Each of the following molecules can be converted to oxaloacetate by a single enzymatic reaction EXCEPT:

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11ecba2d_0eec_b036_a3bf_dfaafb6f9da0_MD0008_00 A metabolic pathway is a series of enzymatic steps that interconvert metabolic intermediates to produce energy (catabolism) or structural molecules (anabolism). Many metabolic pathways intersect and have an intermediate in common. This intermediate may be synthesized from multiple precursors, depending on the pathway used in their synthesis. Oxaloacetate is an intermediate in several metabolic pathways, including gluconeogenesis, the citric acid cycle, and amino acid degradation. Each uses a different precursor and a different enzyme to catalyze the synthesis of oxaloacetate. In the first step of gluconeogenesis, the enzyme pyruvate carboxylase catalyzes the carboxylation of pyruvate to form oxaloacetate (Choice A). In this reaction, CO₂ is added to pyruvate, and ATP is hydrolyzed to provide the necessary energy. In the final step of the citric acid cycle, oxaloacetate is synthesized by the oxidation of malate, a reaction catalyzed by malate dehydrogenase (Choice B). This reaction requires the simultaneous reduction of NAD⁺ to NADH. NADH can then enter the electron transport chain and provide electrons necessary for the production of ATP. In amino acid degradation, amino acids are converted to α-keto acids by transamination, in which an amino group is transferred from an amino acid to α-ketoglutarate to synthesize glutamate (Choice D). The α-keto acid derivative of aspartate is oxaloacetate. Alanine can be deaminated to yield pyruvate, which can then be converted to oxaloacetate by pyruvate carboxylase. This process requires two enzymatic steps instead of one. Educational objective: Metabolic pathways are series of enzymatic reactions that interconvert metabolic intermediates. Many metabolic pathways intersect, and the intermediates at these intersection points may be synthesized from multiple distinct precursors. For example, oxaloacetate is an intermediate in several metabolic pathways: gluconeogenesis, the citric acid cycle, and amino acid degradation.

Each of the Following Molecules Can Be Converted to Oxaloacetate