Citric Acid Cycle

Citric Acid Cycle, series of chemical reactions occurring within the cell, responsible for the final breakdown of food molecules to form carbon dioxide, water, and energy. This process, which is carried out by seven enzymes, is also known as the tricarboxylic acid cycle, or the Krebs cycle. The citric acid cycle is active in all animals and higher plants and in most bacteria. In organisms that have cells with nuclei, the cycle is contained within a membrane-bound organelle called the mitochondrion, a structure often referred to as the power plant of the cell. Discovery of the citric acid cycle is credited to Sir Hans Adolf Krebs, a British biochemist who outlined its essential steps in 1937.

Before a food can enter the citric acid cycle, it must be broken down into small units called acetyl groups. Each acetyl group (CH3CO) contains only two atoms of carbon, along with hydrogen and oxygen. At the start of the cycle, an acetyl group combines with a four-carbon molecule called oxaloacetate to yield a six-carbon compound, citric acid. In the remaining steps of the cycle, the citric acid molecule is gradually rearranged and stripped of two of its carbon atoms, which are given off in the form of carbon dioxide. Four electrons also are released. These travel along a nearby series of carrier molecules in the cell, the electron transport chain, where they produce energy in the form of an energy-rich molecule called adenosine triphosphate, or ATP, before combining with oxygen to form water. An additional product of the cycle is another energy-containing molecule, called guanosinetriphosphate, or GTP. These fuel molecules, ATP and GTP, can be used by the cell for many energy-requiring processes. Another fuel molecule, creatine phosphate, may also be used to provide extra energy to brain and muscle cells. At the end of the cycle, the original molecule of oxaloacetate is regenerated. This molecule can then combine with another acetyl group to start the cycle over again. At each turn of the cycle, more energy is produced.

The citric acid cycle is an economical way of turning the food components in a cell into usable energy. Only the acetyl groups are destroyed in the cycle; the seven enzymes that carry out the various reactions, and the intermediate compounds that these enzymes act on, can be used again and again. Many of the intermediate compounds produced in the citric acid cycle also are of value as starting materials for the synthesis of amino acids, carbohydrates, and other cellular products. See Metabolism; Sugar Metabolism.

Adenosine Triphosphate

Adenosine Triphosphate (ATP), molecule found in all living organisms that is the main immediate source of usable energy for the activities of the cells. ATP is built up by the metabolism of foodstuffs in the cell in special compartments called mitochondria. Because the energy-exchanging function of ATP and the catalytic (work-boosting) function of enzymes are intimately connected, ATP is characterized as a coenzyme. The adenosine part of the molecule is made up of adenine, a nitrogen-containing compound (also one of the principal components of the gene), and ribose, a five-carbon sugar. Three phosphate units (triphosphate), each made up of one phosphorus atom and four oxygen atoms, are attached to the ribose. The two bonds between the three phosphate groups are high-energy bonds, that is, they are relatively weak and yield their energy readily when split by enzymes. With the release of the end phosphate group, 7 kilocalories (7 calories, in common usage) of energy become available for work, and the ATP molecule becomes ADP (adenosine diphosphate). Most of the energy-consuming reactions in cells are powered by the conversion of ATP to ADP; they include the transmission of nerve signals, the movement of muscles, the synthesis of protein, and cell division. Usually, ADP quickly regains the third phosphate unit through the action of cytochrome, a protein that builds it up by using food energy. In vertebrate muscle and brain cells, excess ATP can join with creatine to provide a reserve energy store.

The release of two phosphate groups from ATP by the enzyme adenyl cyclase forms AMP (adenosine monophosphate), a nucleotide component of nucleic acids, the material of DNA; this enzyme is important in many of the body's reactions. The American biochemist Earl Sutherland, Jr., won the 1971 Nobel Prize in physiology or medicine for work showing that one form of AMP called cyclic AMP, created by the action of the enzyme adenyl cyclase, is instrumental in the activities of many hormones, including epinephrine and ACTH.

Plants produce ATP by direct utilization of the energy in sunlight. See Photosynthesis.