© sser ltd.. of all the functions of proteins, one of the most important is that of catalysis in...
TRANSCRIPT
© SSER Ltd.
Of all the functions of proteins, one of the most important is that of catalysis
In the absence of catalysis, most reactions in biological systemswould take place far too slowly to provide products at an
adequate pace for metabolising organisms
The catalysts that serve this function in livingorganisms are called ENZYMES
All enzymes are globular proteins and are the most efficient catalysts known
Enzymes are able to increase the rate of reaction by a factor of up to 1020 over uncatalysed reactions
• They are proteins of high molecular weight
• They are biological catalysts
• They are sensitive to temperature changes,being denatured athigh temperatures
• They are sensitive to pH
• They are generally specific in the reactions they catalyse
• Enzymes possess an active site within which chemical reactions take place
Substratemolecule inthe ACTIVESITE
Enzyme molecule
Substratemolecules
(complementaryshape to active site)
Productmolecules
diffuse awayfrom the
active site
Substrate molecules bind with enzyme molecules at the activesite as a consequence of their complementary shapes. This is the
basis of the LOCK AND KEY MODEL of enzyme activity
Enzyme remainsunchanged
Enzymemolecule
Activesite
Reactionoccurs
In an enzyme-catalysed reaction, the enzyme binds to the substrate to form a complex
An enzyme-substrate complexforms
Products diffuseaway from the
active site
Enzymemolecule
The lock & key model proposes that the substrate binds to the active site which it fits exactly, like a key in a lock
SS
A reactionoccurs
forming anenzyme-product
complex
This model takes into account the fact that proteins (enzymes) have some three-dimensional flexibility
SUBSTRATE
Substrate binds to the enzymeat the active site
Binding of the substrateinduces the enzyme to change
shape such that there is anexact fit once the substrate
has bound
Enzyme Molecule
According to this model, reactions can only take place AFTER induced fit has occurred
Energy barrierwith enzyme
Energy barrierwithout enzyme
Energy levelof substrate
Energy levelof the products
Enzymes are catalystsbecause they lower the
ACTIVATION ENERGY
needed to drive areaction
Substrates need to overcome an energybarrier before they will convert to products
Lower activationenergy
Temperature
pH
Substrate Concentration
Enzyme Concentration
Inhibitors
Activators
As the temperature increases, molecular motion and thus molecular collisions increase
More product molecules are formed in a given time and hencethe reactionrate increases
For many enzymes, the maximum rate of reaction is reached at atemperature between 37°C to 40°CThis is the optimum temperature
The reaction rate doubles for every 10°C rise in temperature
As the temperature increasesbeyond the optimum, bonds that stabilise the enzyme’s tertiary structure are brokenThe enzyme loses its shapes and the active site is altered
Substrate can no longer bind to the enzymeThe enzyme has been DENATURED
Each specific enzyme can only work over a particular range of pH
Each enzyme has its own optimum pHwhere the rate of reaction is maximum
The effects of pH on the rate of enzyme controlled reactions displaycharacteristically bell shaped curves
AB C Enzyme A = amylaseoptimum pH = 7.2
Enzyme B = pepsinoptimum pH = 2.0
Enzyme C = lipaseoptimum pH = 9.0
Changes in pH can affect the ionic and hydrogenbonds responsible for the specific tertiary shape of enzymesExtremes of pH break these bonds and denature the enzyme
Low SubstrateConcentration
Low productconcentration per
unit time
Increased SubstrateConcentration
More product formation;
increased reaction rate
Further increasein substrate
concentration
Excess substrateconcentration
Maximum productformation; maximum
rate of reaction
No further increasein product formation;
maximum reaction ratemaintained
Enzyme concentration
is the LIMITING FACTOR
Increasing concentration of substrate
Rate ofreaction
A
Rate of reactionincreases as the
substrate concentration
increases
Rate of reaction reachesa maximum at substrate
concentration A
No further increase inthe reaction rate despite the increasing substrate
concentration
All the active sites of the enzymes are
occupied -enzyme concentration
is the limiting factor
Rate ofreaction
Increasing concentration of enzyme
The rate of reactionis directly proportional to the
enzyme concentration
As enzyme concentrationincreases, the rate of
reaction increases
In living cells, enzyme concentrations are usuallymuch lower than substrate
concentrations
Substrate concentration israrely a limiting factor
When the substrate concentration is low, theinhibitor competes successfully for the activesite; fewer substrate molecules are converted
into product and the rate of reaction is reduced
Low substrate concentrationInhibitor molecule
The effect of the competitive inhibitor is overcomewhen the high concentration of substrate molecules compete
successfully for the active sites of the enzymes; at high substrateconcentration, maximum reaction rate is achieved
High substrate concentrationInhibitor molecule
maximum rate
At low substrate concentrations,the rate of reaction is reducedin the presence of the inhibitor
withoutinhibitor
inhibitorpresent
The effect of the inhibitoris overcome by very highsubstrate concentrationsAt high substrate concentrations,the inhibitor is out-competed by thesubstrate and the maximum rateof reaction is achieved
Substrate binds to the enzyme when a non-competitive inhibitor is present but
cannot be converted to product; the rate of reaction is reduced
Low substrate concentrationInhibitor molecule
Substrate molecules not converted to product when inhibitormolecules arebound to the enzyme
Substrate molecules convertedinto product when no inhibitor is
attached to the enzyme
High substrate concentrationInhibitor molecule
Substrate molecules convertedinto product when no inhibitor is
attached to the enzyme
At high substrate concentrationall enzyme active sites are occupied
Substrate molecules bound to enzymeswith attachedinhibitor are NOT converted into product - maximumreaction rates are never achieved
The effect of the inhibitor is not overcomeby increasing the substrate concentration.
All the enzyme molecules with bound non-competitive inhibitor do NOT
convert substrate to product; the effectis equivalent to lowering
enzyme concentration
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Non-competitive inhibitors act by preventing bound
substrate being converted into product
with inhibitor; maximumreaction rate never achieved -
the effect of the inhibitor cannotbe overcome by increasing the
substrate concentration
no inhibitor; maximumreaction rate achieved
at high substrateconcentration