Question

The oxaloacetate produced in gluconeogenesis is formed in the matrix. Most of gluconeogenesis occurs in the cytoplasm. Oxaloacetate must be converted to 1 of 3 possible products in order to cross the mitochondrial membrane and then be converted back to oxaloacetate in the cytoplasm. Name ONE of these 3 possible products

Answer 1

Answer :-

Gluconeogenesis is the metabolic process by which organisms produce sugars (namely glucose) for catabolic reactions from non-carbohydrate precursors. Glucose is the only energy source used by the brain (with the exception of ketone bodies during times of fasting), testes, erythrocytes, and kidney medulla. In mammals this process occurs in the liver and kidneys.

The first step in gluconeogenesis is the conversion of pyruvate to phosphoenolpyruvic acid (PEP).

In order to convert pyruvate to PEP there are several steps and several enzymes required. Pyruvate carboxylase, PEP carboxykinase and malate dehydrogenase are the three enzymes responsible for this conversion.

Pyruvate carboxylase is found on the mitochondria and converts pyruvate into oxaloacetate. Because oxaloacetate cannot pass through the mitochondria membranes it must be first converted into malate by malate dehydrogenase. Malate can then cross the mitochondria membrane into the cytoplasm where it is then converted back into oxaloacetate with another malate dehydrogenase. Lastly, oxaloacetate is converted into PEP via PEP carboxykinase.

Therefore, the product that oxaloacetate is converted into is malate.

  

  

Let's see it in steps :

1. Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate by the carboxylation of pyruvate. This reaction also requires one molecule of ATP (Adenosine triphosphate), and is catalyzed by pyruvate carboxylase. This enzyme is stimulated by high levels of acetyl-CoA (produced in β-oxidation in the liver) and inhibited by high levels of ADP (Adenosine diphosphate) and glucose.
2. Oxaloacetate is reduced to malate using NADH (Nicotinamide adenine dinucleotide), a step required for its transportation out of the mitochondria.
3. Malate is oxidized to oxaloacetate using NAD⁺(Nicotinamide adenine dinucleotide - oxidised form) in the cytosol, where the remaining steps of gluconeogenesis take place.
4. Oxaloacetate is decarboxylated and then phosphorylated to form phosphoenolpyruvate using the enzyme PEPCK (Phosphoenolpyruvate carboxykinase). A molecule of GTP (Guanosine-5'-triphosphate) is hydrolyzed to GDP (Guanosine diphosphate) during this reaction.​​​​​​​

  

fig:- Mitochondrial substrate transport in gluconeogenesis

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