Biochemistry MCAT Chapter 8: Carbohydrate Metabolism I Practice Problems (2/2)

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@momenakhatoon5996 9 ай бұрын

Question on #8: If the buffer solution has Citrate, which is made in the mitochondria - why would it accumulate in the cytosol of the cell rather than in the mitochondria?

@professoreman2289 8 ай бұрын

Hi! What a great question. So, first and foremost, I am just going to go over the problem really quickly and then answer your question just so we are on the same page! The problem is asking which enzyme's activity would be most directly affected by the presence of citrate in the assay buffer. Let's go through the options provided: A. Fructose-2,6-bisphosphatase: This enzyme is involved in regulating glycolysis and gluconeogenesis, but it is not directly affected by citrate. B. Isocitrate dehydrogenase: This enzyme catalyzes the oxidative decarboxylation of isocitrate, producing α-ketoglutarate and CO2 in the citric acid cycle (also known as the Krebs cycle or TCA cycle). While it is part of the same cycle as citrate, the enzyme itself is not known to be directly regulated by citrate. C. Phosphofructokinase-1 (PFK-1): This enzyme is a key regulatory step in glycolysis and is known to be inhibited by high levels of citrate. Citrate signifies a high-energy state, indicating that the cell does not need to produce more ATP through glycolysis, so it acts as a negative regulator of the enzyme. D. Pyruvate carboxylase: This enzyme is involved in gluconeogenesis and is not directly regulated by citrate. Given these options, the correct answer is C, Phosphofructokinase-1 (PFK-1), because citrate is an allosteric inhibitor of PFK-1. In the presence of citrate, the activity of PFK-1 would decrease, which is likely why the enzyme activity has greatly decreased in the assay mentioned in the problem. Regarding your question, citrate can accumulate in the cytosol of a cell as part of the cell's normal regulatory mechanisms. While citrate is produced in the mitochondria during the citric acid cycle, it can be transported out into the cytosol through citrate transporters when there is an abundance of metabolic intermediates and ATP (indicating a high-energy state). Once in the cytosol, citrate serves several functions: - It acts as a signaling molecule to regulate energy-producing pathways like glycolysis (as an inhibitor of PFK-1). - It is a source of acetyl-CoA for fatty acid synthesis, which occurs in the cytosol. - It can also inhibit glycolysis, thus preventing the production of excess ATP and directing the excess acetyl-CoA towards fat production for storage. These mechanisms help balance the cell's energy needs by either continuing energy production or shifting to energy storage when appropriate.