Enzyme kinetics is the study of the chemical reactions that are catalysed by enzymes. In enzyme kinetics, the rate is measured and the effects of varying the conditions of the reaction are investigated. Enzymes are usually protein molecules that manipulate other molecules — the enzymes substrates, kinetic studies on enzymes that only bind one substrate, such as triosephosphate isomerase, aim to measure the affinity with which the enzyme binds this substrate and the turnover rate. Some other examples of enzymes are phosphofructokinase and hexokinase, both of which are important for cellular respiration, when enzymes bind multiple substrates, such as dihydrofolate reductase, enzyme kinetics can also show the sequence in which these substrates bind and the sequence in which products are released. An example of enzymes that bind a single substrate and release multiple products are proteases, others join two substrates together, such as DNA polymerase linking a nucleotide to DNA. Although these mechanisms are often a series of steps, there is typically one rate-determining step that determines the overall kinetics. This rate-determining step may be a reaction or a conformational change of the enzyme or substrates. Knowledge of the structure is helpful in interpreting kinetic data. For example, the structure can suggest how substrates and products bind during catalysis, what changes occur during the reaction, not all biological catalysts are protein enzymes, RNA-based catalysts such as ribozymes and ribosomes are essential to many cellular functions, such as RNA splicing and translation. The main difference between ribozymes and enzymes is that RNA catalysts are composed of nucleotides, whereas enzymes are composed of amino acids, ribozymes also perform a more limited set of reactions, although their reaction mechanisms and kinetics can be analysed and classified by the same methods. The reaction catalysed by an enzyme uses exactly the same reactants, like other catalysts, enzymes do not alter the position of equilibrium between substrates and products. However, unlike uncatalysed chemical reactions, enzyme-catalysed reactions display saturation kinetics, the substrate concentration midway between these two limiting cases is denoted by KM. The two most important kinetic properties of an enzyme are how quickly the enzyme becomes saturated with a substrate. Knowing these properties suggests what an enzyme might do in the cell, Enzyme assays are laboratory procedures that measure the rate of enzyme reactions. Because enzymes are not consumed by the reactions they catalyse, enzyme assays usually follow changes in the concentration of substrates or products to measure the rate of reaction. There are many methods of measurement, spectrophotometric assays are most convenient since they allow the rate of the reaction to be measured continuously. Although radiometric assays require the removal and counting of samples they are extremely sensitive. An analogous approach is to use mass spectrometry to monitor the incorporation or release of stable isotopes as substrate is converted into product, the most sensitive enzyme assays use lasers focused through a microscope to observe changes in single enzyme molecules as they catalyse their reactions
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As larger amounts of substrate are added to a reaction, the available enzyme binding sites become filled to the limit of <math>V_\max</math>. Beyond this limit the enzyme is saturated with substrate and the reaction rate ceases to increase.
Progress curve for an enzyme reaction. The slope in the initial rate period is the initial rate of reaction v. The Michaelis–Menten equation describes how this slope varies with the concentration of substrate.