<P> The rate of a reaction is the concentration of substrate disappearing (or product produced) per unit time (mol L s). </P> <P> The% purity is 100% × (specific activity of enzyme sample / specific activity of pure enzyme). The impure sample has lower specific activity because some of the mass is not actually enzyme . If the specific activity of 100% pure enzyme is known, then an impure sample will have a lower specific activity, allowing purity to be calculated . </P> <P> All enzyme assays measure either the consumption of substrate or production of product over time . A large number of different methods of measuring the concentrations of substrates and products exist and many enzymes can be assayed in several different ways . Biochemists usually study enzyme - catalysed reactions using four types of experiments: </P> <Ul> <Li> Initial rate experiments . When an enzyme is mixed with a large excess of the substrate, the enzyme - substrate intermediate builds up in a fast initial transient . Then the reaction achieves a steady - state kinetics in which enzyme substrate intermediates remains approximately constant over time and the reaction rate changes relatively slowly . Rates are measured for a short period after the attainment of the quasi-steady state, typically by monitoring the accumulation of product with time . Because the measurements are carried out for a very short period and because of the large excess of substrate, the approximation that the amount of free substrate is approximately equal to the amount of the initial substrate can be made . The initial rate experiment is the simplest to perform and analyze, being relatively free from complications such as back - reaction and enzyme degradation . It is therefore by far the most commonly used type of experiment in enzyme kinetics . </Li> <Li> Progress curve experiments . In these experiments, the kinetic parameters are determined from expressions for the species concentrations as a function of time . The concentration of the substrate or product is recorded in time after the initial fast transient and for a sufficiently long period to allow the reaction to approach equilibrium . Progress curve experiments were widely used in the early period of enzyme kinetics, but are less common now . </Li> <Li> Transient kinetics experiments . In these experiments, reaction behaviour is tracked during the initial fast transient as the intermediate reaches the steady - state kinetics period . These experiments are more difficult to perform than either of the above two classes because they require specialist techniques (such as flash photolysis of caged compounds) or rapid mixing (such as stopped - flow, quenched flow or continuous flow). </Li> <Li> Relaxation experiments . In these experiments, an equilibrium mixture of enzyme, substrate and product is perturbed, for instance by a temperature, pressure or pH jump, and the return to equilibrium is monitored . The analysis of these experiments requires consideration of the fully reversible reaction . Moreover, relaxation experiments are relatively insensitive to mechanistic details and are thus not typically used for mechanism identification, although they can be under appropriate conditions . </Li> </Ul>

How can product concentration affect the activities of enzymes