The reaction profile of a catalysed reaction compared to an uncatalysed
reaction.
Enzymes reduce the activation energy, the
minimum kinetic energy needed by reactant molecules to react by
breaking bonds and forming new bonds in the product molecules.
So, by reducing the activation energy, at the same
temperature, more molecules can react per unit time (rate), so
increasing the reaction rate of the enzyme reaction compared to the
uncatalysed reaction..
The enzyme helps break reactant molecule bonds more
easily than without a catalyst, so facilitating a faster reaction without
increasing concentration or temperature - the collision rate doesn't
increase, but there is more chance a fruitful collision producing the
product molecules because less kinetic energy is needed.
These arguments apply irrespective of whether the
enzyme is functioning in its optimum conditions or not.
A substrate molecule is a reactant which
is to be changed into the product by way of the specific enzyme.
The substrate molecule (or molecules) must fit neatly
into the active site on an enzyme and weakly bond to it.
The enzyme, or more specifically, the active site, is
referred to as the 'lock', and in an analogy with door locks, the
substrate molecules are referred to as the 'key or keys'.
The action by which enzymes function is
called the 'key and lock' mechanism. This is illustrated below.
The following diagrams illustrate two examples of the
'key and lock' mechanism - how an enzyme works.
The active site is where the chemical change from
substrate to product takes place and its shape is very important.
The mechanism is discussed in more detail in
the next section.
Many biochemistry reactions either involve synthesis
of a larger molecule by joining smaller ones together or breaking down and splitting
a larger molecule into smaller ones.
It is sometimes
quoted as a hypothesis, but there is a vast amount of evidence to show this
mechanism is correct.
Each enzyme is shaped precisely to accept the
substrate molecules, otherwise the reaction will NOT take place. This is
why a particular enzyme can only catalyse a specific reaction. The
substrate must fit into the active site!
The complete molecular structure of some enzymes
has been determined by X-ray crystallography.
From a computer database you generate the
structure of the enzyme and with advanced computer graphics you can 'virtually'
examine the 3D active site.
You can then bring in a substrate molecule
(real or theoretical) to see
how it fits (or not) into the unique structure of the active site.
It is now possible to design drugs to block enzyme
reactions to treat a particular medical condition.
You can then
synthesise the drug and thoroughly test to see if it works AND has no
harmful side effects.
If the enzyme is not the right shape e.g. the protein structure-active site is damaged, the substrate molecule
cannot 'key in' or 'dock in' so the enzyme cannot function and the reaction does not take
place.
This protein structure damage is referred to as a
denaturing of
the enzyme.
Enzyme damage (denaturing) can be caused by too high a temperature or the medium may
be too acid (too low a pH) or too alkaline (too high a pH) - see later
section on factors affecting the rate of enzyme reactions.
Examples of
the 'key
and lock' mechanism for enzyme action
A good example of using a scientific model
and well supported by scientific evidence
(Stage 1)
is the 'docking in' of the substrate molecules into the active site, they
are held there just sufficiently strongly to allow the chemical transformation to take
place.
The active site is considered the 'lock'.
(Stage 2)
happens on the active site where the substrates are catalytically changed to
products which are then released from the enzyme.
The substrate reactant molecule is considered the 'key'.
