Lipmann Discovers Coenzyme A

Date 1951

Fritz Albert Lippman discovered the molecule coenzyme A, the link between the glycolytic and citric acid enzyme cycles in living cells. The discovery represented a breakthrough in the understanding of metabolism and the creation, storage, and use of energy by living things.

Locale Boston, Massachusetts

Key Figures

• Fritz Albert Lipmann (1899-1986), German American biochemist
• Hans Adolf Krebs (1900-1981), German biochemist
• Otto Heinrich Warburg (1883-1970), German biochemist and 1931 Nobel laureate
• Otto Fritz Meyerhof (1884-1951), a German biochemist and physiologist

Summary of Event

All life on Earth is sustained by the constant recycling of matter and energy through the earth’s environment. The energy cycle begins with the absorption of visible light by photosynthetic autotrophs, those organisms (for example, plants, phytoplankton) that can produce their own food (sugar) from sunlight, water, and carbon dioxide. These photosynthetic autotrophs make this food so that the cells of their bodies can later convert the sugar into usable energy to drive chemical reactions essential for life. Organisms that cannot make their own food must scavenge for food; they are heterotrophs.

Heterotrophs need to obtain food for the same reason as autotrophs: to provide energy to drive the cellular chemical reactions necessary for life. Heterotrophs include animals, protozoa, and fungi. Herbivorous heterotrophs eat plants to obtain food the plants made from sunlight. Carnivorous heterotrophs eat herbivores and other carnivores to obtain the food these organisms gained by eating something else. When organisms die, they are decomposed by bacteria and fungi until their component molecules are returned to the soil to be recycled into new plants. The energy cycle starts over again, the entire purpose being to provide energy for two very important life chemical reaction pathways: glycolysis and the citric acid cycle.

These two chemical reaction pathways were deciphered by biochemists during the first half of the twentieth century. In 1897, Eduard Buchner and Hans Buchner discovered that the process of fermentation (alcohol formation) can occur without the presence of living cells. They found that sugar solutions will decompose eventually into ethyl alcohol. Fermentation, however, can be accelerated in the presence of yeast cells. In 1905, William Young and Arthur Harden discovered that an extract of fluids from yeast cells could ferment the sugar glucose. There were certain chemical reactions occurring within the cytoplasm of yeast cells that catalyzed, or sped up, the conversion of glucose into ethyl alcohol. Furthermore, this series of reactions was anaerobic: These chemical reactions did not require oxygen. Phosphate, however, was essential. The next step was to determine what protein enzymes from yeast extract drove the fermentation reactions and to identify the various chemical intermediates in the reaction pathway.

During the 1920’s and 1930’s, biochemists derived most of the chemical reaction steps involved in alcoholic fermentation. The principal scientists involved in these studies were Gustav Embden, Otto Fritz Meyerhof, and Otto Heinrich Warburg. They solved the complete enzymatic pathway for the breakdown of glucose, a series of anaerobic chemical reactions called glycolysis, also called the Embden-Meyerhof Pathway. Glycolysis begins with the conversion of the sugar glucose into glucose-6-phosphate by the enzyme hexokinase. Glucose-6-phosphate is then converted to fructose-6-phosphate by another enzyme. Fructose-6-phosphate is converted to fructose-1,6-diphosphate by a third enzyme. Fructose-1,6-diphosphate is then split into two molecules of glyceraldehyde-3-phosphate. Each glyceraldehyde-3-phosphate is converted to 1,3-diphosphoglycerate, which is converted to 3-phosphoglycerate, which is converted to 2-phosphoglycerate, which is converted to phosphoenolpyruvate. The final glycolytic reaction is the conversion of phosphoenolpyruvate to pyruvic acid.

The net result of glycolysis is the conversion of one six-carbon molecule (glucose) into two three-carbon molecules (two pyruvic acids). Along the reaction pathway, the energy of two adenosine triphosphate (ATP) molecules is expended, four new ATP molecules are produced directly, and four additional ATP molecules are produced indirectly. The purpose of glycolysis in living cells is the production of ATP, a high-energy molecule, from adenosine diphosphate (ADP), a low-energy molecule. ATP is a high-energy molecule, because approximately seventy-three hundred calories of energy are released when one of ATP’s phosphate bonds is broken, when ATP is converted to ADP. When a cell runs out of ATP, it has no energy, chemical reactions cease, and the cell dies. With a supply of glucose, a cell can add phosphates back to ADP to construct ATP by the process of glycolysis. The cell lives.

Pyruvic acid, the end product of glycolysis, can be converted into one of three possible molecules: ethyl alcohol, lactic acid, or acetyl coenzyme A. Acetyl coenzyme A was discovered by one of Meyerhof’s students, Fritz Albert Lipmann. Lipmann, a native of Königsberg, Germany, began his biochemical research in Meyerhof’s laboratory at the University of Heidelberg from 1927 to 1930. He played an important supporting role in Meyerhof’s discovery of glycolysis. In 1941, Lipmann became director of the Biochemistry Research Department at Massachusetts General Hospital in Boston. He continued his studies of energy generation in living organisms, particularly animals. Animal ATP production was much higher than could be explained simply by glycolysis.

Seeking other enzymatic pathways for ATP production, Lipmann concentrated his studies on fluid extract from pigeon liver. Pigeons were primary targets of energy metabolism research because of their higher metabolic rates needed for flight. A number of biochemists were studying pigeon flight muscles and liver in order to isolate the necessary enzymes for ATP production. They were attempting to derive an enzymatic pathway for aerobic energy production (that is, aerobic respiration), ATP production in the presence of oxygen. Among the key scientists in this research effort were Lipmann, Hans Adolf Krebs, Warburg, Albert Szent-Györgyi , Franz Knoop, and Carl Martius.

Krebs had discovered several of the intermediate molecules (for example, citric acid, succinate, fumarate) of aerobic metabolism in the early 1930’s. He had discovered these molecules because of their high reactivity with oxygen in muscles. Szent-Györgyi obtained similar results. After these and other discoveries, Krebs pieced together the various intermediates of aerobic ATP production. By 1937, he had deciphered the enzymatic pathway known as the citric acid cycle, also called the Krebs cycle or the tricarboxylic acid (TCA) cycle.

In 1945, Lipmann discovered a molecule in pigeon liver extracts that accelerated aerobic respiration without being part of Krebs’s famous citric acid cycle. By 1947, he had isolated the molecule and determined its structure. The molecule, which he named coenzyme A (CoA), is a large molecule consisting of four major components: an adenine unit, a five-carbon sugar unit, a pantothenic acid unit, and a betamercaptoethylamine unit. Coenzyme A was important, because it was highly reactive particularly with two-carbon molecules such as acetic acid.

In 1951, Lipmann and colleagues demonstrated that coenzyme A can chemically react with pyruvic acid, the end-product of anaerobic glycolysis, to produce acetyl coenzyme A (coenzyme A plus acetic acid). Acetyl coenzyme A dumps the acetate into the Krebs cycle, thereby leaving coenzyme A free to react with another pyruvic acid. Therefore, coenzyme A is the intermediate between the two most important energy-generating reaction pathways in the cells of living organisms: anaerobic glycolysis and the aerobic Krebs cycle. Coenzyme A routes two-carbon units (acetyl groups) from glycolysis to the Krebs cycle.

Once coenzyme A dumps acetate into the Krebs cycle, the two-carbon acetate combines with a four-carbon molecule called oxaloacetate to produce the six-carbon molecule citric acid. Citric acid is converted to cis-aconitate, which subsequently is converted to isocitrate. Isocitrate, a six-carbon molecule, is converted to a five-carbon molecule called alpha-ketoglutarate, with the lost carbon being carried away by inhaled oxygen, eventually being exhaled by the organism as carbon dioxide. The five-carbon alpha-ketoglutarate is converted to the four-carbon molecule succinyl-coenzyme A, with the lost carbon again being carried away by oxygen in the form of carbon dioxide. The four-carbon succinyl-coenzyme A is converted to succinate. Succinate is converted to fumarate, which is converted to malate, which is converted to oxaloacetate. Oxaloacetate picks up an acetate from coenzyme A to produce citric acid and thus the Krebs cycle begins over again. Each time through the cycle, fifteen new ATP molecules are generated from ADP.

Significance

Fritz Lipmann’s discovery of coenzyme A, his determination of its structure, and his deciphering of its role as acetyl coenzyme A in cellular respiration represent a great breakthrough in twentieth century biochemistry. The discovery of coenzyme A linked the anaerobic and aerobic phases of cellular respiration, helped quantify the amount of energy production for each glucose molecule an organism consumes, and helped link cellular respiration to other metabolic reaction pathways (for example, carbohydrate and protein metabolism). Lipmann and Krebs shared the 1953 Nobel Prize in Physiology or Medicine for their contributions to the understanding of energy metabolism in living organisms.

Cellular respiration is the core of modern biochemistry since the field was established by Krebs, Lipmann, Meyerhof, and others studying energy metabolism in the 1930’s, 1940’s, and 1950’s. The central focus of every introductory biochemistry course is cellular respiration. Lipmann’s contribution is of great value because it helped resolve how cells stay alive.

All cells must have energy in order to survive. They need energy in order to drive the thousands of chemical reactions that occur every minute of each cell’s life. These chemical reactions involve nutrient assimilation, deoxyribonucleic acid (DNA) replication, membrane recycling, intracellular movements, and the like. Energy is released when one phosphate bond is broken from ATP, thus leaving the less energetic molecule, ADP, and a free inorganic phosphate. The purpose of glycolysis and the Krebs cycle is to reunite ADP and inorganic phosphate to produce ATP so that the cell can rebreak ATP to release energy to drive more chemical reactions.

Glycolysis is an anaerobic process; it does not require the presence of oxygen. It is a series of enzyme reactions in the cell cytoplasm that converts one six-carbon molecule, glucose, into two three-carbon molecules, pyruvic acids. During the reaction pathway, there is a net gain of six ATPs (eight in bacteria). Each pyruvic acid then binds to Lipmann’s coenzyme A, with one carbon dioxide released, to produce acetyl coenzyme A, the intermediate between glycolysis and the Krebs cycle. The acetyl unit (two carbons) of acetyl coenzyme A is passed along to oxaloacetate (four carbons) to form the six-carbon citric acid.

The remainder of the circular Krebs cycle is the elimination of two carbons (via carbon dioxide) to make oxaloacetate and stimulate the production of fifteen ATPs. Therefore, for each sugar molecule that starts glycolysis, six ATPs are generated from glycolysis, followed by fifteen more ATPs for each of the two pyruvic acids that enter the Krebs cycle. A total of thirty-six ATPs are produced by the two cycles for each glucose molecule (thirty-eight in bacteria).

Therefore, the work of Lipmann, Krebs, and other biochemists explains why one must eat and breathe. One eats to obtain sugar and other nutrients for ATP production. Oxygen is inhaled to carry away the excess carbons of the Krebs cycle, carbons which originally came from the sugar consumed. This carbon dioxide is exhaled. If sugar or oxygen is not received, the Krebs cycle soon stops in the cells, the cells die from lack of ATP, and death occurs.

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell. 4th ed. New York: Garland Science, 2002. This lengthy introductory molecular biology textbook for undergraduate biology majors is a thorough survey of the science by several leading molecular biologists and biochemists. It contains excellent photographs, diagrams, and reference lists. Chapter 9, “Energy Conversion: Mitochondria and Chloroplasts,” is a clear introduction to cellular respiration.

Berg, Jeremy M., John L. Tymoczko, and Lubert Stryer. Biochemistry. 6th ed. New York: W. H. Freeman, 2007. Classic introductory biochemistry textbook for undergraduate biology and premedical students. The book is clearly written, contains outstanding illustrations, and includes historical sketches. Chapter 12, “Glycolysis,” and chapter 13, “Citric Acid Cycle,” are very understandable presentations of cellular respiration.

Karp, Gerald. Cell and Molecular Biology. 4th ed. Hoboken, N.J.: John Wiley, 2005. This introductory cell biology textbook for undergraduate biology majors emphasizes the relationship between genetics and biochemistry. It contains excellent illustrations and reference lists. Chapter 4, “Energy, Enzymes, and Metabolism,” is a detailed discussion of glycolysis and the Krebs cycle.

Lipmann, Fritz, and Nathan O. Kaplan. “Intermediary Metabolism of Phosphorus Compounds.” Annual Review of Biochemistry 18 (1949): 267-298. This extensive review article, written by Lipmann prior to his discovery of acetyl coenzyme A, describes his research into cellular respiration. The article describes coenzyme A and points toward experiments involving coenzyme A and acetate. More than one hundred references are provided.

Nicholls, David G. Bioenergetics: An Introduction to the Chemiosmotic Theory. New York: Academic Press, 1982. This textbook of energy conversion in cellular respiration is intended for advanced undergraduate and graduate biology students. The book describes cellular energy production with schematic illustrations, cartoons, and mathematical derivations. It includes an extensive list of references.

Raven, Peter H., and George B. Johnson. Biology. 7th ed. Boston: McGraw-Hill, Higher Education, 2005. This outstanding introductory biology textbook for undergraduate biology majors is clearly written with beautiful photographs and illustrations. Chapter 8, “Cellular respiration,” is a simple introduction to glycolysis and the Krebs cycle that is understandable to the layperson.

Zubay, Geoffrey L. Biochemistry. 4th ed. Dubuque, Iowa: Wm. C. Brown, 1998. This comprehensive introductory biochemistry textbook for advanced undergraduate and graduate biology students is a strong survey of the subject. The book is very detailed, but it includes excellent diagrams and illustrations. Chapter 8, “Anaerobic Production of ATP,” and chapter 9, “Aerobic Production of ATP,” are complete discussions of cellular respiration.