Transcription
Hi friends, welcome to biology exams4u.com.
Today's topic is an interesting topic: how Hans Krebs discovered the crop cycle. In the last video, we discussed about eight steps of the Krebs cycle, and I was very curious to find out how he discovered this pathway. I would like to share a five-minute summary of the experiment.
First of all, let's begin with an interesting fact. Krebs submitted his work or paper to the journal Nature, as you all know, one of the most reputed journals in biological sciences, in 1937. And Nature rejected his paper, citing that there were many submissions that needed to be reviewed. So, even without reviewing, Nature rejected the paper, and that work eventually got a Nobel Prize in 1953.
Krebs was a German-born biologist, and he moved to England as a Jewish refugee during Hitler's time. He joined the University of Sheffield. And this was the title of his paper: "The Role of Citric Acid in Intermediate Metabolism in Animal Tissues." He published this paper along with his student, William Johnson, from the Department of Pharmacology, University of Sheffield. He published that article in the journal "Die Naturwissenschaften" in 1937.
What was the knowledge about cellular respiration in the 1930s? It was known that during cellular respiration, oxygen is used, and carbon dioxide is released, and the process is responsible for synthesizing energy or ATP that drives all activities of the cell. This is known. Glycolysis is known by 1930. So, this is the citric acid cycle, and this part is not known. How pyruvate is connected to citrate, uh, this isocitrate, alpha-ketoglutarate, succinyl-CoA, etc. These intermediates are known. There were many publications citing these intermediates in cellular respiration, but often considered this as a linear pathway or not clear about the order of the reaction. So, this was the primary question for Hans Krebs: how to link the pyruvate to citrate? So, how to link glycolysis to the next step?
Now we know that it is the citric acid cycle. The experimental model was pigeon breast muscle, which is a powerful muscle necessary for flight. At that time, many experiments on cellular respiration used this model. And this was the methodology for Lord Krebs. He used tissue slices, then spectrophotometry, and manometry. Manometer is a way to precisely measure gases consumed and produced by living cells. He learned this technique from his mentor, the famous enzymologist Otto Warburg, and he often credits Otto Warburg and his mentorship for all his accomplishments, or even this discovery of the Krebs cycle. He learned all these techniques from Warburg's laboratory in Germany. The main technique he used is this manometry, and he's standing near to his beloved apparatus, the Warburg apparatus. Using this simple apparatus, he discovered this world pathway.
Then, what was the discovery? First, along with William Johnson, they made citrate, a C6 compound, by feeding oxaloacetate to the cell, which is a C4 compound. So, they could connect these two. Oxaloacetate is used to form citrate. Then, from where the two other two carbons come from? That was a question. So, oxaloacetate is a C4 compound, and citrate is a C6 compound. And this was the hypothesis: from glycolysis, pyruvate is formed as the end product, that was known. So, pyruvate contributes the carbon to oxaloacetate for the formation of citrate. So, how pyruvate, which is a three-carbon compound, contributes two carbons during this process? That was not known. So, by conducting many experiments, they confirmed that pyruvate is responsible for the formation of citrate by providing carbon atoms in the reaction. This is not known. The involvement of acetyl-CoA is not known, which is the two-carbon compound, the intermediate that connects glycolysis to the citric acid cycle. That was not known. Later, Fritz Lipmann discovered coenzyme A and its role in the oxidation of pyruvate. Later, the compound involved was found to be acetyl-CoA, which is a two-carbon compound that reacts with oxaloacetate, forming citrate.
Krebs could make alpha-ketoglutarate, succinyl-CoA, succinate, all from oxaloacetate, and he proposed a cyclic pathway that connects glycolysis to the Krebs cycle, or the citric acid cycle. At that time, it was called the citric acid cycle, as the first stable compound was citrate in the pathway. So, now we know that this is what is happening. Pyruvate is converted to acetyl-CoA, and the role of coenzyme A was discovered by Fritz Lipmann, and the enzyme involved is pyruvate dehydrogenase. For that discovery, Fritz Lipmann, along with Hans Krebs, got the Nobel Prize in 1953.
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