Krebs cycle

Categories: Cellular biology; photosynthesis and respiration; physiology

Every living organism must process chemical energy to survive. The series of metabolic pathways known as cellular respiration, which obtains most of the energy needed for cellular metabolism, consumes both organic fuel and oxygen. Respiratory processes ultimately produce the adenosine triphosphate (ATP) that drives metabolic processes. The Krebs cycle, named for biochemist Hans A. Krebs, is a basic chemical process that is found in the mitochondria of all eukaryotic cells.

The Krebs cycle is the crucial second part in the breakdown of glucose to water and carbon dioxide—a part of cellular respiration. Cellular respiration begins in the cytoplasm (the fluid within the cell) with the breakdown of glucose to form pyruvic acid, in a process known as glycolysis. Pyruvic acid is then transported across the mitochondrial membranes into the matrix, where it loses a molecule of carbon dioxide and is converted into acetyl coenzyme A (acetyl CoA). The Krebs cycle completes the breakdown of glucose by joining the acetyl portion of acetyl CoA to an organic acid which then, through a series of steps, releases the equivalent of what was left of the glucose as carbon dioxide.

Together, these steps supply energized electrons, which are necessary for the final step, oxidative phosphorylation, where the bulk of the cell’s ATP is produced. In addition to its central role in catabolism, or the breakdown of organic molecules, the Krebs cycle plays a central role in anabolism, or the synthesis of organic molecules. Many of the intermediate molecules in the Krebs cycle can be used in other biochemical pathways to produce amino acids, carbohydrates, and lipids.

Oxidation and Electron Transfer

The fuel for running the Krebs cycle is the two-carbon fragments known as acetyl groups. The overall chemistry of the Krebs cycle involves the oxidation of the acetyl group’s two carbon atoms to two molecules of carbon dioxide. As oxidation occurs in the Krebs cycle, electrons are released, in the form of hydrogen atoms, and picked up by electron carriers. The release of these electrons, which have a high energy content, is the primary goal of the Krebs cycle. They are used later as the energy source for oxidative phosphorylation.

The electron acceptors are two coenzymes similar to coenzyme A; nicotinamide adenine dinucleotide (NAD⁺) and flavin adenine dinucleotide (FAD). Both NAD⁺ and FAD have a ring containing nitrogen that shares four electrons with an adjacent carbon atom. Such arrangements are especially suitable for accepting electrons and protons and then releasing them later. In their oxidized states, NAD⁺ and FAD are each capable of accepting two electrons, donated initially as two hydrogen atoms. When NAD⁺ reacts with two hydrogen atoms, it keeps one and strips away the electron from the other, releasing what is left as a free proton (H⁺). In the process NAD⁺ is made into its reduced form, NADH. When FAD reacts with two hydrogen atoms, it accepts them both, along with their two electrons, to become FADH₂. Thus, the coenzymes NAD⁺ and FAD, as NADH and FADH₂, serve as electron transfer agents, connecting the Krebs cycle with the electron transport system embedded in the inner mitochondrial membrane.

Principal Steps

After years of research, a detailed picture of the chemistry of the Krebs cycle is available. For each of the eight principal steps, the structure of the reactants and products, as well as the enzymes that catalyze the reactions, has been determined. During one turn of the Krebs cycle, the equivalent of one acetyl group is converted into two carbon dioxide molecules.

The first step of the Krebs cycle occurs when acetyl CoA reacts with oxaloacetate, the ionic form of oxaloacetic acid, to form citrate. (All of the acids in the Krebs cycle occur in their ionic forms.) This first product is tricarboxylic acid; hence, one of the other names of this cycle, the tricarboxylic acid cycle. In addition to citrate, the first step in the Krebs cycle releases a molecule of coenzyme A, which is ready to react with another acetyl group from a pyruvate molecule. While the overall result of the Krebs cycle is degradation, this initial step is one of building up, or synthesis.

In the second step, citrate is made into isocitrate by a complex rearrangement involving the loss of a molecule of water and then the addition of a water molecule. The net effect is to move a hydroxyl or alcohol group from one carbon to an adjacent one. The starting citrate and the product, isocitrate, have the same molecular formula but have different molecular structures. Such molecules are called isomers.

In the third step, isocitrate is oxidized. It passes two hydrogen ions to NAD⁺, thus reducing it to NADH and releasing a free proton. Isocitrate also loses a molecule of carbon dioxide and becomes alpha-ketoglutarate. With the loss of this carbon dioxide molecule, the equivalent of only one of the two original acetyl carbon atoms remains.

The fourth step involves the loss of another carbon, in the form of carbon dioxide, equivalent to another of the original two acetyle carbon atoms. Alpha-ketogluterate bonds with a molecule of coenzyme A to form succinyl CoA. The remaining steps involve the remaking of oxaloacetate so the cycle can occur again with another acetyl group. In the process, a few more high-energy electrons are passed off to electron carriers.

The fifth step involves splitting succinyl CoA to produce free coenzyme A and succinate. The splitting of this bond releases enough energy to drive a substrate-level phosphorylation reaction which takes place in two steps. First, a molecule of guanosine diphosphate (GDP) reacts with inorganic phosphate to form guanosine triphosphate (GTP). Then GTP transfers its phosphate to adenosine diphosphate (ADP), to produce ATP. These two nucleotides are very similar in having high-energy phosphate bonds, but they differ in their nitrogenous bases; GTP has guanine, and ATP has adenine.

In step six, succinate is oxidized to a fumarate. In the process two hydrogen atoms, with their high-energy electrons, are passed to FAD to form FADH₂.

In step seven, fumarate is transformed to malate by the addition of a molecule of water.

In the last step, malate is made into oxaloacetate, which is ready to start the process all over again. A further consequence of this reaction is the release of another two hydrogen atoms, with high-energy electrons, that are picked up by NAD⁺, with the consequent production of the usual free proton.

Advantages

Detailed studies of these chemical reactions reveal that the carbon atoms that are actually oxidized and released as carbon dioxide come from the oxaloacetate portion of the citrate ion rather than from the acetyl group. The acetyl group is now one-half of the new oxaloacetate and, after another turn of the Krebs cycle, these acetyl carbons will be released as carbon dioxide.

The mechanics of the Krebs cycle have additional advantages. The cycle’s chief function of obtaining energy for the cell’s needs is accomplished in small, discrete increments rather than in one large burst. This stepwise process allows finer control of the entire reaction sequence, with a large number of points at which control can be exercised. Finally, as the acetyl group passes through a cycle, a variety of molecules are produced, which can provide raw materials for the synthesis of essential biological molecules.

Bibliography

Gilbert, Hiram F. Basic Concepts in Biochemistry: A Student’s Survival Guide. 2d ed. New York: McGraw-Hill, 2000. Part of McGraw-Hill’s Basic Sciences series; includes bibliography and index.

Igelsrud, Donald E. “How Living Things Obtain Energy: A Simpler Explanation.” The American Biology Teacher 51, no. 2 (February, 1989): 89-93. This well-presented discussion is important for two reasons: It is written from a biologist’s point of view, and it makes the chemistry much easier for the nonspecialist to grasp. While it is directed at biology teachers and includes valuable classroom suggestions, it can be appreciated by anyone interested in biochemical oxidation and energy transfer.

Krebs, Hans A. “The History of the Tricarboxylic Cycle.” Perspectives in Biology and Medicine 14 (Autumn, 1979): 154-170. While this journal is probably not easily available in public libraries, it is worthwhile to seek it out at a college. Krebs not only describes the origin of his ideas and their development but also provides rare insight concerning his success. A revealing analysis of how one scientist found a solution which others, equally brilliant, had overlooked.

Metzler, David. Biochemistry. 2d ed. New York: Harcourt, 2002. Provides a detailed and technical view of all aspects of biochemistry. Readable by high-school students, with some background, and by scientists.