Enzyme_Kinetics_of_Beta galactosidase

Enzyme_Kinetics_of_Beta galactosidase - Biology 301...

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Biology 301, Exercise #2 Enzyme Kinetics of β -Galactosidase Introduction Enzymes as Catalysts . Enzymes are a subgroup of proteins which catalyze biochemical reactions. Without enzymes most necessary chemical reactions in our bodies would be unable to proceed. An enzyme can increase the rate of a reaction but it cannot change the equilibrium of the reaction. For example, if substrate S is converted to product P and the equilibrium constant , K eq , for this reaction is equal to 100, then the concentration of P at equilibrium is 100 times greater than that of S. However, it may take a long time to reach this equilibrium in the absence of an enzyme. An enzyme increases the rate of the reaction by decreasing the activation energy (E a ). Without an enzyme, a graph of free energy versus progress of the reaction is as shown in Figure A (see below). A large input of energy is necessary to make the reaction proceed. However, when an enzyme is present, it lowers the energy of activation so that the reaction can proceed with a much smaller amount of energy (Figure B). Enzyme Reaction . The simplest reaction that can be catalyzed by an enzyme is one in which a single substrate is converted to a single product. In this reaction, the substrate binds to the active site of the enzyme, a relatively small three dimensional area that is specific for the substrate, to form an enzyme-substrate complex . The "binding" is usually by weak bonds such as hydrogen bonds, electrostatic or hydrophobic interactions, or van der Waals forces. The enzyme then converts the substrate to product, which is released to give free enzyme again. This reaction is described by the following equation in which E, S and P represent enzyme, substrate and product, respectively, and the terms k 1 , k 2 , and k 3 are the rate constants for the indicated reactions: k 1 k 3 E + S ES E + P k 2 Enzyme Kinetics . The rate of catalysis or velocity (v), increases as the substrate concentration is increased. However, the velocity eventually reaches a maximum (V max ) 1
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when all the available active sites are filled, and the enzyme is said to be saturated . If more enzyme is added, a larger number of active sites are available for catalysis and the V max increases in proportion to the enzyme added. K m is a measure of the strength of binding of the substrate to the active site and can be defined in two ways. First, it is the substrate concentration at which half of the active sites are filled. Since V max occurs when all the sites are filled, K m is the substrate concentration ([S]) at which V is equal to one half of V max (see below). The second way to define K m is by the rate constants of the individual steps: K m = (k 2 + k 3 ) / k 1 . One application of this occurs when k
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This note was uploaded on 04/08/2008 for the course BIOL 301 taught by Professor Tepperman during the Winter '08 term at University of Cincinnati.

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Enzyme_Kinetics_of_Beta galactosidase - Biology 301...

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