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Enzymes  � What  is  needed  for  chemical  reactions  to  proceed  in  the  body  (  and  thus  metabolism  to  proceed)  is  the  presence  of  specific  proteins  called  ENZYMES.  

� Most  chemical  reactions  do  not  proceed  spontaneously  and  they  need  help  from  a  catalyst.  

� An  energy  barrier  must  be  overcome  before  a  chemical  reaction  can  begin.  This  energy  is  called  the  activation  energy  (because  it  activates  the  reactants).  

Enzymes  

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Products

Reactant

Ener

gy

Activation energy barrier

Without enzyme

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Enzymes  

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Reactant

Products

Enzyme

Ener

gy

Activation energy barrier reduced by enzyme

With enzyme

Enzymes  Lower  the  Ac1va1on  Energy  Barrier  

Reactants

Progress of the reaction Products

a

b

c Ener

gy

The activation energy “a” is lowered to “b” when an enzyme is involved in this reaction. Thus the reaction will proceed much easier with an enzyme.

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A  specific  enzyme  catalyzes  each  cellular  reac1on  � An  enzyme  

�  is  very  selective  in  the  reaction  it  catalyzes    �  has  a  shape  that  determines  the  enzyme’s  specificity  for  the  substrate  

�  produces  a  very  specific  product    

� The  specific  reactant  that  an  enzyme  acts  on  is  called  the  enzyme’s  substrate.  

� A  substrate  fits  into  a  region  of  the  enzyme  called  the  active  site.  

A  specific  enzyme  catalyzes  each  cellular  reac1on  � Enzymes  are  specific  because  only  a  specific  substrate  molecules  fits  into  their  active  site  and  only  a  specific  product  is  made  

� The  fact  that  enzymes  are  specific  is  also  reflected  in  their  names  

� An  enzyme  is  usually  named  after  the  substrate  that  it  uses  followed  with  -­‐ase  �  SUCRASE  :  uses  sucrose  as  a  substrate    �  LACTASE  :  uses  lactose  as  a  substrate  

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Enzyme (sucrase)

Active site

The enzyme available with an empty active site

1

Example of the action of an Enzyme

Enzyme (sucrase)

Substrate binds to enzyme with induced fit.

Substrate (sucrose)

Active site

The enzyme available with an empty active site

1

2

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The substrate is converted to products

H2O

Enzyme (sucrase)

Substrate binds to enzyme with induced fit.

Substrate (sucrose)

Active site

The enzyme available with an empty active site

1

2

3

Glucose

Fructose

The products are released

The substrate is converted to products

H2O

Enzyme (sucrase)

Substrate binds to enzyme with induced fit.

Substrate (sucrose)

Active site

The enzyme available with an empty active site

1

2

3

4

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Enzymes  have  op1mal  condi1ons  for  func1oning  �  For  every  enzyme,  there  are  optimal  conditions  under  which  it  is  most  effective.  

� Temperature  affects  molecular  motion.  �  An  enzyme’s  optimal  temperature  produces  the  highest  rate  of  contact  between  the  reactants  and  the  enzyme’s  active  site.  

� Most  human  enzymes  work  best  at  35–40°C.  � The  optimal  pH  for  most  enzymes  is  near  neutrality  (pH  ~  7)  

� The  optimal  pH  and  temperature  tells  us  the  environment  where  the  enzyme  functions  best.  

Enzymes  have  op1mal  condi1ons  for  func1oning    Since  enzymes  are  proteins,  they  are  vulnerable  to  denaturation.    The  3-­‐  dimensional  framework  of  the  enzyme  falls  apart  and  they  lose  efficiency.    Examples  are  changes  in  Temperaure  and  pH.  

Enzyme 1 will denature at pH 7 and Enzyme 2 will denature at pH 2.

1 2

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A  specific  enzyme  catalyzes  each  cellular  reac1on  � Many  enzymes  require  non-­‐protein  helpers  called  cofactors,  which  �  bind  to  the  active  site  and  �  function  in  catalysis.  

�  Some  cofactors  are  inorganic,  such  as  the  ions  of  zinc,  iron,  magnesium,  or  copper.  

�  If  a  cofactor  is  an  organic  molecule,  such  as  most  vitamins,  it  is  called  a  coenzyme.  

©  2015  Pearson  Education,  Inc.  

Enzyme  inhibi1on  can  regulate  enzyme  ac1vity  in  a  cell  A  chemical  that  interferes  with  an  enzyme’s  activity  is  called  an  inhibitor.  � Competitive  inhibitors  

�  block  substrates  from  entering  the  active  site  and  �  reduce  an  enzyme’s  productivity.  

� Noncompetitive  inhibitors  �  bind  to  the  enzyme  somewhere  other  than  the  active  site,    

�  change  the  shape  of  the  active  site,  and  �  prevent  the  substrate  from  binding.  

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Substrate

Enzyme

Active site

Normal binding of substrate

Competitive inhibitor

Noncompetitive inhibitor

Enzyme inhibition

Enzyme  inhibi1on  can  regulate  enzyme  ac1vity  in  a  cell  � Enzyme  inhibitors  are  important  in  regulating  cell  metabolism.  

�  In  some  reactions,  the  product  may  act  as  an  inhibitor  of  one  of  the  enzymes  in  the  pathway  that  produced  it.  This  is  called  feedback  inhibition.  

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Feedback inhibition

Enzyme 1

Reaction 1 A

Starting molecule

Product

–

Enzyme 2

Reaction 2 B

Enzyme 3

Reaction 3 C D

Many  drugs,  pes1cides,  and  poisons  are  enzyme  inhibitors  � Many  beneficial  drugs  act  as  enzyme  inhibitors,  including  �  ibuprofen,  which  inhibits  an  enzyme  involved  in  the  production  of  prostaglandins  (messenger  molecules  that  increase  the  sensation  of  pain  and  inflammation),  

�  some  blood  pressure  medicines,  �  some  antidepressants,  � many  antibiotics,  and    �  protease  inhibitors  used  to  fight  HIV.  

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Many  drugs,  pes1cides,  and  poisons  are  enzyme  inhibitors  

� Enzyme  inhibitors  have  also  been  developed  as  �  pesticides  and  deadly  poisons  for  chemical  warfare.    

� Natural  occurring  poisons  also  can  function  as  enzyme  inhibitors  

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Many  drugs,  pes1cides,  and  poisons  are  enzyme  inhibitors