Chapter 12: Anabolism – Biosynthesis & Energy Consumption

Anabolism, defined as the biological process of creating order by constructing complex molecules, organelles, and cells from simple inorganic substances, requires substantial energy and follows six fundamental principles, including efficient macromolecule construction from monomers, the dual function of many enzymes in amphibolic pathways, and the separation of catabolic and anabolic reactions through compartmentation. The primary starting materials for biosynthesis are the twelve precursor metabolites, which are essential carbon skeletons derived from the central metabolic pathways. Autotrophic organisms initiate construction by fixing carbon dioxide using high-energy processes like the Calvin-Benson cycle, which is utilized by most aerobic bacteria and eukaryotes, or alternative anaerobic pathways such as the reductive TCA cycle. The synthesis of carbohydrates often involves gluconeogenesis to create glucose from non-sugar precursors, with other sugars and polysaccharides manufactured using nucleoside diphosphate carriers like UDPG. A key bacterial anabolic process is peptidoglycan synthesis, a complex sequence spanning the cytoplasm and periplasmic space that employs the carriers uridine diphosphate (UDP) and the lipid bactoprenol to assemble the NAG-NAM-pentapeptide repeat units before final cross-linking via transpeptidation. Amino acid formation requires the remodeling of precursor metabolites and the assimilation of key nutrients, such as inorganic nitrogen, which can be incorporated as ammonia via glutamate dehydrogenase or the GS-GOGAT system, or through the highly energy-intensive process of nitrogen fixation catalyzed by nitrogenase. To ensure adequate supply of intermediates, particularly those feeding the TCA cycle, microorganisms employ anaplerotic reactions like the glyoxylate cycle to replenish necessary precursor metabolites. Nucleotides are also synthesized from precursor metabolites, with purines built directly on ribose 5-phosphate, while pyrimidines are constructed before the ribose sugar is attached. Finally, lipid synthesis, vital for cell membranes, involves the construction of fatty acids from acetyl-CoA and malonyl-ACP by the fatty acid synthase complex, and these fatty acids are then incorporated into triacylglycerols and phospholipids via intermediates like phosphatidic acid and CDP-diacylglycerol. In Gram-negative bacteria, lipopolysaccharide (LPS) assembly is completed via a branched pathway before the complex molecule is transferred across the periplasmic space to the outer membrane via the dedicated Lpt pathway.