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Diversity of Metabolism in Procaryotes (page 8)

(This chapter has 8 pages)

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Biosynthesis

The pathways of central metabolism (i.e., glycolysis and the TCA cycle), with a few modifications, always run in one direction or another in all organisms. The reason - these pathways provide the precursors for the biosynthesis of cell material. When a pathway, such as the Embden-Meyerhof pathway or the TCA cycle, functions to provide energy in addition to chemical intermediates for the synthesis of cell material, the pathway is referred to as an amphibolic pathway. Pathways of glycolysis and the TCA cycle are amphibolic pathways because they provide ATP and chemical intermediates to build new cell material. The main metabolic pathways, and their relationship to biosynthesis of cell material, are shown in Figure 25 below.

Biosynthesis or intermediary metabolism is a topic of biochemistry, more so than microbiology. It will not be dealt with in detail here. The fundamental metabolic pathways of biosynthesis are similar in all organisms, in the same way that protein synthesis or DNA structure are similar in all organisms. When biosynthesis proceeds from central metabolism as drawn below, some of the main precursors for synthesis of procaryotic cell structures and components are as follows.

Polysaccharide capsules or inclusions are polymers of glucose and other sugars.

Cell wall peptidoglycan (NAG and NAM) is derived from glucose phosphate.

Amino acids for the manufacture of proteins have various sources, the most important of which are pyruvic acid, alpha ketoglutaric acid and oxalacetic acid.

Nucleotides (DNA and RNA) are synthesized from ribose phosphate. ATP and NAD are part of purine (nucleotide) metabolism.

Triose-phosphates are precursors of glycerol, and acetyl CoA is a main precursor of lipids for membranes

Vitamins and coenzymes are synthesized in various pathways that leave central metabolism.  In the example given in Figure 25, heme synthesis proceeds from the serine pathway, as well as from succinate in the TCA cycle. 



Figure 25. The main pathways of biosynthesis in procaryotic cells



END OF CHAPTER

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