Factors that affect enzyme action.
The effect of enzyme concentration.
Enzymes contain active sites in which the substrates fit to be broken down to
products. The greater the concentration of enzyme, the faster the rate of
reaction because they are more frequents the collisions between enzyme and
substrate due to more active sites.
Initially, the rate of reaction increase steeply with increasing concentration of
enzyme. This is because there are more active sites for the substrates to slot in
and be broken down into products. The rate of reaction gradually increases
with increase in enzyme concentration because the substrate concentration is
reducing due to many active sites. Thus the substrate concentration becomes a
limiting factor.
The effect of substrate concentration.
The higher the concentration of the substrate, the faster the rate of reaction
because more substrates are available to be broken down into products due
high collisions between the substrates and enzymes.
As substrates concentration increases, the initial rate of reaction also increases
because there are more substrates which increase the rate collision with the
enzymes` active sites.
If the substrate concentration is increased but that of enzyme kept constant,
there comes a point where every enzyme active site is working continuously.
The enzyme simply cannot work faster thus substrate molecules have to queue
up to wait for the active site become vacant. The point at which the enzyme is
working at its maximum possible rate is known as Vmax.
Or. With further increase in substrate concentration, the rate becomes
constant because there are few active sites and so substrate have to wait for
active sites to be free so that they are worked on. But if at this point the
enzyme concentration is increased, the rate of reaction increases steeply.
Temperature and enzyme activity
At low temperatures, the reaction takes of place only very slowly. This is
because molecules are moving relatively slowly. Substrate molecules will not
often collide with the active site, and so binding between substrate and enzyme
is rare.
As temperature rises, the enzyme and substrate molecules move faster.
Collisions happen more frequently, so that substrate molecules enter the active
site more often increasing the rate of reaction.
As temperature continues to increase, the speed of movement of the substrate
and enzyme molecules also continues to increase. Above the optimum
temperature, the structure of the enzyme molecule vibrates so energetically
that some of the bonds (hydrogen bonds) holding the enzyme molecule in its
precise shape begins to break. The enzyme molecule begins to lose its shape
and activity, and is said to be DENATURED. The substrate molecule fits less
well into the active site of the enzyme, so the rate of the reaction begins to slow
down and eventually the substrate no longer fits at all and the rate becomes
zero.
Optimum temperature- temperature at which an enzyme catalyses a reaction
at the maximum rate. Most human enzymes have optimum temperature
around 400C. Enzymes form other organisms have different optimum
temperatures.
PH.
Most enzymes work fastest at a pH of fairly neutral conditions. Some such as
the protease pepsin work in acidic conditions and have a different optimum
pH.
The lower the pH, the higher the hydrogen ion concentration. These ions can
interact with R groups of amino acids affecting the charges of the groups. This
affects the ionic bonding between the groups which in turn affects the threedimensional arrangement of the enzyme molecule. The shape of the active site
may change and therefore reduce the chances of the substrate molecule fitting
into it.
A pH which is very different form the optimum pH can cause denaturation of
an enzyme.
ENZYME INHIBITORS
Competitive, reversible inhibition.
If some other molecule has a very similar shape as the active site, this will bind
to an enzyme`s active site and inhibit the enzyme`s function. When the
inhibitor binds briefly to the active site, there is competition between it and the
substrate for the site.
In competitive inhibition, the inhibitor has the same shape as the active site
and so both the inhibitor and substrate compete for the same active site. The
rate of enzyme active will slow down since few substrates are able to collide
with the active site. If there is much more of the substrate present than the
inhibitor, the substrate molecules can easily bind to the active site in the usual
way and so the enzyme`s function is unaffected. If the concentration of the
inhibitor rises, or that of the substrate falls, it becomes less and less likely that
the substrate will collide with an empty site.
Competitive inhibition is said to be reversible because it can be reversed by
increasing the concentration of the substrate.
Non-competitive inhibition
A molecule can bind to another part of the enzyme rather than the active site.
While the inhibitor is bound to the enzyme it can seriously disrupt the normal
arrangement of hydrogen bonds and hydrophobic interactions holding the
enzyme molecule in its three-dimensional shape. The resulting distortion
ripples across the molecule to the active site making the enzyme unsuitable for
the substrate. Thus the enzymes action is blocked no matter how much
substrate is present.
Inhibition of enzyme function may be lethal but in many situations it is
essential. For example metabolic reactions must be very finely controlled and
balanced, so no single enzyme can be allowed to run wild leading to more and
more products.
One way of controlling metabolic reactions is to use the end-product of a chain
of reactions as a non-competitive, reversible inhibitor. As the enzyme converts
substrate to product, it is slowed down because the end-product binds to
another part of the enzyme and prevents more substrate binding.
However, the end-product can lose its attachment to the enzyme and go to be
used elsewhere, allowing the enzyme reform into its active state. As product
levels fall, the enzyme is able to top them up again. This is termed end-product
inhibition.
As levels of product 3 rise, there is increasing inhibition of enzyme 1. So, less
product 1 is made and hence less product 2 and 3. Falling levels of the product
3 allow increased function of enzyme 1 so products 1, 2 and3 rise again and
the cycle continues. This end-product inhibition finely controls levels of
product 3 between narrow upper and lower limits and is an example of a
feedback mechanism.