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Cytology

           Induced fit theory
           The induced-fit theory assumes that the substrate plays a role in determining the final
           shape of the enzyme; thus, the enzyme is partially flexible (Figure 1.44). This explains
           why certain compounds can bind to the enzyme but do not react because the enzyme
           has been distorted too much. Other molecules may be too small to induce the proper
           alignment. Therefore, they cannot react, since only the specific substrate is capable of
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           inducing the proper alignment of the active site.

                             Enzyme


                                            Substrate entering the active
                                                site of enzyme

                  Free  enzyme    +   Substrate
                        E           S









               Enzyme-substrate complex   Enzyme-products complex   Free enzyme   +  Products
                        ES                       EP                  E                P


                                   Figure 1.44: Model of the induced fit theory
           Factors governing rate of enzyme           This point  is called  enzyme  saturation
           activity                                   point. Thus, extra substrates have to wait
           Several  factors  affect  enzyme  activities.  for the enzyme to release the product and
           These include substrate concentration,  become  free to accommodate  another
           enzyme concentration,  temperature,  and  substrate molecule (Figure 1.45).
           pH level.
                                                               Maximum reaction rate
           (a) Substrate concentration
           At  fixed  concentration  of  enzymes,  an
           increase  in  substrate  concentration  will                        Rate approaching
                                                                               maximum rate
           increase  the  rate  of reaction.  This is
           because more substrate molecules  will      Rate of reaction  Rate increases directly with
           be colliding with enzyme molecules.              substrate concentration
           Therefore, more products will be formed
           at a time.  However, after a certain              Substrate concentration
           concentration,  any increase will have          Figure 1.45: The effect of substrate
           no effect  on the rate  of reaction,  since   concentration on the rate of enzyme-catalysed
           all  enzyme’s active  sites  have  bound to                reaction
           substrate, and the remaining substrate will
           be unable to bind to enzymes.              (b) Enzyme concentration


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