Summary

Molecular biology and gene expression regulation examine the molecular organization, function, and interactions of DNA, RNA, and proteins. This collection covers the mechanisms through which genetic information is replicated, transcribed, processed, translated, and regulated, including transcriptional control, RNA metabolism, post-transcriptional regulation, protein synthesis, and molecular signaling.

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Heme-Thiolate Enzyme
A heme-thiolate enzyme is an enzyme whose heme iron has an axial sulfur ligand supplied by a cysteine thiolate in the protein. This coordination arrangement contributes to the oxygen-activation chemistry of cytochrome P450 enzymes [1]. A high-valent iron–oxo species, compound I, can cleave substrate carbon–hydrogen bonds during the catalytic cycle [2]. Thiolate donation alters the electronic properties of iron-bound oxygen intermediates and helps support the highly reactive species used in substrate oxidation [3]. In many members, successive reduction and protonation steps lead to oxygen–oxygen bond cleavage and formation of a high-valent iron–oxo intermediate. Reaction with the bound substrate then transfers oxygen or removes hydrogen equivalents, with the surrounding protein controlling substrate orientation and access [4].
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Topic Review
Heme-Containing Enzymes
Heme-containing enzymes are enzymes that incorporate an iron–porphyrin cofactor within their catalytic machinery. Changes in iron oxidation state and coordination permit formation of reactive intermediates, including iron–oxygen species involved in substrate oxidation [1][2]. Heme-containing catalytic systems also participate in anaerobic nitrogen transformations, where c-type heme proteins mediate electron transfer and reactions involving nitrogenous substrates [3]. Binding of nitric oxide, oxygen-derived ligands, or other small molecules alters the electronic and geometric properties of the iron center [4]. The protein supplies axial ligands, substrate-binding regions, and a local environment that organizes these reactions. Heme association can also couple catalytic activity to regulatory changes in the protein [5]. 
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Topic Review
Cofactors
Cofactors are chemical components that associate with proteins and supply properties required for particular enzyme activities. Inorganic iron–sulfur clusters participate in catalysis and electron transfer; cellular assembly machinery forms them on scaffold proteins before insertion into recipient proteins [1][2]. Metal–organic cofactors combine a metal center with an organic ligand, as in molybdenum bound within a pterin-containing catalytic group [3]. Cofactor synthesis itself can require specialized enzymatic reactions, including radical chemistry mediated by iron–sulfur proteins and S-adenosylmethionine [4]. Organic and organometallic cofactors can transfer chemical groups or generate reactive intermediates. Vitamin B12 derivatives, for instance, support methyl-group transfer or radical-mediated rearrangement through distinct chemical forms of the cobalt-containing cofactor. Protein binding positions these components relative to substrates and other catalytic groups [5][6].
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Topic Review
Carboxylating Enzymes
Carboxylating enzymes catalyze reactions that incorporate inorganic carbon into organic substrates as a carboxyl group. They participate in biosynthetic pathways and carbon-fixation cycles with distinct substrate and energy requirements [1]. Biotin-dependent carboxylases couple ATP-driven activation of bicarbonate to carboxylation of enzyme-bound biotin, followed by transfer of the carboxyl group to an acceptor substrate [2]. Ribulose-1,5-bisphosphate carboxylase/oxygenase fixes carbon dioxide through a different active-site reaction. In bacterial carboxysomes, it is enclosed with carbonic anhydrase in a protein shell that elevates carbon dioxide availability near the enzyme [3]. Carboxylation systems thus combine an organic acceptor-binding site with machinery for inorganic-carbon utilization. Mobile cofactor-bearing regions can connect separate catalytic sites within a single carboxylase assembly [4].
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Topic Review
Apoenzyme
An apoenzyme is the protein component of a cofactor-requiring enzyme before the required cofactor has been incorporated or after it has been removed. Cofactors such as iron–sulfur clusters, heme, and metal ions supply chemical functions within the assembled enzyme [1]. For complex iron–sulfur enzymes, maturation machinery builds the cofactor on an assembly scaffold and transfers it to a recipient protein lacking that component [2]. Iron–sulfur cluster maturation incorporates an assembled inorganic cluster into a recipient apoprotein through interactions with dedicated biogenesis components [3][4]. Assembly proteins and intracellular trafficking pathways coordinate this process for iron–sulfur proteins. The apoenzyme retains the polypeptide framework that accommodates the missing component, while full catalytic function requires completion of the corresponding active-site assembly [4][5].
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Topic Review
Enzyme Stability
Enzyme stability is the persistence of an enzyme's catalytically competent structure and activity under stated conditions over time. Immobilization can preserve activity by restricting conformational changes, preventing dissociation of subunits, or creating a favorable local environment [1]. Structural stability includes the energetic resistance of the folded protein to unfolding, while kinetic stability concerns the rate of irreversible inactivation. Melting temperature and activity half-life describe different aspects of these properties [2]. Amino acid substitutions can modify packing, interactions among residues, or conformational flexibility and thereby change stability [3]. The support and surrounding solution also participate in stabilization of immobilized preparations. In adsorbed lipases, solvent or detergent exposure can cause desorption, and unfolding can further reduce catalytic activity during operation [4].
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Topic Review
Enzyme Adsorption
Enzyme adsorption is the accumulation of enzyme molecules at a material surface through noncovalent interactions between the protein and the interface. Adsorbed enzymes form a carrier-associated catalytic preparation in which the support retains protein during reaction and separation [1][2]. Surface area, pore architecture, and protein–surface contacts determine how enzyme molecules occupy the material and which portions remain exposed to solution [3]. Hydrophobic supports can bind lipases through interfacial activation, stabilizing a surface-associated conformation with an accessible catalytic region. Changes in solvent composition or the presence of detergents can release the adsorbed enzyme [4]. Adsorption therefore produces a reversible association. Enzyme loading, the proportion of activity retained, and accessibility of substrates describe distinct properties of the resulting immobilized preparation [5].
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Topic Review
Enzyme Inhibition
Enzyme inhibition is the reduction of an enzyme-catalyzed reaction rate through an interaction that restricts substrate binding, catalytic turnover, or the availability of active enzyme. Reversible inhibitors associate with particular states of the catalytic cycle, producing kinetic patterns related to their binding relationships with enzyme and substrate [1]. Covalent inhibitors form a chemical bond with the target protein and can produce persistent inhibition after free inhibitor has been removed [2]. Inhibition can be examined by measuring product formation or by testing competition with a probe that labels an active enzyme. Such competition measurements also reveal inhibition across enzyme populations in a biological sample [3][4]. The chemical reactivity and recognition properties of a covalent inhibitor govern which residues and target proteins it modifies [5].
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Topic Review
Pan-Genome
A pan-genome is a representation of the combined genomic diversity within a specified group of organisms. Sequence-based representations can integrate multiple genome assemblies and retain alternative versions of genomic regions, including structural variants and sequences absent from an individual reference [1]. Phased assemblies can further preserve the separate chromosome copies contributed by each sampled individual [2]. Gene-based representations group corresponding sequences into gene families and record their distribution across the sampled genomes. Computational representations include collections of sequences and graphs encoding alternative sequence paths [3]. Core genes occur throughout the sampled collection, whereas variable genes occur in only some members. In plants, this variable component includes gene presence–absence differences among accessions. The sampled genomes and criteria for grouping sequences must accompany interpretation of the reported repertoire [4][5].
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Topic Review
Plasmid Curing
Plasmid curing is the loss of a resident plasmid from a microbial cell and the generation of descendants lacking that plasmid. Loss can arise through interference with plasmid replication or segregation, competition between incompatible plasmids, or targeted destruction of plasmid DNA [1][2][. Plasmid-encoded maintenance systems can oppose this process. Toxin–antitoxin systems, for instance, suppress the survival or growth of plasmid-free segregants after loss of the genetic element [3]. In these systems, disappearance of a relatively unstable antitoxin leaves toxin activity insufficiently neutralized, disrupting cellular processes [4]. At population level, the abundance of plasmid-free cells also reflects their relative growth and the reacquisition of plasmids through horizontal transfer. These inheritance, survival, and transmission processes distinguish initial plasmid loss from its persistence across subsequent generations [5].
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