|IUPAC name||2-oxopropanoic acid|
|Other names||α-ketopropionic acid; acetylformic acid; pyroracemic acid; Pyr|
|Molar mass||88.06 g/mol|
|Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Pyruvic acid (C3H4O3 (CH3COCO2H)) is a three-carbon, keto acid that plays an important role in biochemical processes. At the pH levels of the human body, pyruvic acid is usually ionized to pyruvate; the two terms are used essentially synonymously.
Pyruvic acid is formed as an end product of glycolysis, a process that breaks down glucose (a six-carbon molecule) into two molecules of pyruvate (a three-carbon molecule) and simultaneously yields a small net gain of the universal energy storage molecule adenosine triphosphate (ATP), used to power cellular function. The pyruvate is then further processed in a variety of ways, depending on conditions, especially the oxygen level, within the cell.
Pyruvate is a key intersection in the network of metabolic pathways. It can be converted to carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA (acetyl coenzyme A), to lactic acid, to the amino acid alanine, and to ethanol. Therefore, it unites several key metabolic processes. The central role of pyruvic acid and the various metabolic pathways among cells of great diversity suggests a harmony and connectivity among organisms and great antiquity for the process. Furthermore, these pathways themselves involve a great deal of complex coordination.
The name pyruvic comes from the international scientific vocabulary pyr- combined with the Latin uva for grapes, reflecting its importance in fermentation process (Merriam-Webster 2008).
Pyruvic acid (CH3COCO2H) is a type of carboxylic acid; that is, it is an organic acid characterized by the presence of one or more carboxyl groups. A carboxyl group comprises a carbon atom attached to an oxygen atom with a double covalent bond, to a hydroxyl group by a single covalent bond, and to the connecting carbon of a hydrocarbon side chain. The chemical formula of the carboxyl group may be written as -C(=O)OH, -COOH, or -CO2H.
More specifically, pyruvic acid is a type of keto acid, which is any organic acid containing a ketone functional group and a carboxylic acid group. A ketone functional group is characterized by a carbonyl group (O=C) linked to two other carbon atoms. An alpha-keto acid, or 2-oxo acid, such as pyruvic acid, has the keto group adjacent to the carboxylic acid.
Pyruvic acid is a colorless liquid with a smell similar to that of acetic acid. It is miscible with water, and soluble in ethanol and diethyl ether. In the laboratory, pyruvic acid may be prepared by heating a mixture of tartaric acid and potassium hydrogen sulfate, or by the hydrolysis of acetyl cyanide (CH3COCN), formed by reaction of acetyl chloride with potassium cyanide:
Pyruvate is the carboxylate anion of pyruvic acid. A carboxylate anion is an ion with negative charge that contains the group -COO−.
Pyruvate is an important chemical compound in biochemistry. It is the output of the breakdown of glucose known as glycolysis. Glycolysis is a series of biochemical reactions by which one molecule of the six-carbon sugar glucose (Glc) is oxidized to two molecules of the three-carbon pyruvic acid (Pyr), two molecules each of the energy-carrying molecules ATP and NADH, and two molecules of water. ATP is used by all cells as the main molecule for intracellular energy transfer and as the principal energy source for endergonic, or energy-requiring, reactions and NADH is the main electron donor beginning the electron transport chain of oxidative phosphorylation. Glycolysis, through anaerobic respiration, is the main energy source in many prokaryotes, eukaryotic cells devoid of mitochondria (for example, mature erythrocytes), and eukaryotic cells under low-oxygen conditions (for example, heavily-exercising muscle or fermenting yeast).
Pyruvate, produced by glycolysis, then is used to provide further energy in one of two ways. Under aerobic conditions, pyruvate is converted into acetyl-coenzyme A, which is the main input for a series of reactions known as the Krebs cycle, which produces useful energy. In eukaryotes, pyruvate moves into the mitochondria, where it is converted into acetyl-CoA (acetyl coenzyme A) and enters the Krebs cycle. 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 called the citric acid cycle, because citric acid is one of the intermediate compounds formed during the reactions. The citric acid cycle is the "power plant" that energizes metabolism and thus, life itself. Pyruvate also is converted to oxaloacetate, which can either replenish one of the Krebs cycle intermediates or be used for gluconeogenesis (generation of glucose).
If insufficient oxygen is available, pyruvic acid is broken down anaerobically, creating lactic acid in animals and ethanol in plants. Pyruvate from glycolysis is converted by anaerobic respiration to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation, or to acetaldehyde and then to ethanol in alcoholic fermentation.
Gluconeogenesis is a metabolic pathway that generates glucose from non-carbohydrate carbon substrates such as pyruvate, lactate, glycerol, and glucogenic amino acids. This pathway comprises eleven enzyme-catalyzed reactions. It can begin in the mitochondria or in the cytoplasm, depending on the substrate being used. Many of the reactions are reversible steps found in glycolysis. Several non-carbohydrate carbon substrates can enter the gluconeogenesis pathway. One common substrate is lactic acid, formed during anaerobic respiration in skeletal muscle. Lactate is transported back to the liver where it is converted into pyruvate by the Cori cycle using the enzyme lactate dehydrogenase. Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used to generate glucose (Garrett and Grisham 2002).
While most steps in gluconeogenesis are the reverse of those found in glycolysis, three regulated and strongly exergonic reactions are replaced with more kinetically favorable reactions. Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and PEP carboxykinase. This system of reciprocal control allows glycolysis and gluconeogenesis to inhibit each other and prevent the formation of a futile cycle.
Medically, in humans, the oxidation of pyruvate to acetyl coenzyme A is dependent on thiamine, and both pyruvate and lactate blood levels rise in the case of thiamine deficiency (Bender and Bender 2005). The pyruvic acid derivative bromopyruvic acid is being studied for potential cancer treatment applications by researchers at Johns Hopkins University in ways that would support the Warburg hypothesis on the cause(s) of cancer (Pederson 2004).
As the foundation of both aerobic and anaerobic respiration, glycolysis is the archetype of universal metabolic processes known and occurring (with variations) in many types of cells in nearly all organisms.
In glycolysis, phosphoenolpyruvate (PEP) is converted to pyruvate by pyruvate kinase. This reaction is strongly exergonic and irreversible; in gluconeogenesis it takes two enzymes, pyruvate carboxylase and PEP carboxykinase to catalyze the reverse transformation of pyruvate to PEP. The arrow indicating a reverse reaction in the Figure below is incorrect.
Pyruvate decarboxylation by the pyruvate dehydrogenase complex produces acetyl-CoA.
|pyruvate||pyruvate dehydrogenase complex||acetyl-CoA|
|CoA + NAD+||CO2 + NADH + H+|
Carboxylation by the pyruvate carboxylase produces oxaloacetate.
|ATP + CO2||ADP + Pi|
Reduction by the lactate dehydrogenase produces lactate.
Current evolutionary theory on the origin of life posits that the first organisms were anaerobic because the atmosphere of prebiotic Earth was almost devoid of oxygen. As such, requisite biochemical materials must have preceded life and recent experiments indicate that pyruvate can be synthesized abiotically. In vitro, iron sulfide at sufficient pressure and temperature catalyzes the formation of pyruvate. Thus, argues Günter Wächtershäuser, the mixing of iron-rich crust with hydrothermal vent fluid is suspected of providing the fertile basis for the formation of life.
|Glycolysis Metabolic Pathway|
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