Enzymes

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Chapter 5

• Enzymes

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Enzymes as organic catalysts

• Enzyme is for lowering of activation energy.

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No. of substrate particles
Energy level of substrate particles
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No. of substrate particles
E Activation energy when enzyme is not added.
EActivation energy when enzyme is added.
Energy level of substrate particles
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Lock and key hypothesis
Active site
substrate
Products
Enzyme-substrate complex
Enzyme
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Induced fit model

• The active site is induced by the substrate to
  change its shape to fit the shape of the
  substrate.

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Enzyme can catalyze both forward and backward
reactions. It does not alter the equilibrium
position of a biochemical reaction. The normal
progress of a biochemical reaction is due to the
immediate removal of the end product.
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Specificity

• The degree of specificity varies from one enzyme
  to another.
• Intracellular enzyme works on one particular
  substrate.
• Extracellular enzyme works on a range of related
  substrates.

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Factors affecting enzymatic activity
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Temperature affects the energy content and hence
the mobility of the enzyme and substrate
molecules. Higher temperature enables enzyme and
substrate molecules to collide more frequently
and therefore facilitates the substrate molecule
to bind with the active site of the enzyme. If
the temperature is too high (above optimum), the
polypeptide chains made up the enzyme will
vibrate too much so that the structure will be
disrupted, and the enzyme is said to be
denatured.
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Temperature
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Temperature coefficient Q10

• Q10 Reaction rate at (X10)oC
• Reaction rate at XoC

When the temperature is below the optimum
temperature, Q10 usually is 2. When the
temperature is above the optimum temperature, Q10
usually is less than 1.
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No. of substrate particles
T2gtT1
Energy level of substrate particles
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pH

• Changes in pH alter the ionic charge on the
  enzyme surface, thus causing change in shape of
  its active site, thus diminishing its catalytic
  activity. If extremes of pH is encountered, the
  enzyme is denatured and loses its catalytic
  function.

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Reversible inhibitors The effect is temporary.
The inhibiting effect can be eliminated if the
reversible inhibitor is removed.
Irreversible inhibitors This kind of inhibitor
disrupts the bonds of the polypeptides which form
the enzyme. Therefore the shape of the active
site is altered permanently and loses its
catalytic properties.
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Reversible inhibitor(I) Competitive Inhibitor
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Usually the end product of a biochemical reaction
acts as competitive inhibitor. This is known as
end-product inhibition. Immediate removal of the
end product facilitates enzyme reaction.
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Non-competitive Inhibitor
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Example of competitive inhibitor
Sulphonamides have very similar molecular shape
to aminobenzoic acid, a compound essential for
growth of many pathogenic bacteria. Therefore,
sulphonamide acts as competitive inhibitor to
check the growth of many bacteria in treatment of
many diseases caused by bacteria.
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Example of non-competitive inhibitor
Diisopropylflurophosphate (DFP), a nerve gas used
in warfare is a non-competitive inhibitor. DFP
acts on enzyme cholinesterase which is involved
in synaptic transmission. DFP causes prolonged
muscle contraction and death is the end result.
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Enzyme concentration and substrate concentration
affects the concentration of the enzyme-substrate
complex and hence the rate of enzyme-catalysed
biochemical reaction.
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Enzyme cofactorIt is non-protein substance which
makes some enzyme to function more efficiently.
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Enzyme activator

• It is an inorganic ion which loosely bound to an
  enzyme or an substrate to increase the chance to
  an enzyme-substrate complex forming. Some times,
  the inorganic ions are free and does not bind to
  the enzyme or substrate,
• e.g. copper, iron, calcium, Cl- etc.

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Coenzyme

• It is an organic non-protein molecule firmly
  associated with the enzyme and is essential for
  enzyme activity.
• It acts a carrier for transferring chemical
  groups or atoms from one enzyme to another.
• Many coenzymes are synthesized from vitamins.
• e.g. NAD

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Prosthetic group

• It is an organic non-protein group which is
  tightly bound to the enzyme or as an integral
  part with the enzyme.
• It can act as carriers of atoms or electrons
  while transferring one compound to another in an
  overall metabolic pathway.
• e.g. FAD, haemoglobin etc.

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Application of enzymes
Some washing powders are biologically active.
They contain enzyme protease to digest the
protein dirt.
Meat tenderizer contains enzyme protease which
break down and soften the texture of meat.