Summary

Biosynthesis and synthetic biology explore the enzymatic formation of biological molecules and the design, engineering, and reconstruction of biological systems. This collection covers biosynthetic pathways, enzyme-mediated reactions, metabolic engineering, genetic circuits, engineered microorganisms, cellular factories, genome editing, and the production of valuable compounds through programmable biological processes.

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Polyketide Biosynthesis
Polyketide biosynthesis is the enzymatic assembly of carbon skeletons from short-chain acyl precursors through repeated carbon–carbon bond formation [1]. Polyketide synthases select starter and extender units and catalyze decarboxylative condensations; optional processing of the resulting intermediates establishes their oxidation states and substitution patterns [2]. Modular assembly lines can terminate chains through cyclization, while additional tailoring enzymes modify the initially assembled scaffold through oxidation, ring formation, or other reactions [3]. Carrier-protein domains tether intermediates through a phosphopantetheine-linked thioester and deliver them to successive catalytic domains. Domain recognition and movement coordinate extension and modification within the enzyme system. The order of these reactions, together with precursor selection and chain-release chemistry, specifies the structural framework of the resulting polyketide [4][5].
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  • 25 Oct 2026
Topic Review
Polyamine Biosynthesis
Polyamine biosynthesis is the enzymatic production of small organic polycations containing multiple amino groups. The pathway forms putrescine and converts it into higher polyamines through successive aminopropyl-transfer reactions. Spermidine and spermine arise from addition of aminopropyl groups supplied by decarboxylated S-adenosylmethionine [1][2]. Biosynthetic enzyme expression, substrate supply, and feedback regulation are coordinated with polyamine degradation, uptake, and export to maintain intracellular pools [3]. Ornithine decarboxylase produces putrescine in a major biosynthetic route and is subject to targeted degradation mediated by antizyme [4]. The supply of ornithine and activated aminopropyl donors connects successive steps to broader amino acid and methionine metabolism. The resulting network connects amino acid metabolism with the synthesis and interconversion of putrescine, spermidine, and related polyamines [5].
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  • 25 Oct 2026
Topic Review
Purine Biosynthesis
Purine biosynthesis is the enzymatic formation of purine nucleotides through sequential construction of the purine ring on an activated ribose framework. The de novo pathway uses phosphoribosyl pyrophosphate together with amino-acid-derived atoms, carbon dioxide, and folate-associated one-carbon units to produce inosine monophosphate [1]. Physical association among pathway enzymes can facilitate transfer of intermediates between successive reactions [2]. In mammalian cells, dynamic assemblies called purinosomes organize enzymes of this pathway and associate with cellular sources of required metabolites [3]. Inosine monophosphate branches toward adenine and guanine nucleotides, with additional reactions establishing their phosphorylation states. Feedback inhibition, enzyme expression, and precursor supply coordinate purine production with cellular demand [4]. 
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  • 25 Oct 2026
Topic Review
Nucleotide Biosynthesis
Nucleotide biosynthesis is the enzymatic production of nucleotides from metabolic precursors. De novo pathways combine sources of carbon and nitrogen with activated ribose, consuming energy to construct the purine or pyrimidine nucleotide framework [1]. Pyrimidine synthesis forms the ring through successive reactions before generating uridine monophosphate, which supplies further pyrimidine nucleotide conversions [2]. In bacteria, the resulting nucleotide pools support nucleic-acid synthesis, energy transfer, and signaling, linking biosynthetic activity to broader cellular metabolism [3]. Regulation of phosphoribosyl pyrophosphate synthesis links activated ribose supply with nucleotide production and cellular biosynthetic demand [4]. Subsequent phosphorylation produces higher phosphate forms, and ribonucleotide reduction supplies deoxyribonucleotide precursors. Salvage reactions incorporating preformed bases or nucleosides also contribute to the pools that are maintained alongside de novo synthesis [5].
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  • 25 Oct 2026
Topic Review
Antibiotic Biosynthesis
Antibiotic biosynthesis is the enzyme-mediated formation of biologically produced compounds with antibacterial activity. In producing microorganisms, regulatory networks coordinate expression of biosynthetic enzymes with signals associated with growth and specialized metabolism [1]. Biosynthetic routes construct diverse molecular frameworks through reactions that assemble precursors, introduce functional groups, and modify intermediates [2]. Peptide-derived antibiotics can undergo extensive maturation, including cyclization and chemical modification of an initially synthesized precursor [3]. Genes encoding pathway enzymes, transport functions, and regulatory components are often organized in biosynthetic gene clusters. Some clusters remain weakly expressed or inactive under particular culture conditions, while changes in regulation can permit production of their encoded metabolites. The final molecular structure reflects the sequence and substrate specificity of the pathway's assembly and tailoring reactions [4].
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  • 25 Oct 2026
Topic Review
Amino Acid Biosynthesis
Amino acid biosynthesis is the enzymatic formation of amino acids from carbon skeletons and assimilated nitrogen. Interconnected pathways use intermediates of central metabolism and distribute amino groups or amide nitrogen among successive reactions [1]. Aromatic amino acid synthesis proceeds through the shikimate pathway to chorismate, which branches toward phenylalanine, tyrosine, and tryptophan [2]. Glutamine synthetase and glutamate synthase connect nitrogen assimilation to the production of amino acid pools [3]. In plastids, protective enzymes remove reactive intermediates that can otherwise damage enzymes of branched-chain amino acid synthesis. Feedback inhibition and regulation of enzyme expression coordinate amino acid production with cellular consumption. Transport between compartments connects spatially separated reactions, while shared precursors couple the pathways to the broader carbon and nitrogen metabolic network [4][5].
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  • 25 Oct 2026
Topic Review
Ethanol Fermentation
Ethanol fermentation is the metabolic conversion of organic substrates into ethanol through reactions coupled to cellular energy generation and redox balance. In yeast, sugar catabolism supplies pyruvate, which is converted through acetaldehyde to ethanol [1]. Reduction of acetaldehyde reoxidizes NADH to NAD+, sustaining continued glycolytic turnover and its associated substrate-level phosphorylation [2]. The sugars entering this process must be accessible to the organism's uptake and metabolic pathways; complex polysaccharides require prior hydrolysis, and pentose utilization requires the appropriate enzymatic routes [3]. Fermentative carbon flow can coexist with respiratory metabolism, and high glycolytic flux can promote ethanol formation in Crabtree-positive yeasts. The process therefore links carbon conversion, nucleotide-cofactor recycling, and allocation of substrate between growth and fermentation products [4][5].
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  • 26 Oct 2026
Topic Review
Immobilized Cell Bioreactor
An immobilized cell bioreactor is a reaction vessel containing cells retained through attachment, entrapment, aggregation, or physical confinement. The retained cells remain metabolically active and can show physiological states associated with their immobilized organization [1]. A carrier supplies the physical structure for retention, and its arrangement affects contact between cells and the surrounding liquid [2]. Gel entrapment confines cells within a hydrated polymer network through which dissolved substrates and products can move [3]. Viable immobilized cells sustain intracellular enzyme systems while exchanging nutrients and metabolites with the medium. Fluid flow and diffusion connect the bulk liquid to the retained biomass, producing local concentration gradients when reaction outpaces transport. The reactor therefore contains interacting cellular, support, and liquid phases throughout the biochemical conversion [4].
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  • 25 Oct 2026
Topic Review
Anaerobic Bioreactors
Anaerobic bioreactors are engineered vessels that maintain biological transformations in an environment without supplied molecular oxygen. Their microbial populations convert organic material through anaerobic pathways, including digestion processes that produce methane from biomass [1]. Accumulating compounds such as ammonia, sulfide, or organic inhibitors can alter the activity of the participating microorganisms [2]. Biomass can remain suspended or associate with particles and surfaces, creating spatially organized communities in which intermediate products pass between metabolic groups [3]. Reactor processes can support methanogenesis, hydrogen production, or formation of soluble fermentation products, according to the organisms and operating conditions. Feed delivery, mixing, gas removal, and biomass retention connect the reactor's physical organization to substrate conversion and the maintenance of its anaerobic community [4][5].
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  • 25 Oct 2026
Topic Review
Dark Fermentation
Dark fermentation is microbial conversion of organic substrates into hydrogen and other fermentation products without a requirement for light energy. Anaerobic substrate breakdown generates reducing equivalents that can be transferred to hydrogen-producing enzymes [1]. The culture may contain multiple microbial populations whose metabolic activities distribute carbon and electrons among hydrogen, organic acids, and other products [2]. Complex feedstocks must first yield compounds accessible to the fermenting organisms, connecting substrate hydrolysis to downstream hydrogen formation [3]. Competing metabolic branches consume the same intermediates and reducing equivalents, so changes in pathway flux alter the product distribution [4]. Hydrogen leaves the liquid phase through gas transfer, while soluble fermentation products remain in the medium. These linked biological and transport processes establish the evolving composition of the fermenting culture [5].
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