© sser ltd.. of all the functions of proteins, one of the most important is that of catalysis in...

© 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

XXXXXX XX

XX XX

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