Enzymes

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Enzymes

• L. Scheffler

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Enzymes

• Enzymes are catalysts and increase the speed of a
  chemical reaction without themselves undergoing
  any permanent chemical change.
• Enzymes are neither used up in the reaction, nor
  do they appear as reaction products.

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Enzymes

• Enzymes are protein molecules that catalyze
  biochemical reactions
• The substance on which the enzyme acts is known
  as the substrate

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Discovery of Enzymes

• 1825 Jon Jakob Berzelius discovered the catalytic
  effect of enzymes.
• 1926 James Sumner isolated the first enzyme in
  pure form.
• 1947 Northrup and Stanley together with Sumner
  were awarded the Nobel prize for the isolation of
  the enzyme pepsin.

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Enzyme Characteristics

• High molecular weight proteins with masses
  ranging from 10,000 to as much as 2,000,000
  grams per mole
• Substrate specific catalysts
• Highly efficient, increasing reaction rates by a
  factor as high as 108

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Enzyme Nomenclature

• The earliest enzymes that were discovered have
  common names
• i.e. Pepsin, Renin, Trypsin, Pancreatin
• The enzyme name for most other enzymes ends in
  ase
• The enzyme name indicates the substrate acted
  upon and the type of reaction that it catalyzes

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Enzyme Names

• Examples of Enzyme Names
• Glutamic Oxaloacetic Transaminase (GOT)
• L-aspartate 2-oxoglutarate aminotransferase.
• Enzyme names tend to be long and complicated.
  They are often abbreviated with acronyms

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Enzyme Specificity

• Enzyme specificity may be characterized as
• Absolute The enzyme catalyzes on one reaction
• Group Specific The enzyme acts only on
  molecules having a particular functional group
• Linkage Specific The enzyme acts on a
  particular type of chemical bond
• Stereo-chemical Specific The enzyme acts on a
  particular stereo or optical isomer

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Enzyme Specificity

• The action of an enzyme depends primarily on the
  tertiary and quaternary structure of the protein
  that constitutes the enzyme.
• The part of the enzyme structure that acts on the
  substrate is called the active site.
• The active site is a groove or pocket in the
  enzyme structure where the substrate can bind.

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Cofactors

• Cofactors are other compounds or ions that
  enzymes require before their catalytic activity
  can occur.
• The protein portion of the enzyme is referred to
  as the apoenzyme.
• The enzyme plus the cofactor is known as a
  holoenzyme.

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Cofactors

• Cofactors may be one of three types
• Coenzyme A non protein organic substance that
  is loosely attached to the enzyme
• Prosthetic Group A non protein organic substance
  that is firmly attached to the enzyme
• Metal ion activators K, Fe2, Fe3, Cu2,
  Co2, Zn2, Mn2, Mg2, Ca2, or Mo2,

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Enzymes and Cofactors
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Cofactors

• Enzymes have varying degrees of specificity.
• One cofactor may serve many different enzymes.

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Enzyme Mechanisms

• Enzymes lower the activation energy for
  reactions and shorten the path from reactants to
  products

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Enzyme Mechanisms

• The basic enzyme reaction can be represented as
  follows

E S ? ES ? E P
Enzyme Substrate Enzyme substrate
Enzyme Product
complex
The enzyme binds with the substrate to form the
Enzyme-Substrate Complex. Then the substrate are
released as the product.
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Enzyme Mechanics

• An enzyme-substrate complex forms when the
  enzymes active site binds with the substrate
  like a key fitting a lock. 
• The shape of the enzyme must match the shape of
  the substrate.
• Enzymes are therefore very specific they will
  only function correctly if the shape of the
  substrate matches the active site.

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Induced Fit Theory
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Induced Fit Theory

• The substrate molecule normally does not fit
  exactly in the active site.
• This induces a change in the enzymes conformation
  (shape) to make a closer fit.
• In reactions that involve breaking bonds, the
  inexact fit puts stress on certain bonds of the
  substrate.
• This lowers the amount of energy needed to break
  them.

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Induced Fit Theory

• The enzyme does not actually form a chemical bond
  with the substrate. After the reaction, the
  products are released and the enzyme returns to
  its normal shape.
• Because the enzyme does not form chemical bonds
  with the substrate, it remains unchanged.
• The enzyme molecule can be reused repeatedly
• Only a small amount of enzyme is needed

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Enzymes and Reaction Rates

• Factors that influence reaction rates of
  Enzyme catalyzed reactions include
• Enzyme and substrate concentrations
• Temperature
• pH

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Enzymes and Reaction Rates

• At low concentrations, an increase in substrate
  concentration increases the rate because there
  are many active sites available to be occupied
• At high substrate concentrations the reaction
  rate levels off because most of the active sites
  are occupied

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Substrate concentration

• The maximum velocity of a reaction is reached
  when the active sites are almost continuously
  filled.
• Increased substrate concentration after this
  point will not increase the rate. 
• Vmax is the maximum reaction rate

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Substrate concentration

• Vmax is the maximium reaction rate
• The Michaelis-Menton constant , Km is the
  substrate concentration when the rate is ½ Vmax
• Km for a particular enzyme with a particular
  substrate is always the same

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Effect of Temperature

• Higher temperature increases the number of
  effective collisions and therefore increases the
  rate of a reaction.
• Above a certain temperature, the rate begins to
  decline because the enzyme protein begins to
  denature

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Effect of pH

• Each enzyme has an optimal pH at which it is most
  efficient
• A change in pH can alter the ionization of the R
  groups of the amino acids.
• When the charges on the amino acids change,
  hydrogen bonding within the protein molecule
  change and the molecule changes shape.
• The new shape may not be effective.

Pepsin is most efficient at pH2.5-3 while Trypsin
is much higher
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Inhibitors

• Enzyme inhibitors are substances which alter the
  catalytic action of the enzyme and consequently
  slow down or stop catalysis.
• There are three common types of enzyme inhibition
  
• Competitive inhibiters
• Non-competitive inhibiters
• Substrate inhibition.

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Competitive Inhibitors

• Competitive inhibition occurs when the substrate
  and a substance resembling the substrate are both
  added to the enzyme.
• The inhibitor blocks the active site on the
  enzyme stopping its catalytic action

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Non-competitive Inhibitors

• Non-competitive inhibitors deactivate the active
  site of the enzyme.
• They alter the enzyme so that it can no longer
  bind to the substrate

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Substrate Inhibitors

• Substrate inhibition occurs when excessive
  amounts of substrate are present.
• Additional substrate sometimes interferes with
  the ability of substrate molecules to find active
  sites on enzymes
• In these cases the reaction velocity decreases
  after the maximum velocity has been reached.

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Effect of inhibitors on the Reaction Rate

• For non-competitive inhibitors Vmax is lower but
  Km is the same.
• For competitive inhibitors, Vmax is the same but
  Km is increased.