Bacteria glyoxylate cycle

Glyoxylate cycle A modification of the Krebs cycle, which occurs in some bacteria. Acetyl coenzyme A is generated directly from oxidation of fatty acids or other lipid compounds. 

Some plants and bacteria that can use acetate as their sole source of carbon are able to oxidize acetyl-CoA via the citric acid cycle, or the acetate can be converted to carbohydrates via a pathway that is a modification of the citric acid cycle. This pathway is known as the glyoxylate cycle (Fig. 12-10)... 

The Glyoxylate Cycle Enables Plants and Bacteria to Grow on Acetate... 

Many bacteria and plants are able to subsist on acetate or other compounds that yield acetyl CoA. They make use of a metabolic pathway absent in most other organisms that converts two-carbon acetyl units into four-carbon units (succinate) for energy production and biosyntheses. This reaction sequence, called the glyoxylate cycle, bypasses the two decarboxylation steps of the citric acid cycle. Another key difference is that two molecules of acetyl CoA enter per turn of the glyoxylate cycle, compared with one in the citric acid cycle. 

The glyoxylate cycle enhances the metabolic versatility of many plants and bacteria. This cycle, which uses some of the reactions of the citric acid cycle, enables these organisms to subsist on acetate because it bypasses the two decarboxylation steps of the citric acid cycle. 

Theme and variation. Propose a reaction mechanism for the condensation of acetyl CoA and glyoxylate in the glyoxylate cycle of plants and bacteria. 

Plants and bacteria use the glyoxylate cycle to convert two acetyl-CoA molecules into ... 

The glyoxylate cycle, found in plants and some fungi, algae, protozoans, and bacteria, is a modified version of the citric acid cycle in which two-carbon molecules, such as acetate, are converted to precursors of glucose. 

Plants and some bacteria contain two enzymes (isocitrate lyase and malate synthase) that enable them to synthesize sugars by using the glyoxylate cycle, a variant form of the citric acid cycle. Notice in Figure 14.20 that the glyoxylate cycle uses some of the same enzymes as the citric acid cycle, but that the steps in which decarboxylations occur are bypassed. One of the intermediates in the bypass is glyoxylate, which gives the cycle its name. 

THE GLYOXYLATE CYCLE ENABLES PLANTS AND BACTERIA TO GROW ON ACETATE... 

The glyoxylate cycle also occurs in bacteria. This point is far from surprising because many types of bacteria can live on very limited carbon sources. They have metabolic pathways that can produce all the biomolecules they need from quite simple molecules. The glyoxylate cycle is one example of how bacteria manage this feat. 

In plants and bacteria, the glyoxylate cycle is a pathway that bypasses the two oxidative decarboxylations of the citric acid cycle. As a result of this pathway, plants can convert acetyl-CoA to carbohydrates, which animals cannot do. 

Bacteria that have a glyoxylate cycle can convert the acetic acid to amino acids, carbohydrates, and lipids, but humans can use the acetic acid only as an energy source or to make lipids. 

The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the final oxidative pathway for carbohydrates, lipids, and amino acids. It is also a source of precursors for biosynthesis. The authors begin Chapter 17 with a detailed discussion of the reaction mechanisms of the pyruvate dehydrogenase complex, followed by a description of the reactions of the citric acid cycle. This description includes details of mechanism and stereospecificity of some of the reactions, and homologies of the enzymes to other proteins. In the following sections, they describe the stoichiometry of the pathway including the energy yield (ATP and GTP) and then describe control mechanisms. They conclude the chapter with a summary of the biosynthetic roles of the citric acid cycle and its relationship to the glyoxylate cycle found in bacteria and plants. 

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