Biochemistry and Molecular Biology - Belyasova, N. A. 2002

Metabolism. Processes Requiring Energy Input
Features of Biosynthesis. Carbohydrate Biosynthesis

Biosynthesis of cellular components constitutes one of the major metabolic “expenses,” consuming the largest share of energy stored as ATP, reduction equivalents, and building blocks. Biosynthetic reactions are part of anabolism, the features of which and its interrelation with Other Aspects of METABOLISM are described in Chapter 8. It is worth adding a few more important regularities of biosynthetic processes to this information: 1) the pathways for synthesizing complex cellular compounds often run parallel to the corresponding Catabolic pathways that generate the building blocks—biosynthetic precursors. However, the reaction sequences in these reversible pathways never completely coincide, nor do the sets of Enzymes involved in these processes; 2) the Regulation of Biosynthetic pathways and catabolic processes is carried out independently, at the level of different enzymes; 3) The entry of substances into biosynthetic pathways requires their activation, which most often occurs via phosphorylation involving ATP; 4) The biosynthesis of structurally and functionally related compounds often represents “branched” pathways, where the product of one series of reactions serves as a substrate for the Synthesis of Other products; 5) The Role of precursors in the biosynthesis of most cellular components is played by a limited number of substrates that are formed during major Catabolic and anabolic processes and belong to the key metabolic Intermediates of the Cell.

Most of these listed regularities of biosynthesis can be observed, for instance, when comparing the processes of Glycolysis and Gluconeogenesis. The latter will be discussed in this chapter.