Dear Colleagues,

Enzymes are key metabolic elements of all living systems. Even primitive single cellular organisms have a large number of catalytically active biomolecules that are important for the metabolic homeostasis of these systems. Enzymes function as biocatalysts and convert specific substrates to corresponding reaction products. Although they speed up chemical reactions by lowering the activation energy, they do not alter the chemical equilibrium of the catalyzed reaction. By doing this, some enzymes allow a chemical reaction that would normally require millions of years in the absence of enzyme to occur in just milliseconds. According to the recommendations of the International Union of Biochemistry and Molecular Biology, enzymes can be classified into seven major groups, with each of these being further classified into subgroups. Oxidoreductases catalyze electron-transfer reactions. Transferases transfer functional groups from a donor to an acceptor. Hydrolases split chemical bonds using water as a second substrate. Lyases cleave chemical bonds by mechanisms other than hydrolysis or oxidation. Isomerases change the structure of substrates without altering their elementary composition. Ligases join two substrate molecules via the formation of covalent bonds. Translocases catalyze the movement of molecules or ions across biomembranes.

Catalysis in mono-substrate reactions is initiated when an enzyme (E) binds its specific substrate (S) to form the enzyme-substrate complex (ES). This complex is then converted to the enzyme-product complex (EP) via the intermediate formation of a transition state (ES*). The enzyme-product complex (EP) subsequently dissociates, thereby releasing the product from the enzyme’s active site. The enzyme is then ready to bind another substrate molecule and thus initiate another round of substrate turnover. Each of the described elementary reactions is characterized by a pair of kinetic constants (on-constant, off-constant) that may become rate-limiting under different experimental conditions.

E + S ⇄ ES ⇄ ES* ⇄ EP ⇄ E + P

Multi-substrate reactions follow other kinetic mechanisms in which it is important how and where the different substrates bind, and in which order the substrate binding occurs. For a two-substrate reaction for example, substrate A first binds to its binding site to form the enzyme-substrate complex (EA). Substrate B then binds to its specific binding site to form the ternary enzyme substrate complex (EAB). Alternatively, substrate B may first bind to its binding site and the subsequent binding of substrate A completes formation of the ternary complex (EAB). Although the enzyme-substrate complexes are structurally similar, the kinetics of their formation may be different. A third option is that the two substrates bind simultaneously to form a related ternary enzyme-substrate complex, but here again the kinetics may be different. In-depth kinetic analyses of such complex enzymatic reactions are challenging and may only be possible when the concentration of one substrate is kept constant.

This special issue aims to publish advanced reports on the kinetic characterization of enzymes and related biocatalysts. These should include detailed in vitro activity assays of purified native or recombinant enzymes, as well as state-of-the-art kinetic modeling. The resulting kinetic constants should be placed in the context of the metabolic network of the corresponding cells, thus allowing a better understanding of the regulatory principles of cellular metabolism.

Prof. Dr. Hartmut Kuhn
Guest Editor