The production of ATP is accompanied by a sequence of reactions. In this sequence, the hydrogen atoms shed by the foodstuff are passed between compounds known as hydrogen carriers. In fact, some of the compounds do not accept hydrogen atoms. They accept only electrons from the hydrogens. This results in the release of hydrogen ions—electrically charged hydrogen atoms. These compounds are called electron carriers. They contain iron or copper atoms that take up or pass on the electrons. Hydrogen and electron carriers first receive two hydrogen atoms or electrons. They then return to their original form by passing the atoms or electrons on to the next carrier.
The first carrier is called nicotinamide adenine dinucleotide (NAD), derived from the vita-
min nicotinic acid. This accepts hydrogen from foodstuffs, then passes it on to the next carrier. This carrier is a derivative of vitamin B2 (riboflavin) called flavine mononucleotide (FMN). This in turn accepts the hydrogen and then passes it on to the next carrier. The final set of carriers is in the enzyme cytochrome oxidase, which transfers electrons to oxygen atoms. The resulting oxygen ions then combine with hydrogen ions taken up from the medium to form water. This entire respiratory chain is built into the structure of certain membranes in the cell. The chain is subdivided into three spans. As two hydrogens or electrons cross each span, the energy released is used to make one molecule of ATP. Thus, the complete oxidation just described yields three molecules of ATP.
The first phase in the breakdown of glucose is called glycolysis. In the absence of oxygen, glycolysis is the only method by which most organisms can obtain energy. This first phase takes place in the cytoplasm of the cell. Cytoplasm is the substance inside a cell not including the nucleus. Glycolysis results in the formation of a three-carbon molecule called pyruvic acid. If oxygen is available, the pyruvic acid is moved into the mitochondria, where the next stage occurs. Mitochondria are sausage-shaped structures found in the cytoplasm of cells. Without oxygen, glycolysis can keep going only if pyruvic acid is continuously converted into lactic acid (in animals) or etha-nol (in some other organisms). Lactic acid is the substance that accumulates in muscles. It causes fatigue when oxygen is used up during strenuous exercise. Both lactic acid and etha-nol are potentially toxic. They are formed only as a temporary measure until the oxygen is replenished. In contrast, some bacteria called anaerobes use this method as the source of all their energy. For them, oxygen is toxic.
Glycolysis is an inefficient means of converting the energy in glucose into useful pack-
Some types of bacteria and algaeare unusual in the way they obtain energy for their metabolic processes. They utilize inorganic sources such as hydrogen sulfide or iron salts. For example, the rust colored area in the foreground (below) contains large numbers of iron bacteria. These bacteria are responsible for the rust color. They derive their energy by oxidizing the inorganic iron.
118 Biochemistry: Biochemical energy
One molecule of six-carbon sugar (glucose)
One molecule of six-carbon sugar (fructose-1,6-bisphosphate)
Two, three-carbon sugar derivatives
4 ADP 4 ATP/"-
Two molecules of pyruvic acid (a three-carbon compound)
Glycolysisis the first sequence of reactions in the breakdown of glucose to obtain energy (in the form of ATP). The main stages are illustrated in the diagram. Those that take place in the cytoplasm are on a pale brown background. Those that occur in the mitochondria are on a gray background. The first four stages are the same in all organisms that utilize glycolysis. Overall, these stages generate two molecules of ATP for every one molecule of glucose. Thereafter, one of two pathways is possible. If oxygen is absent, or if the organism is an anaerobe (that is, one that cannot use oxygen), the pyruvic acid is converted to either lactic acid or ethanol. There is no further generation of ATP. If, however, oxygen is present, the pyruvic acid is broken down to acetyl coenzyme A (acetyl CoA). During this process, six molecules of ATP are produced from the one original glucose molecule. The acetyl CoA then enters the citric acid (or Krebs) cycle. This results in yet more ATP being produced. The principal stages in this cycle, which can also produce energy from fats and proteins, are illustrated in the diagram on the opposite page. The details are described in the main text.
Two molecules of carbon dioxide
Two molecules of ethanol (a two-carbon compound)
Two molecules of lactic acid (a three-carbon compound)
ets of ATP energy. This is because there is a net gain of only two ATP's per glucose molecule used up. The method begins with the input of two ATP's. The first is used to convert the sugar into glucose. The second is used to form fructose. This is then split into two three-carbon sugar derivatives. These, in turn, are converted to pyruvic acid with the manufacture of four ATP's. Hydrogen atoms are also produced. These are taken up by NAD, as previously described. But without oxygen, these hydrogen atoms do not pass along the hydrogen carrier system. The NAD can be regenerated to maintain glycolysis only by transferring the hydrogen to the pyruvic acid, thus forming lactic acid.
Acetyl coenzyme A
Before oxidation can proceed to the second stage, called the Krebs, or citric acid cycle, the pyruvic acid must lose another carbon atom (as carbon dioxide). This reaction produces two hydrogen atoms that pass along the hydrogen-carrier chain. This yields three molecules of ATP and one molecule of acetyl coenzyme A (acetyl-CoA). This latter substance is extremely important in the breakdown of carbohydrates and the oxidation of fats and proteins. Acetyl coenzyme is formed by linking the remaining two-carbon molecule of acetic (ethanoic) acid with coenzyme A, a derivative of the B-group vitamin pantothenic acid. Just as the coenzyme NAD may act as a carrier for the transfer of hydrogen atoms, CoA is a carrier for acetyl groups in a number of reactions. An acetyl group is composed of carbon, hydrogen, and oxygen.