Advances in the Understanding of Energy Metabolism
Advances in the Understanding of Energy Metabolism
Investigation of the chemistry of life had begun in the nineteenth century with the discovery that organic molecules, the chemicals found in living things, were larger and more complex than inorganic molecules, those in the non-living world. By the end of the century, the major classes of organic molecules—the carbohydrates, lipids, proteins, and nucleic acids—had all been identified, but little was known about the cellular processes involved in making and breaking them down. Metabolism is a general term for the sum of all the chemical reactions that make up these processes. During the first half of the twentieth century, biochemists and biologists began to make significant progress in working out the steps in these chemical reactions, which turned out to be much more complex than had been originally suspected. The study of the reactions involved in providing energy for organisms resulted in some of the most important advances.
In 1897 bacteriologist Hans Buchner (1850-1902) discovered that fermentation, the process by which yeast cells break down sugar, could occur even when the yeast cells had been ruptured; it could in fact occur in a soluble extract from the cells. Until that time, there had been a lively debate over whether or not intact cells were needed for the chemical processes related to life. The great bacteriologist Louis Pasteur (1822-1895) had argued that intact cells were necessary; but Buchner's finding proved him wrong, making it clear that biochemists could study the reactions of life in test tubes, thus calling into question the concept of vitalism, the idea that there was some life force necessary for chemical reactions in organisms.
Shortly after Buchner's discovery, Franz Hofmeister (1850-1922) presented the enzyme theory of life, arguing that all cellular reactions are controlled by specific chemicals called enzymes. This replaced the older protoplasmic theory, which held that the whole cell is needed for such processes. As the century progressed the idea of the enzyme became more clearly delineated, as it was found that enzymes were large protein molecules. It was in 1930 that the first enzyme was crystallized—that is, fully purified from all other cell components. Enzymes are catalysts, which means that they speed chemical reactions in living systems, but are the same chemically at the end of the reaction as they were at the beginning. So a single enzyme molecule can perform the same reaction many times, and thus in many cases not much of a particular enzyme is needed in living tissue. This posed a particular problem in early biochemical research because it meant that enzymes were often difficult to find with the crude detection methods then available, and often large quantities of tissue yielded only very small amounts of an enzyme after purification.
The disruption of cells and the isolation of specific enzymes were important tools in discovering how cells break down sugar, most commonly glucose, and release chemical energy from the bonds between the atoms in the glucose molecule. It became evident quite early in these investigations that this process in energy metabolism was a complex one, involving not a single reaction, but a series of reactions. It took several decades to work out all the steps in these reactions, and German biochemists made the most significant contributions to this research. Beginning in 1908, Otto Warburg (1883-1970) studied the use of oxygen in energy-releasing processes in cells, developing a number of techniques to carefully measure the uptake of this gas. His introduction of quantitative methods brought a level of precision to biochemistry that was essential to working with small amounts of material and to detecting subtle chemical changes. It was also Warburg who began experiments on tissue slices, a technique that was later used in studies of energy use in liver and muscle cells.
Working in the same Berlin research institute as Warburg, Gustav Embden (1874-1932) and Otto Meyerhof (1884-1951) discovered the steps by which the six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvic acid. This series of reactions is anaerobic (does not require oxygen), and in yeast, the pyruvic acid is converted to alcohol, ethanol, and carbon dioxide in a process called alcoholic fermentation. In muscle cells, however, where an almost identical set of reactions occurs, if oxygen is insufficient, the pyruvic acid is converted to lactic acid, the buildup of which tires muscle. If oxygen is present, the complete breakdown of pyruvic acid can occur. Hans Krebs (1900-1981), who began his career working with Warburg, discovered the sequence of reactions involved in this process.
Krebs found that in the presence of oxygen, a pyruvic acid molecule entered a cycle of reactions, the first of which was the reaction of pyruvic acid with oxalacetic acid to form citric acid. This series of reactions thus became known as the citric acid cycle. It is also called the Krebs cycle in honor of his work, which was a significant achievement because it involved putting together findings from other researchers and then doing painstaking experiments to discover how the steps in the process fit together.
As Krebs later recalled, he built upon the work of another biochemist, Albert Szent-Györgyi (1893-1986), who had developed a way to separate intact mitochondria, the cell organelles involved in energy production, from the flight muscle of pigeons. Szent-Györgyi used the line of reasoning that if the addition of a substance to such a preparation greatly increased the chemical activity as measured by the use of oxygen, then this indicated that the substance was an intermediate in the breakdown of carbohydrate for energy. Of the many substances tested, only a few were active, but these substances, all four-carbon acids, did not bear any resemblance to the sugars found in food.
It was Krebs who discovered that these acids acted as carriers for pyruvic acid. Then, through a series of reactions, pyruvic acid was broken down and the energy in the molecule released. The four-carbon acids returned to their original form by the end of the reaction series, thus allowing for their recycling, and this is why the whole series of reactions is called a cycle. There are a number of such cycles found in metabolism. In fact, Krebs himself had discovered another cycle a few years earlier, one that creates urea, the form of nitrogen that is excreted from the body in urine. Krebs received the 1953 Nobel Prize for Physiology or Medicine for his research. Warburg received the same honor in 1931, indicating the importance of research on energy metabolism.
While some biochemists were discovering the intermediate steps in the breakdown of glucose, others were investigating the chemicals that served as the repositories for the energy stored in the chemical bonds in glucose. There is a great deal of energy in these bonds, and what happens in the breakdown of glucose is that this energy is distributed to other molecules, in packets of energy of more useable size. It was Fritz Lipmann (1899-1986) who identified adenosine triphosphate (ATP) as the high-energy product of the breakdown of glucose and other food stuffs, such as proteins and fats. As its name implies, there are three phosphate, or phosphorous-containing, groups in ATP, and the last phosphate is attached with a high-energy bond created using the energy from the Krebs cycle and from other energy-releasing processes. When glucose is completely broken down to carbon dioxide and water, 38 ATP molecules are produced, and it is ATP that provides the energy for muscle contraction, nerve impulse conduction, and most other energy-requiring processes in cells and in organisms.
The breakdown of lipids and proteins also results in the production of ATP by processes with final stages identical to those in the breakdown of sugar. It was again Lipmann who discovered the common denominator, a substance called acetyl CoA. Like Krebs, Lipmann was interested in how pyruvic acid was broken down in animal tissue. He discovered that a derivative of the B vitamin, pantothenic acid, was necessary for the process. It turned out to be, like other B vitamins, a coenzyme, that is, a small molecule that must be present in order for a specific enzyme to function. Lipmann called it coenzyme A, or CoA, which reacts with pyruvic acid to form acetyl CoA. This molecule is the form of pyruvic acid that enters the Krebs cycle by reacting with oxalacetic acid to form citric acid. Lipmann found that acetyl-CoA is formed not only in the breakdown of sugars, but in the breakdown of proteins and lipids as well. This means that, ultimately, these nutrients as well as carbohydrates can be broken down completely through the Krebs cycle and the chemical energy in these molecules converted to the high-energy bond in ATP.
Because acetyl-CoA proved so crucial in energy metabolism, Lipmann was awarded the Nobel Prize for Physiology or Medicine in 1953, the same year that Krebs received the award. The work of these men, along with that of many others, some of whom are mentioned above, accomplished a great deal during the first half of the twentieth century. In 1900 the concept of the enzyme as a highly specific protein molecule had yet to be developed, and the complexity of energy metabolism was unknown. By 1950 the basic steps in the breakdown of all the energy-rich nutrients—carbohydrates, lipids, and proteins—had been worked out by biochemists, the function of a number vitamins in these reactions was understood, and the crucial role of ATP appreciated. Biochemists had not only identified many chemicals involved in energy metabolism, but had also discovered how these chemicals related to each other, how one was converted into another, and in many cases had identified the enzymes responsible for these conversions.
MAURA C. FLANNERY
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