Pathways, anabolic biosynthetic

Metabolism Consists of Catabolism (Degradative Pathways) and Anabolism (Biosynthetic Pathways)... 

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... 

Anabolic metabolic pathways are the flip side of catabolic ones. Anabolic reactions are biosynthetic that is, they create complex molecules out of simpler ones. Anabolic pathways are reductive in nature and consume energy. In all these ways, anabolic pathways stand in contrast to catabolic ones. It is frequently the case that the end product of an anabolic pathway will inhibit the first enzyme in the same pathway. This makes a good deal of sense. Anabolic pathways require energy and if there is enough end product available there is little reason to keep making more of it. So an excess of the end product simply turns off the pathway by inhibiting the first enzyme ... 

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... 

As was pointed out in Chapter 10, routes of biosynthesis (anabolism) often closely parallel pathways of biodegradation (catabolism). Thus, catabolism begins with hydrolytic breakdown of polymeric molecules the resulting monomers are then cleaved into small two- and three-carbon fragments. Biosynthesis begins with formation of monomeric units from small pieces followed by assembly of the monomers into polymers. The mechanisms of the individual reactions of biosynthesis and biodegradation are also often closely parallel. However, in most instances, there are clear-cut differences. A first principle of biosynthesis is that biosynthetic pathways, although related to catabolic pathways, differ from them in distinct ways and are often catalyzed by completely different sets of enzymes. 

Fig. 5.15. Anabolic pathways (synthetic) and central catabolic pathways 14. Only the main biosynthetic routes, and their principal connections to catabolic routes, are shown, in simplified version. The relationships between energy (ATP), and redox (NAD4, NADP ) metabolism, nitrogen...

Many biologically important routes of amino acid utilization, other than those leading to incorporation into proteins, are known. Some of these routes are distinctly anabolic pathways in which the amino acids serve as an initial substrate in an independent biosynthetic pathway. Other simple pathways involve the conversion of one amino acid to another, such as the formation of tyrosine from phenylalanine. The utilization of glycine in the formation of porphyrin derivatives occurs by very complex highly branched pathways. Some other biologically important pathways lead to the biosynthesis of small peptides as in the biosynthesis of glutathione. 

The catabolic and anabolic pathways that are responsible for the formation of many of the biomarker compounds discussed in this chapter occur through an intermediary metabolism via glycolysis and the citric acid cycle (Voet and Voet, 2004). The biosynthetic pathways of these compounds can be divided into primary and secondary metabolism (figure 9.4). Many of these compounds are not used as chemical biomarkers in estuarine research but are shown here simply to illustrate their relationship with the biomarkers discussed in this chapter. For more details on the biosynthetic pathways illustrated here, please refer to Voet and Voet (2004) and Engel and Macko (1993). 

Algae can adjust the intracellular concentration of DMSP through the biosynthetic (anabolic) or the degradation (catabolic) pathways. DMSP-lyase enzymes facilitate the degradation pathway, in which DMSP is cleaved to DMS, acrylate and a proton. What controls the activity of DMSP-lyases in phytoplankton is still unknown. Stefels (2000) suggested... 

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 ). 

In addition to the obvious difference in the direction of their metabolic goals, anabolism and catabolism differ in other significant ways. For example, the various degradative pathways of catabolism are convergent. That is, many hundreds of different proteins, polysaccharides and lipids are broken down into relatively few catabolic end products. The hundreds of anabolic pathways, however, are divergent. That is, the cell uses relatively few biosynthetic precursor molecules to synthesize a vast number of different proteins, polysaccharides and lipids. 

One of the most striking features of the common fatty acids is that all have an even number of carbon atoms (Table 27.1). This even number results because all fatty acids are derived biosynthetically from the simple two-carbon precursor, acetyl CoA. The anabolic pathway by which orgam sms synthesize fatty acids is shown in Figure 29.8. 

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