Metabolism amphibolic pathways

Amphibolic pathway. A metabolic pathway that functions in both catabolism and anabolism. 

Apart from the production of NADH and FADH2, which are the high-energy fuels of electron transport, the citric acid cycle has two other major functions. Several of its intermediate compounds are used to synthesize other cell constituents. This, the provision of molecules for other metabolic or biosynthetic pathways, is the anabolic function of the cycle (Table 12.1). Alternatively, certain other processes occurring within the cell may produce intermediates of the citric acid cycle. These compounds enter the reactions of the cycle, and their degradation involves the catabolic role of the cycle. These two major capabilities classify the citric acid cycle as an amphibolic pathway (Greek amphi meaning both sides ). 

Clearly, the reactions of glycolysis and the citric acid cycle are central to both anabolic and catabolic cellular activities. Metabolic pathways that function in both anabolism and catabolism are called amphibolic pathways. Consider for a moment the difficulties that the dual nature of these pathways could present to the cell. When the cell is actively growing, there is a great demand for biosynthetic precursors to build new cell structures. A close look at Figure 22.13 shows us that periods of active cell growth and biosynthesis may deplete the supply of citric acid cycle intermediates. The problem is, the processes of growth and biosynthesis also require a great deal of ATP ... 

Biochemical pathways may be described as catabolic, anabolic (biosynthetic), amphibolic or anaplerotic. The principal function of a catabolic sequence is to degrade (usually by an oxidative process) simple organic molecules derived from the breakdown of polymers (e.g. amino acids from proteins) and retain some of the free energy released in a biologically useful form. Anabolic pathways consume energy and synthesize (usually by a reductive process) the simple molecules which are assembled into proteins, nucleic acids, carbohydrate polymers and lipids. Amphibolic pathways, such as the tricarboxylic acid cycle, have both catabolic and anabolic properties. They are central metabolic pathways which furnish, from catabolic sequences, the intermediates which form the substrates of anabolic processes. The... 

Amphibolic pathway A metabolic pathway that can yield either energy or building blocks for synthetic reactions. 

Certain of the central pathways of intermediary metabolism, such as the citric acid cycle, and many metabolites of other pathways have dual purposes—they serve in both catabolism and anabolism. This dual nature is reflected in the designation of such pathways as amphibolic rather than solely catabolic or anabolic. In any event, in contrast to catabolism—which converges to the common intermediate, acetyl-CoA—the pathways of anabolism diverge from a small group of simple metabolic intermediates to yield a spectacular variety of cellular constituents. 

We should note at this point that the TCA cycle is more than just a means of producing NADH for oxidative phosphorylation. The pathway also provides a number of useful intermediates for other, often synthetic, pathways. For example, citrate is the starting substance for fat synthesis (Chapter 9) succinyl-CoA is required for haem production and 2-oxoglutarate and oxaloacetate in particular are involved with amino acid and pyrimidine metabolism. Pathways which have dual catabolic/anabolic functions are referred to as amphibolic . 

The word amphibolic is often applied to those metabolic sequences that are part of a catabolic cycle and at the same time are involved in a biosynthetic (anabolic) pathway. Another term, anaplerotic, is sometimes used to describe pathways for the synthesis of regenerating substrates. This word, which was suggested by H. L. Komberg, comes from a Greek root meaning "filling up."80... 

Most major metabolic pathways are considered to be either mainly anabolic or catabolic. Which of the following pathways is most correctly considered to be amphibolic ... 

Metabolism consists of catabolism, which is the oxidative breakdown of nutrients, and anabolism, which is reductive synthesis of biomolecules. The citric acid cycle is amphibolic, meaning that it plays a role in both catabolism and anabolism. Although the citric acid cycle is a part of the pathway of aerobic oxidation of nutrients (a catabolic pathway see Section 19.7), some of the molecules that are included in this cycle are the starting points of biosynthetic (anabolic) pathways (see Section 19.8). Metabolic pathways operate simultaneously, even though we talk about them separately. We should always keep this point in mind. 

Amphibolic p ways metabolic pathways serving both degradation and synthesis. See Metabolic cycle. 

M.c. are anabolic, catabolic or amphibolic The Calvin cycle (see) is an anabolic (synthetic) cycle. A truly catabolic cycle, which does not supply intermediates for biosynthesis, probably does not exist. The THcar-boxylic acid cycle (see) is an important central metabolic pathway, serving both the terminal oxidation of substrates and the provision of intermediates for biosynthesis (e.g. biosynthesis of porphyrins and certain amino acids) it is therefore an amphibolic M.c. Similarly, the Pentose phosphate cycle (see) has a catabol-... 

Between the clearly defined anabolic and catalx>lic pathways lies a central area of metabolism in which various simple compounds are interconverted. The pathways are said to be amphibolic since they have a dual function (amphi = both) and provide material which may be used either for synthesis or for breakdown. All the metabolic pathways involve the uptake or release of energy. Energy is required for ... 

The tricarboxylic acid (TCA) cycle is an essential metabolic pathway in plants. This cycle is amphibolic and functions not only in catabolism to supply energy as ATP, but also to generate precursors for anabolic pathways. Certain intermediates of the cycle, particularly a-ketoglutaric acid and oxaloacetic acid, are utilized... 

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