So, roles of enzymes within organisms.
The recognition and catalytic abilities of
enzymes come about through specific interactions
with functional groups in the active sites.
And if we had to take, for example, esterase
enzymes which are found in the blood stream
and indeed throughout the body, these enzymes
are responsible for the hydrolysis of ester
Therefore, any drug or substrate containing
an ester which enters the blood stream is
normally hydrolyses... hydrolysed by these
systemic esterases. Esterases have the ability
to bind to a wide variety of substrates provided
they have this ester linkage.
As we will be discussing a little later on
when we come on to the pro-drugs lecture,
pro-drugs themselves are often activated by
esterases. And the enzyme works by binding
the substrate and also, crucially, a molecule
of water that ultimately performs the hydrolysis
itself. Acid in basic groups in the active
site catalyse the reaction, as we will see.
So, here let’s have a look and idealise hydrolysis
of an ester. The black line is to represent
the pocket or active site of our enzyme. In
this case, an esterase active site, which
is written in black.
The ester, which you should recognise from
the Module III discussion of functional groups,
is shown here in green.
Note, carbonyl carbon attached to the oxygen
from that carbonyl and then you have got the
two alcohol groups on either side. Also note,
in red, we show that the participating water
is also present in the active site.
In the first instance, we have a NH2 side
chain, a part of the active site,
shown in yellow, and a carboxylic acid side
chain is part of the active site, shown in
Now, the origins of this side chain can obviously
vary. But, for example, the blue carboxylic
acid side chain could belong to the amino
acid: glutamic acid or indeed, aspartic acid.
If we look at the NH2 side chain, shown in
yellow, this could be the side chain from,
for example, lysine.
What’s happening, in this case, is the electron
pair from a lysine nitrogen is donated onto
a hydrogen, breaking open that bond and releasing,
to all intents and purposes, hydroxide anion.
This, then, attacks the carbonyl at the ester
and furthermore, a proton is abstracted from
the glutamic or aspartic acid showed in blue.
This results in converting what is essentially
an sp2 hybridised carbon and oxygen system
to a tetrahedral one, which is shown here.
Here you can see sp3 hybridised carbon in
the center, shown as green, and the water
OH, which was originally red, is now attached
to that carbonyl carbon. From here, as you
can appreciate, couple of different reactions
Either we lose the OH- again and the ester
is reformed or as shown in this case, we lose
the alkoxy group, shown as O-R1, on our particular
diagram, thus producing a free alcohol and
alcohol carboxylic acid, which is shown on
the right hand side of the board.
If you can pair this mechanism to an ester
hydrolysis with a typical lab based system,
you will find there isn’t a great lack of
similarity here. You just have to make sure
that all of the actors in this particular
reaction are closed together. And by binding
a system close to where the catalytic region
is, it’s possible to actually achieve a high
turnover of ester hydrolysis then it would
be in the laboratory.