carbon compound alpha-ketoglutarate. The carbon removed is lost in the form of carbon dioxide.
Next we again drop 1 carbon in the form of carbon dioxide to produce 4-carbon succinyl-CoA. And from here on forward, the carbon count remains the same till oxaloacetate, succinate, fumarate, malate, and oxaloacetate all have four carbons until oxaloacetate combines with another molecule of acetyl CoA to form citrate. Now, as I mentioned earlier, the whole purpose of Krebs cycle is to produce energy.
This energy is carefully extracted in the form of hydrogen ions and stored in energy carrying molecules known as FAD and NAD. The hydrogen ions reduce NAD and FAD to form energy shuttles NADH and FADH2. which are used in electron transport chain.
Click on the card above to learn more about the electron transport chain. Now you can remember that the NADH production takes place at the carbon drops of the cycle and at the final step of the cycle that is conversion of malolate to oxaloacetate. These steps are facilitated by dehydrogenase enzymes.
We will learn more about these enzymes in just a bit. The only step using a dehydrogenase enzyme but not producing NADH is the step facilitated by succinate dehydrogenase. This step produces QH2 which then donates its hydrogen to FADH2. So, two NADH are produced where the carbon dioxide is produced in the cycle and one NADH is produced at the final step of the cycle that is conversion of malate to oxaloacetate.
In these reactions, dehydrogenase enzymes are used and one FADH is produced. in the conversion of succinate to fumarate. So, now let's look at how to remember the enzymes of Krebs cycle. By now, I assume you can recall the mnemonic on how to remember the key molecules in Krebs cycle. So, first using the mnemonic, we will write all the key molecules of Krebs cycle.
Next, use this mnemonic to remember the enzymes that catalyze the reactions of Krebs cycle. So, at disco, devils sip down 5 drinks. All the S in this mnemonic stand for Synthases and all the D in this mnemonic stand for Dehydrogenase enzymes. The only two unique names you have to worry about are aconitase and fumarase.
For rest of the enzymes, you can get the complete name of the enzyme by looking at which molecule is the enzyme acting on. The first letter in this mnemonic corresponds to the enzyme that catalyzes the reaction of combination of acetal-CoA with oxaloacetate to produce citrate. Like we just discussed, all the S alphabets of this mnemonic mean the enzyme will be a synthase and to get the complete name, we look at which reaction the enzyme is catalyzing. For example, in this case, the first reaction, the citrate is being produced.
So, the enzyme will be named as citrate synthase. Next the enzyme is aconitase which converts citrate to isocitrate. Now again, all the Ds of this mnemonic will mean the enzyme will be a dehydrogenase. Now since this enzyme will cause dehydrogenation of isocitrate to form alpha-ketoglutarate, the complete name of the enzyme will be isocitrate dehydrogenase. Now similarly the next enzyme will also be a dehydrogenase and in this case the enzyme is acting on alpha-ketoglutarate so the enzyme will be named as alpha-ketoglutarate dehydrogenase.
Similarly we have succinyl-CoA synthase since the enzyme is acting on succinyl-CoA. Then we have succinate dehydrogenase and the next you know it is fumarase. And the last molecule will also be a dehydrogenase which acts on malate so it will be known as malate dehydrogenase.
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