What Is Biosynthesis?

Biosynthesis is the set of enzyme-catalyzed processes by which living cells assemble complex molecules from simpler precursors. It is the constructive half of metabolism, paired with catabolism, which breaks molecules down to release energy. Because building an ordered molecule from small units lowers entropy, every biosynthetic sequence must be driven by an energy source, typically the hydrolysis of adenosine triphosphate or a related nucleotide, and most reductive steps must be supplied with electrons carried by NADPH. Cells run these processes continuously to replace turnover, to grow, and to produce the specialized compounds that mediate defense, signaling, and competition.

A biosynthetic pathway is a stepwise sequence in which the product of one enzyme becomes the substrate of the next. Each step is catalyzed by a specific enzyme classified under the Enzyme Commission numbering scheme maintained by the International Union of Biochemistry and Molecular Biology, and the ENZYME repository operated by the SIB Swiss Institute of Bioinformatics records the reaction, cofactor requirements, and associated nomenclature for each characterized entry. Organizing metabolism this way lets a pathway be traced across organisms even when the participating genes differ.

Primary Metabolism

Primary biosynthetic pathways make the molecules every cell requires. Amino acids are built from carbon skeletons drawn out of glycolysis and the citric acid cycle with nitrogen introduced through glutamate and glutamine. Purine and pyrimidine nucleotides are assembled on a ribose phosphate scaffold, with purines built atom by atom onto the sugar and pyrimidines built as a ring and then attached. Fatty acid synthesis condenses acetyl and malonyl units on an acyl carrier protein, extending the chain two carbons at a time and reducing each intermediate with NADPH. Gluconeogenesis reverses most of glycolysis while bypassing its irreversible steps with distinct enzymes, a pattern typical of biosynthesis: the constructive route is rarely the simple reversal of the degradative one, which allows the two directions to be regulated independently. Reference maps of these routes, including the interconnections between them, are curated in the KEGG PATHWAY database as wiring diagrams covering carbohydrate, lipid, nucleotide, amino acid, and cofactor metabolism.

Secondary Metabolite Pathways

Secondary metabolites are compounds that are not required for growth but confer an ecological advantage, and their biosynthesis follows a small number of recurring architectures. Polyketide synthases and nonribosomal peptide synthetases operate as modular assembly lines in which each module adds one extender unit and optionally modifies it, so the product structure is encoded in the order and content of the modules. Terpenoids are built from five-carbon isoprene units supplied by either the mevalonate pathway or the methylerythritol phosphate pathway, then cyclized by terpene synthases into thousands of skeletons. Ribosomally synthesized and post-translationally modified peptides begin as ordinary gene products that tailoring enzymes then cyclize and decorate. In bacteria and fungi the genes for a given pathway usually sit together in a biosynthetic gene cluster, which makes them tractable to genome mining. Experimentally established pathways of both primary and secondary metabolism are collected in MetaCyc, a curated database holding several thousand pathways alongside their reactions, enzymes, and metabolites.

Regulation and Compartmentalization

Flux through a pathway is controlled chiefly at its committed step, the first reaction that leads irreversibly toward the end product. Allosteric feedback inhibition by that end product provides fast control on a timescale of seconds, while transcriptional repression and enzyme degradation adjust capacity over hours. Eukaryotic cells add spatial control by placing opposing pathways in different compartments, so that fatty acid synthesis in the cytosol does not run against fatty acid oxidation in the mitochondrion. Substrate channeling within multienzyme complexes further protects unstable intermediates from diffusing away.

Applications

Biosynthesis has applications in a range of fields, including:

• Metabolic engineering and synthetic biology for microbial production of drugs and chemicals
• Antibiotic and natural product discovery through genome mining
• Industrial fermentation of amino acids, vitamins, and organic acids
• Biofuel and bioplastic production, including polyhydroxyalkanoates
• Crop improvement targeting oil, starch, and defensive compound content
• Biocatalysis and enzyme engineering for chemical manufacturing