substrate-level phosphorylation

The central metabolic pathways are catabolic, but the precursor metabolites and reducing power they generate can also be diverted for use in biosynthesis. To reflect the dual role of these pathways, they are sometimes called amphibolic pathways (amphi meaning "both kinds'').

The most common pathway that initiates the breakdown of sugars is glycolysis (glycos means "sugar'' and lysis means "dissolution"). This pathway is also called the Embden-Meyerhoff pathway to honor the scientists who described it, or the glycolytic pathway. This multistep pathway gradually oxidizes the 6-carbon sugar glucose to form two molecules of pyruvate, a 3-carbon compound. At about midpoint in the pathway, a 6-carbon derivative of glucose is split into two 3-carbon molecules. Both of these latter molecules then undergo the same series of transformations to produce pyruvate molecules. Glycolysis provides the cell with a small amount of energy in the form of ATP, some reducing power in the form of NADH, and a number of precursor metabolites. Some bacteria have a different pathway called the Entner-Doudoroff pathway instead of or in addition to the glycolytic pathway; some archaea have a slightly modified version of the Entner-Doudoroff pathway. Like glycolysis, the Entner-Doudoroff pathway generates pyruvate, but it uses different enzymes, generates reducing power in the form of NADPH, and yields less ATP.

The pentose phosphate pathway also breaks down glucose, but its primary role in metabolism is the production of compounds used in biosynthesis, including reducing power in the form of NADPH and precursor metabolites. It operates in conjunction with other glucose-degrading pathways (glycolysis and the Entner-Doudoroff pathway). Most intermediates it generates are drawn off for use in biosynthesis, but one compound is directed to a mid-point step of glycolysis for further breakdown.

Pyruvate generated in any of the preceeding pathways must be converted into a specific 2-carbon fragment to enter the Krebs cycle. This is accomplished in a complex reaction called the transition step, which removes CO2, generates reducing power, and joins the resulting acetyl group to a compound called coenzyme A, forming acetyl-CoA. Note that the transition step is repeated twice for each molecule of glucose that is broken down.

The TCA cycle accepts the 2-carbon acetyl group of acetyl-CoA, initiating a series of oxidation steps that result in the release of two molecules of CO2. For every acetyl-CoA that enters the TCA cycle, the cyclic pathway "turns" once. Therefore, it must "turn'' twice to complete the oxidation of one molecule of glucose. The TCA cycle generates precursor metabolites, a great deal of reducing power, and ATP.

Respiration uses the reducing power accumulated in gly-colysis, the transition step, and the TCA cycle to generate ATP by oxidative phosphorylation (table 6.4). The electron carriers NADH and FADH2 transfer their electrons to the electron

6.1 Principles of Metabolism 137

transport chain, which ejects protons to generate a proton motive force. This transfer of electrons also serves to recycle the carriers so they can once again accept electrons from catabolic reactions. In aerobic respiration, electrons are ultimately passed to molecular oxygen (O2), the terminal electron acceptor, producing water. Some organisms use anaerobic respiration. This is analogous to aerobic respiration, generating ATP by oxidative phosphorylation, but it uses an inorganic molecule other than O2, such as nitrate (NO3:), as a terminal electron acceptor. Organisms that use respiration, either aerobic or anaerobic, are said to respire.

Cells that cannot respire are limited by their relative inability to recycle reduced electron carriers. A cell only has a limited number of carrier molecules; if electrons are not removed from the reduced carriers, none will be available to accept electrons. As a consequence, subsequent catabolic processes cannot occur. Fermentation provides a solution to this problem, but it results in only the partial oxidation of glucose. Thus, compared with respiration, fermentation produces relatively little ATP. It is used by facultative anaerobes when a suitable inorganic terminal electron acceptor is not available and by organisms that lack an electron transport chain. These cells must stop short of oxidizing glucose completely to avoid generating even more reducing power. Otherwise, they would run out of the oxidized form of the electron carriers very quickly. Instead of oxidizing pyruvate in the TCA cycle, they use pyruvate or a derivative of it as a terminal electron acceptor. By transferring the electrons carried by NADH to pyruvate or a derivative, NAD+ is regenerated so it can once again accept electrons in the steps of glycolysis. Note that fermentation always uses an organic molecule as the terminal electron acceptor. Although fermentation does not use the TCA cycle, organisms that ferment still employ certain key steps of the cycle in order to generate the precursor molecules required for biosynthesis. ■ facultative anaerobes, p. 89

Table 6.4 ATP-Generating Processes of Prokaryotic Chemoorganoheterotrophs

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