Metabolism catabolic reactions

Metabolism is the sum of all chemical reactions in the body. Reactions that break down large molecules into smaller fragments are called catabolism reactions that build up large molecules from small pieces are called anabolism. Although the details of specific biochemical pathways are sometimes complex, all the reactions that occur follow the normal rules of organic chemical reactivity. 

With reference to the free energy as a characteristic of metabolism one may say that catabolic reactions proceed with a release of energy and anabolic ones, with a consumption of energy The anabolic reactions can proceed only as closely coupled to the catabolic reactions. High-energy, or macroergic, compounds act as energetic mediators between these two types of reactions. 

Introduction to metabolism Definition of catabolic and anabolic pathways INTRODUCTION TO METABOLISM (p. 89) Most pathways can be classified as either catabolic (they degrade complex molecules to a few simple products, such as C02, NH3, and water) or anabolic (they synthesize complex end-products from simple precursors). Catabolic reactions also capture chemical energy in the form of ATP from the degradation of energy-rich molecules. Anabolic reactions require energy, which is generally provided by the breakdown of ATP. 

Some catabolic reactions depend upon ADP, but under most conditions its concentration is very low because it is nearly all phosphorylated to ATP. Reactions utilizing ADP may then become the rate-limiting pacemakers in reaction sequences. Depletion of a reactant sometimes has the effect of changing the whole pattern of metabolism. Thus, if oxygen is unavailable to a yeast, the reduced coenzyme NADH accumulates and reduces pyruvate to ethanol plus C02 (Fig. 10-3). The result is a shift from oxidative metabolism to fermentation. 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

The biological roles of phosphorus include (1) anabolic and catabolic reactions, as exemplified by its essentiality in high-energy bond formation, e.g., ATP (adenosine triphosphate), ADP (adenosine diphosphate), etc., and the formation of phosphorylated intermediates in carbohydrate metabolism ... 

Biochemically, folacin functions in vivo as coenzymes and carriers of one-carbon units for a number of enzyme reactions, including synthesis of amino acids, proteins, and nucleic acids (58,120,122). Folacin participates in both anabolic and catabolic reactions, and its metabolism is cyclic in nature. Greater detail on the biochemistry of folacin is available (120,122). 

Metabolic reactions are of two general kinds. Catabolism includes all reactions that break down large molecules into smaller ones with a release of energy, as in the breakdown of carbohydrates. Catabolic reactions are also called destructive metabolism. 

Some catabolic reactions of amino acid carbon chains are easy transformations to and from TCA cycle intermediates—for example, the transamination of alanine to pyruvate. Reactions involving 1-carbon units, branched-chain, and aromatic amino acids are more complicated. This chapter starts with 1-carbon metabolism and then considers the catabolic and biosynthetic reactions of a few of the longer side chains. Amino acid metabolic pathways can present a bewildering amount of material to memorize. Perhaps fortunately, most of the more complicated pathways lie beyond the scope of an introductory course or a review such as this. Instead of a detailed listing of pathways, this chapter concentrates on general principles of amino acid metabolism, especially those that occur in more than one pathway. 

Tissues carry out an enormous number of chemical reactions. Collectively these are referred to as metabolism. Some reactions or reaction sequences (often referred to as metabolic pathways) create complex molecules from simpler molecules, and these processes are then called anabolism. Many reactions, however, serve to break down complex molecules into simple molecules, and these processes are called catabolism. Among the latter is the degradation of foodstuffs, such as proteins, carbohydrates, and fats. Often these are degraded to C02 and HzO, which are excreted by the organism. The purpose of doing this is to acquire useful energy, which is in turn required for the various life processes, including... 

The PDC is a junction box for cellular metabolism and energetics in which substrates may be diverted toward anabolic pathways (e.g., carbohydrate synthesis) or catabolic reactions and... 

Folic acid functions in the transfer of one-carbon fragments in a wide variety of biosynthetic and catabolic reactions it is therefore metaboUcaUy closely related to vitamin B12, which also functions in one-carbon transfer. Deficiency of either vitamin has similar clinical effects, and it seems likely that the main effects of vitamin B12 deficiency are exerted by effects on folate metabolism. 

Metabolic reactions are categorized into two types, anabolic and catabolic. Anabolic reactions construct complex molecules from simple molecules, usually while using up energy that becomes stored in chemical bonds. Catabolic reactions break down complex molecules into multiple simple molecules, usually while releasing energy. 

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