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The rate of a chemical reaction can be measured by measuring the change in the disappearance of the reactants or the appearance of the products per unit time (LeMay 2000). The rate of a reaction can be increased through the use of a catalyst. The activation energy for a reaction, the initial input of energy to start a reaction, can be lowered by a catalyst, which is a substance that speeds up the rate of a reaction. In biological systems, a catalyst is a protein called an enzyme. The enzyme has a specific attraction for certain molecules called the substrate which bind to the enzyme in a particular area called the active site. An enzyme is unique from a reactant in that it is not used up in a reaction. In fact, once the a reaction is catalyzed by an enzyme, that enzyme can catalyze another reaction until there are no more reactants. In order for a reaction to occur, the activation energy threshold must be overcome and the reactants, or substrate, must have the appropriate orientation. Enzymes lower the activation energy for a reaction because they help to orient the substrate correctly and can increase the probability of contact between substrate molecules by bringing them closer together. (LeMay 2000) Enzymes are specific for a particular substrate so only the enzyme will increase the rate of a specific reaction for a specific substrate (Campbell 1996). Lactase, for example, is an enzyme that catalyzes the breakdown of lactose into glucose and galactose, two smaller molecules. In order to measure the maximum rate of reaction, vmax, a plot of the initial velocities, vo, is plotted against the substrate concentration, [S]. The horizontal asymptote is the maximum rate of the reaction, vmax, which is the rate at which all of the enzymes contain substrate and are converting them to product. Therefore, increasing [S] at vmax will have no impact on the rate. The point at which the velocity is equal to 1/2 is called the Michaelis-Menten constant, Km, for that reaction. The higher the Km, the affinity for the enzyme and the substrate is lower compared to a lower Km value that would indicate that the affinity of the enzyme and the substrate is higher. Since the value for vmax can only be approached on the graph, but never reached, and Km is estimated from the graph, the exact values for vmax and Km cannot be determined from this graph. If the Michaelis-Menten equation vo= vmax[S]/(Km + [S]), is rearranged to 1/vo = (Km/vmax)(1/[S]) + 1/vmax which is analogous to y=mx + b. If 1/ vo is plotted against 1/[S], then the y-intercept is 1/ vmax and the x-intercept is 1/ Km. This plot is called a Lineweaver-Burk plot and can be used to exactly determine the value of vmax and Km. (Horton 1996)
Campbell, Neil A. Biology. 4th Edition. The Benjamin/Cummings
Publishing Co., 1996, Horton, Robert. Principles of Biochemistry. 2nd Edition. Prentice Hall, 1996, 123-127. LeMay, Eugene et al. Chemistry: Connections to Our Changing
World. 2nd Edition. |
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