Before a pyruvic acid molecule enters the citric acid cycle, which takes place in the mitochondria, a molecule of carbon dioxide is removed and a molecule of NADH is produced, leaving an acetyl fragment. The 2-carbon fragment is then bonded to a large molecule called coenzyme A. Coenzyme A consists of a combination of the B vitamin pantothenic acid
and a nucleotide. Pantothenic acid is one of several B vitamins essential to respiration in both plants and animals; others include thiamine (vitamin Bj), niacin, and riboflavin. The bonded acetyl fragment and coenzyme A molecule is referred to as acetyl CoA. The following equation summarizes the fate of the two pyruvic acid molecules following glycolysis and leading to the citric acid cycle:
e nz ym es
In addition to pyruvic acid, fats and amino acids can also be converted to acetyl CoA and enter the process at this point. The NADH molecules donate their hydrogen to an electron transport system (discussed in the section, "Electron Transport and Oxidative Phosphorylation"), and the acetyl CoA enters the citric acid cycle (see Fig. 10.14).
The Citric Acid (Krebs) Cycle Reexamined
In the citric acid cycle, acetyl CoA is first combined with oxaloacetic acid, a 4-carbon compound, producing citric acid, a 6-carbon compound. The citric acid cycle is kept going by oxaloacetic acid, which is produced in small amounts, but is an intermediate product rather than a starting substance or an end product of the cycle. As the cycle progresses, a carbon dioxide is removed, producing a 5-carbon compound. Then another carbon dioxide is removed, producing a 4-carbon compound. This 4-carbon compound, through additional steps, is converted back to oxaloacetic acid, the substance with which the cycle began, and the cycle is repeated.
Each full cycle uses up a 2-carbon acetyl group and releases two carbon dioxide molecules while regenerating an oxaloacetic acid molecule for the next turn of the cycle. Some hydrogen is removed during the process and is picked up by FAD and NAD. One molecule of ATP, three molecules of NADH, and one molecule of FADH2 are produced for each turn of the cycle. The citric acid cycle may be summarized as follows:
oxaloacetic + acetyl + ADP + P + 3 NAD + FAD enzy mes) acid CoA
oxaloacetic + CoA +ATP + 3 NADH + H+ + FADH2 + 2 CO2 acid
The hydrogen carried by NAD and FAD can mostly be traced to the acetyl groups and to water molecules added to some compounds in the citric acid cycle. The FAD and FADH2 are now known to be intermediate compounds. Ubiquinol, a component of the electron transport system, receives electrons from either NADH or FADH2.
After completion of the citric acid cycle, the glucose molecule has been totally dismantled, and some of its energy has been transferred to ATP molecules. A considerable portion of the energy was transferred to NAD and FAD when they were used to pick up hydrogen and electrons from the molecules derived from glucose as they were broken down during glycolysis and the citric acid cycle. This energy is released as the hydrogen and electrons are passed along an electron transport system. This system, like the electron transport system of photosynthesis, functions something like a high-speed bucket brigade in passing along electrons from their source to their destination. Several of the electron carriers in the transport system are cytochromes. They are very specific and, as electrons flow along the system, they can transfer their electrons only to other specific acceptors. When the electrons reach the end of the system, they are picked up by oxygen and combine with hydrogen ions, forming water.
Part of the energy that is released during the movement of electrons along the electron transport system can be used to make ATP in a process called oxidative phosphorylation. If hydrogen ion and electron transport begins with NADH, which was produced inside the mitochondria (i.e., during the conversion of pyruvic acid to acetyl CoA and during the citric acid cycle), enough energy is produced to yield three ATP molecules from each NADH molecule. Similarly, if hydrogen ion and electron transport begins either with FADH2 or with NADH, produced outside the mitochondria (i.e., during glycolysis), two ATP molecules are produced.
The manner in which ATP is produced during the operation of the respiratory electron transport system involves essentially the same chemiosmotic concept that was applied earlier to proton movement across thylakoid membranes.
The chemiosmosis theory concerning electron transport and proton movement across membranes was proposed in the 1960s by Peter Mitchell, a British biochemist, and is now widely accepted as the explanation for the movements in both photosynthesis and respiration. In respiration, oxidative phosphorylation is energized by a gradient of protons (H+) that flow by chemiosmosis across the inner membrane of a mitochondrion. Mitchell, who received a Nobel Prize for his work in 1978, surmised that protons are "pumped" from the matrix of the mitochondria to the region between the two membranes (see Fig. 3.13) as electrons flow from their source in NADH molecules along the electron transport system, which is located in the inner membrane. The protons are believed to "diffuse" back into the matrix via channels provided by an enzyme complex known as the F1 particle (an ATPase), releasing energy that is used to synthesize ATP.
If we retrace our steps through the entire process of aerobic respiration, we find that glycolysis yields four molecules of ATP and two molecules of NADH (from which more molecules of ATP are formed), for a total of eight ATP from the conversion of glucose to two pyruvic acid molecules. Two ATP are used in the process, however, leaving a net gain of six ATP.
When two pyruvic acid molecules are converted to two acetyl CoA in the mitochondria, two more NADH molecules (which will generate six molecules of ATP) are produced. The two acetyl CoA molecules metabolized in the citric acid
Stern-Jansky-Bidlack: I 10. Plant Metabolism I Text I I © The McGraw-Hill
Introductory Plant Biology, Companies, 2003
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