Featured Article Summary #114

Enzyme kinetics studies how fast enzyme-catalysed reactions proceed and how changing reaction conditions affects that rate. By measuring reaction rates, scientists can learn an enzyme’s catalytic mechanism, its role in metabolism, how its activity is regulated, and how drugs or molecules that inhibit or activate it change the reaction speed. Enzymes work by binding a substrate to their active site, forming an enzyme–substrate complex, and then converting it through a series of steps to products before the enzyme is ready to start again. Even when multiple substrates are involved, kinetics can reveal the order of binding and product release, though usually one step is rate-determining and sets the overall speed—often a chemical event or a change in the shape of the enzyme or its substrates. Understanding the enzyme’s structure helps interpret kinetic results, including how binding and structural changes occur during the reaction and what specific amino acids do. Finally, not all biological catalysts are proteins: RNA catalysts such as ribozymes and ribosomes also drive important cellular processes, and although they are made of nucleotides rather than amino acids, their mechanisms and kinetics can still be analysed. Enzymes use the same reactants and produce the same products as the uncatalysed reaction and do not change the equilibrium between substrates and products. Instead, they show saturation kinetics: at low substrate levels the rate rises roughly in proportion to substrate concentration, but at high levels the enzyme active sites become full and the rate levels off at a maximum. KM is the substrate concentration where the reaction speed is half the maximum, reflecting how readily the enzyme saturates and what maximum rate it can reach. Enzyme assays are lab tests that measure how quickly an enzyme reaction proceeds by tracking changes in substrate or product concentration, using methods such as spectrophotometry (often continuous and convenient) or radiometric approaches (very sensitive but usually discontinuous). More advanced techniques can even watch single enzyme molecules using laser-based microscopy, revealing dynamics that differ from traditional measurements that average over large populations. In typical assays, product forms at an initial rate that is nearly linear for a short time, so experiments are often done after only a few percent of the reaction has finished, and progress-curve analysis can be used when full non-linear curves are more informative. The Standards for Reporting Enzymology Data provide required details for reporting kinetic and equilibrium results in a consistent, reliable way. Some enzymes work with only one substrate and use single-substrate mechanisms, such as isomerases and certain lyases, including triosephosphate isomerase and the hammerhead ribozyme. However, some single-substrate enzymes are not truly single-substrate in mechanism: catalase first reacts with one hydrogen peroxide molecule to form an intermediate, then the enzyme is regenerated using a second molecule of hydrogen peroxide, making it a ping–pong mechanism. Because enzyme-catalysed reactions saturate, the initial reaction rate rises with substrate concentration at first, but levels off at Vmax when the enzyme becomes fully occupied. In the Michaelis–Menten model, substrate first forms an enzyme–substrate complex (ES); after this, the rate depends on ES and behaves like a unimolecular, zero-order step at high substrate. The Michaelis–Menten equation links the initial rate v0 to the substrate-binding equilibrium and rate constant, and KM is the substrate concentration where the rate is half of Vmax (often related to the ES dissociation constant when substrate binding/dissociation is faster than the rate-determining step).