Write the reactions that convert pyruvate (or alanine) to phosphoenolpyruvate. I
ID: 101627 • Letter: W
Question
Write the reactions that convert pyruvate (or alanine) to phosphoenolpyruvate. Indicate which reactions occur in the cytosol and which reactions occur in the mitochondria. Compare these reactions to the reactions that occur when lactate is the precursor. Write the reactions that convert pyruvate (or alanine) to phosphoenolpyruvate. Indicate which reactions occur in the cytosol and which reactions occur in the mitochondria. Compare these reactions to the reactions that occur when lactate is the precursor. Write the reactions that convert pyruvate (or alanine) to phosphoenolpyruvate. Indicate which reactions occur in the cytosol and which reactions occur in the mitochondria. Compare these reactions to the reactions that occur when lactate is the precursor.Explanation / Answer
Pyruvate to phosphoenolpyruvate conversion occurs during gluconeogenesis. The conversion occurs in two steps. In the first step, pyruvate is converted to oxaloacetate with the help of the enzyme pyruvate carboxylase by incorporation of CO2. This reaction requires ATP hydrolysis.
Pyruvate + CO2 +H2O + ATP -> Oxaloacetate + 2H+ + ADP + Pi
This reaction occurs in mitochondria. For the conversion of oxaloacetate to phosphoenolpyruvate, the oxaloacetate needs to be transported to the cytoplasm. However, mitochondria does not have transporters for oxaloacetate. Hence oxaloacetate is first converted to malate by malate dehydrogenase (with the help of NADH as the reducing power). The malate is then transported to cytoplasm, where it is converted back to oxaloacetate (with regeneration of NADH). Subsequently, oxaloacetate is converted to phosphoenolpyruvate (PEP) with the help of enzyme PEP carboxykinase (phosphoenolpyruvate carboxykinase). GTP acts as phosphate donor for the reaction and the CO2 is released.
Oxaloacetate +GTP -> Phosphoenolpyruvate + GDP + CO2
This reaction occurs in the cytosol.
When lactate is used as precursor, it is first converted into pyruvate. This conversion is carried out in liver. For this reaction, the lactate (generally produced in muscles during vigorous exersice) reaches the liver cells via the bloodstream. In liver, the lactate is acted upon by the lactate dehydrogenase enzyme and results into oxidation of lactate into pyruvate along with generation of NADH.
Lactate + NAD+ -> Pyruvate +NADH + H+
The rest of the steps of conversion of pyruvate to phosphenol pyruvate occurs in the manner alreay described.
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