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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Oxygenase Enzymes
Oxygenase enzymes constitute a class of oxidoreductases that catalyze the direct incorporation of one or both atoms of molecular oxygen (O₂) into organic substrates, thereby oxidizing the substrate through a bona fide oxygen-transfer mech[1]anism rather than through electron or hydrogen abstraction [2]. In the Enzyme Commission nomenclature, oxygenases are assigned to EC 1.13 (acting on single donors with incorporation of molecular oxygen) and EC 1.14 (acting on paired donors with incorporation or reduction of molecular oxygen) [3]. The class is partitioned into two functional categories according to oxygen stoichiometry: monooxygenases incorporate one atom of O₂ into the substrate while reducing the second oxygen atom to water, whereas dioxygenases incorporate both atoms of molecular oxygen into the substrate . Catalysis by oxygenases generally requires reductive activation of O₂ at a redox-active metal center or organic cofactor, with heme iron, flavin, mononuclear non-heme iron, Rieske iron–sulfur clusters, and Fe(II)/2-oxoglutarate centers representing the principal cofactor systems that support substrate oxygenation [4][5]. The essential components of an oxygenase-catalyzed reaction are therefore the catalytic cofactor or metal center, the organic substrate that accepts the oxygen atom(s), and, for many members, an electron-delivery system that sustains the reduced cofactor state required for O₂ activation [3][4].
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Topic Review
Biocatalysis
Biocatalysis is the use of biological catalysts—enzymes in isolated form or catalytic systems within intact cells—to convert chemical substrates into products. The three-dimensional architecture of the catalyst organizes substrate recognition and the chemical steps of bond making and breaking, giving rise to chemo-, regio-, and stereoselectivity that are characteristic of biological catalysis [1]. The catalyst may operate in a soluble state or be confined within a solid support or matrix through adsorption, encapsulation, or cross-linking, which determines its spatial distribution, substrate accessibility, and recoverability within the reaction system [2]. Biocatalytic conversions can proceed in water or in organic-containing media, provided the catalyst retains sufficient activity and stability under those conditions [3]. The accessible reaction type follows from the catalytic machinery of the biocatalyst, so that different enzyme classes support distinct bond-forming or bond-cleaving transformations through their active-site chemistry [4]. Multiple biocatalytic steps can be connected, together with auxiliary reactions such as cofactor regeneration, into an overall biochemical transformation [5]. 
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Topic Review
Enzyme Catalysis
Enzyme catalysis is the acceleration of a chemical reaction by an enzyme through interactions that reduce the free-energy barrier to reaction. The active-site environment positions reacting groups and provides electrostatic stabilization of configurations along the reaction pathway. Networks of interacting residues and protein motions can couple substrate binding to changes in the catalytic region [1][2]. Catalytic turnover comprises substrate association, chemical conversion, and product release. Transition-state stabilization, proton or electron transfer, and quantum-mechanical contributions can have different roles in particular reactions [3]. The spatial arrangement of the active site also constrains which substrates and product configurations are accessible. Changes in enzyme sequence or structure can therefore alter catalytic activity and selectivity, and multiple enzyme-catalyzed steps can be connected into a reaction pathway [4].
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Topic Review
Carbohydrate-Active Enzymes
Carbohydrate-active enzymes are enzymes that synthesize, cleave, or chemically modify carbohydrates and carbohydrate-containing molecules. Their substrate recognition accommodates the diversity of sugar residues, linkages, and branching patterns found in complex glycans [1]. Major catalytic groups include glycoside hydrolases, glycosyltransferases, polysaccharide lyases, carbohydrate esterases, and enzymes with auxiliary oxidative activities [2]. Lytic polysaccharide monooxygenases use a surface-accessible copper center to catalyze oxidative cleavage of polysaccharide chains, complementing hydrolytic reactions [3][4]. Glycoside hydrolases and glycosyltransferases organize substrates within active sites that position the reacting bonds and stabilize intermediates or transition states. Distortion of a bound sugar ring can form part of the catalytic mechanism. Individual enzymes combine these chemical activities with binding regions that recognize particular carbohydrate structures [5][6].
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Topic Review
Biotin-Dependent Enzymes
Biotin-dependent enzymes are enzymes that use covalently attached biotin to transfer a carboxyl group during catalysis. The biotin cofactor is linked to a lysine residue within a carrier region of the protein and moves between distinct catalytic sites [1]. In biotin-dependent carboxylases, one site uses ATP and bicarbonate to carboxylate biotin, and another transfers the carboxyl group to an organic acceptor [2]. Domain movements carry this activated intermediate between the sites; in multimeric enzymes, the participating sites can lie on different polypeptide chains [3][4]. The holoenzyme can distribute biotin carboxylase, carrier, and carboxyltransferase functions among separate subunits. Their assembly positions the mobile biotin-bearing domain for successive reactions and links cofactor movement to substrate-specific carbon–carbon bond formation [5].
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Topic Review
Recombinant Protein Tagging
Recombinant protein tagging is the genetic fusion of a peptide or protein sequence to a target protein, producing a polypeptide with an additional recognition or functional element. Tags can provide binding sites and can alter the solubility, yield, or folding of the fusion protein [1]. An affinity tag interacts with a corresponding ligand, allowing the tagged product to be selectively captured from a mixture [2]. Tag sequences can also encode epitopes, localization reporters, or elements that recruit cellular activities; their placement must preserve the reading frame of the target coding sequence [3]. In split-fluorescent-protein systems, a short fused fragment associates with a complementary fragment to assemble a fluorescent complex, linking detection to the presence of the tagged protein [4].
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Topic Review
Recombinant Enzymes
Recombinant enzymes are catalytically active proteins produced through expression of engineered DNA that encodes the enzyme in a biological host. The coding sequence is incorporated into an expression construct whose regulatory elements support transcription and translation in the selected organism. The encoded protein may retain a naturally occurring sequence or contain deliberately introduced sequence changes [1]. Formation of an active recombinant enzyme can require folding, assembly of subunits, incorporation of cofactors, proteolytic processing, or other post-translational modifications [2][3]. These requirements interact with the biochemical capabilities of the expression host [2]. The enzyme may accumulate within cells or enter a secretion pathway, and its mature molecular form depends on both the encoded sequence and host-mediated processing [4]. The term identifies the recombinant origin of the enzyme and does not specify a single catalytic reaction, protein family, or expression organism [1][2].
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Topic Review
Recombinant Expression Systems
A recombinant expression system comprises a biological host and a set of engineered genetic elements that together direct the synthesis of a specified gene product. Its functional architecture integrates a coding sequence with transcriptional and translational control elements—including promoters, ribosome binding sites, untranslated regions, and terminators—that operate within the host's molecular machinery to govern when and to what extent the target polypeptide is produced [1][2]. The expression construct may be maintained episomally on a plasmid or integrated into the host genome, and its regulatory configuration, together with host physiology, determines the efficiency of transcript production, translation, folding, and any post-translational processing the host can perform [3]. The system may further incorporate auxiliary components such as signal peptides for secretion, molecular chaperones for folding support, or genetic modifications that enhance protein maturation [4]. The operational integrity of a recombinant expression system depends on the functional compatibility between the engineered construct and the chosen host, and on the coordinated sequence of molecular events that convert the introduced genetic information into the molecular state of the final product [5].
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Topic Review
Shikimate Pathway
The shikimate pathway is a biosynthetic sequence that converts phosphoenolpyruvate and erythrose 4-phosphate into chorismate, a branching precursor for aromatic amino acids and additional aromatic metabolites. It operates in plants, fungi, bacteria, and certain other organisms, linking central carbon metabolism to the formation of aromatic structures [1][2]. The canonical pathway comprises seven enzymatic reactions, beginning with the condensation of its two carbon substrates and proceeding through dehydroquinate, dehydroshikimate, shikimate, shikimate 3-phosphate, and 5-enolpyruvylshikimate 3-phosphate before chorismate formation [3][4]. Its reactions include carbon–carbon bond formation, ring formation, dehydration, reduction, phosphorylation, and elimination. Individual catalytic activities may occur in separate proteins or within multifunctional proteins, depending on the organism. Carbon supply, cofactor availability, and feedback regulation connect pathway activity to the metabolic demand for its products and downstream branches [1][3].
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Topic Review
Non-Mevalonate Pathway
The non-mevalonate pathway is a sequence of enzyme-catalyzed reactions that forms the five-carbon isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate from pyruvate and glyceraldehyde 3-phosphate. It is also termed the methylerythritol phosphate pathway and occurs in many bacteria and in the plastids of plants and several other eukaryotic lineages [1]. The pathway begins with formation of 1-deoxy-D-xylulose 5-phosphate, followed by reductive rearrangement to 2-C-methyl-D-erythritol 4-phosphate. Subsequent cytidylylation, phosphorylation, cyclization, and reductive transformations generate the terminal diphosphate products [2]. Its final two enzymes contain iron–sulfur clusters and require electron transfer for catalysis [3]. Pathway flux depends on precursor supply, enzyme activity, energy availability, and reducing equivalents [4]. The resulting activated five-carbon units participate in downstream chain elongation and the formation of structurally diverse isoprenoid molecules [1][2].
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