5.3B: Pyruvic Acid and Metabolism
- Outline the metabolic processes that involve pyruvate
Pyruvic acid (CH 3 COCOOH; is an organic acid, a ketone, and the simplest of the alpha-keto acids. The carboxylate (COO − ) anion of pyruvic acid. The Brønsted–Lowry conjugate base, CH 3 COCOO − , is known as pyruvate, and is a key intersection in several metabolic pathways.
Pyruvic acid can be made from glucose through glycolysis, converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA. It can also be used to construct the amino acid alanine, and it can be converted into ethanol.
Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle) when oxygen is present (aerobic respiration); when oxygen is lacking, it ferments to produce lactic acid. Pyruvate is an important chemical compound in biochemistry. It is the output of the anaerobic metabolism of glucose known as glycolysis. One molecule of glucose breaks down into two molecules of pyruvate, which are then used to provide further energy in one of two ways. Pyruvate is converted into acetyl- coenzyme A, which is the main input for a series of reactions known as the Krebs cycle. Pyruvate is also converted to oxaloacetate by an anaplerotic reaction, which replenishes Krebs cycle intermediates; also, oxaloacetate is used for gluconeogenesis. These reactions are named after Hans Adolf Krebs, the biochemist awarded the 1953 Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes. The cycle is also known as the citric acid cycle or tri-carboxylic acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.
If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by fermentation to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation. Alternatively it is converted to acetaldehyde and then to ethanol in alcoholic fermentation.
Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine, and to ethanol. Therefore, it unites several key metabolic processes.
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Key Points
- Pyruvic acid can be made from glucose through glycolysis, converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA.
- Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle ) when oxygen is present (aerobic respiration); it ferments to produce lactic acid when oxygen is lacking ( fermentation ).
- Pyruvate is the output of the anaerobic metabolism of glucose known as glycolysis.
- Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine, and to ethanol.
Key Terms
- pyruvic acid : A colourless liquid; an important intermediate in the metabolism of proteins and carbohydrates, and in fermentation.
- conjugate base : Any compound, of general formula Xn+, which can be transformed into a conjugate acid HX(n+1)+ by the gain of a proton.
- Krebs cycle : A series of enzymatic reactions that occurs in all aerobic organisms; it involves the oxidative metabolism of acetyl units and serves as the main source of cellular energy.