Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Metabolic loops and biosynthetic families
Key intermediates and biosynthetic families

When examining biosynthetic pathways, It is important to identify at least some of the intermediate products (intermediates). One of these is 3-phosphoglycerate. Since 3-phosphoglycerate is the primary product of Photosynthesis, it can legitimately be considered the starting material from which all other carbon-containing compounds are formed. In most organisms, phosphoglycerate can easily be converted into glucose and phosphoenolpyruvate, which in turn can yield phosphoglycerate once again. Any of these three compounds can serve as a precursor in the Synthesis of Other Organic compounds. The First stage of Biosynthesis involves reactions that yield 3-phosphoglycerate (or phosphoenolpyruvate) derived either from CO₂, formate, acetate, and Lipids, or from Polysaccharides

Subsequent stages of biosynthesis leading from 3-phosphoglycerate to the myriad of various compounds found within Cells—such as Amino Acids, NUCLEOTIDES, lipids, and so on—are highly complex and multi-step processes. Fig. 11-1 illustrates THE ORIGIN OF many compounds, specifically twenty amino acids (which make up all Proteins), nucleotides, and lipids. Among the additional key biosynthetic precursors shown in the diagram, special attention should be drawn to glucose-6-phosphate, Pyruvate, oxaloacetate, acetyl-CoA, a-ketoglutarate, and succinyl-CoA.

The amino acid Serine is derived almost directly from 3-phosphoglycerate, aspartate from oxaloacetate, and glutamate from a-ketoglutarate. Each of these Three amino acids gives rise to a "family" of Other Compounds [1]. A relatively modest effort spent understanding and memorizing the relationships between these different families can greatly facilitate The Study of biochemistry. Alongside the serine, aspartate, and glutamate-ketoglutarate families, we should note a prominent fourth family originating directly from pyruvate, and a fifth family (consisting primarily of lipids) derived from acetyl-CoA. Aromatic Amino acids are synthesized from erythrose-4-phosphate and phosphoenolpyruvate, with Chorismic acid playing The Role of a key intermediate [equation (7-50)]. Other families trace their origins back to glucose-6-phosphate and pentose phosphates. All these groups of compounds are indicated in Fig. 11-1 by bold, curved lines.

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FIG. 11-1. Some of the most important biosynthetic pathways. Key Intermediates are enclosed in rectangular boxes, and the 20 Proteinogenic Amino Acids are circled. Dashed lines indicate reactions comprising the gluconate pathway and Gluconeogenesis.