Enzymes are proteinaceous substances that act as catalysts in biochemical reactions, speeding up reaction rates without being used themselves. Enzymes operate through small amounts in cells but each enzyme speeds up reactions by many orders of magnitude through their active sites. The rates of enzyme-controlled reactions are affected by factors like substrate and enzyme concentration, temperature, pH, and salinity, which influence the active site. Inhibitors can also reduce reaction rates by either competitively or non-competitively binding to enzymes and interfering with their catalytic activity.

1. ENZYMES
Enzymes are proteinaceous chemical substances that act as catalyst in the biochemical reactions.
They speed up reaction rate without themselves being used up.Enzymes are only present in very
small amounts in cells but this is all that is needed. Because each enzyme speeds up a reaction by
many orders of magnitude. For example catalase is an enzyme found in the liver and in potatoes.
It catalyses the breakdown of hydrogen peroxide to oxygen and water.
The rates of enzyme controlled reactions are affected by a number of different factors. In most
cases the effect of a factor on the rate of the reaction is due to its influence on the active site.
Substrate concentration
For a given amount of enzyme, an increase in substrate will result in an increase in the rate of
reaction. The more substrate molecules there are, the faster the reaction takes place as more of the
active sites are used. This continues until the concentration of substrate is so high that all the
enzyme molecules are working at their maximum capacity. At this point, however much more
substrate is added the reaction will not go any faster.
Graph to show effect of substrate concentration on rate of an
enzyme controlled reaction with enzyme in excess
Enzyme Concentration
Increasing Enzyme Concentration will increase the rate of reaction, as more enzymes will be
colliding with substrate molecules.However, this too will only have an effect up to a certain
concentration, where the Enzyme Concentration is no longer the limiting factor.

2. Graph to show effect of enzyme concentration on rate of
an enzyme controlled reaction
Temperature
As the temperature rises, reacting molecules have more and more kinetic energy. This increases
the chances of a successful collision and so the rate increases. There is a certain temperature at
which an enzyme's catalytic activity is at its greatest. This optimal temperature is usually around
human body temperature (37.5 oC) for the enzymes in human cells.
Graph to show the effect of temperature on enzyme catalyzed reactions

3. In most chemical reactions an increase in temperature causes an increase in reaction rate. As the
reactants are heated, the particles move faster and are more likely to collide with sufficient energy
to overcome the activation energy of the substances reacting.
Initially the rise in temperature above the optimum temperature breaks some of the hydrogen bonds
that hold the shape of the active site. This reduces the efficiency of the enzyme and the reaction
rate falls.If the temperature falls again the hydrogen bonds reform and the activity of the enzyme
is restored.
If the temperature continues to rise the disulfide bonds and covalent bonds are broken. This
permanently destroys the shape of the active site and so the catalytic activity of the enzyme is lost
permanently. The rise in temperature permanently denatures the enzyme.
pH
The pH of the environment has a major effect on the rate of enzyme controlled reactions. The
intermolecular bonds – particularly the hydrogen bonds – that maintain the tertiary structure and
the active site are very vulnerable to changes in hydrogen ion concentration. Each enzyme has an
optimum pH at which it works at its optimum rate. A change in pH causes a change in the shape
of the active site. It is not so easy for the reactants to fit in and bind to the active site. The activity
of the enzyme is reduced and the rate of reaction slows. If the change is too great, the substrate
will not be able to bind to the active site at all and the enzyme can no longer catalyse the reaction.
In most cases the activity of the enzyme is restored as the pH returns to its optimum level.
Effect of pH on rate of enzyme controlled reaction
Salinity
Salt concentration could affect the activities of enzymes in a couple of ways.First, salt
concentration can have an effect on the equilibrium constant for the reaction being catalyzed,
which does not involve the enzyme per se but will affect the outcome of the enzyme's activity.

4. Second, depending on how the enyzme performs its catalysis of the reaction(general acid-base,
hydrogen bonding, etc), the increased salt may enhance or decrease the effect of the enzyme's
chemistry. Third, some proteins are more or less stable as the salt concentration increases.
Inhibition of enzyme activity
Inhibitors
A number of substances may cause a reduction in the rate of an enzyme catalysed reaction. Some
of these (e.g. urea) are non-specific protein denaturants. Others, which generally act in a fairly
specific manner, are known as inhibitors. They may be competitive or non competitive.
Competitive inhibitors
Competitive inhibition occurs when the substrate and a substance resembling the substrate are both
added to the enzyme. In competitive inhibition, at any given moment, the enzyme may be bound
to the inhibitor, the substrate, or neither, but it cannot bind both at the same time.
Non-competitive inhibitors
Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity
of the enzyme and binds equally well to the enzyme whether or not it has already bound the
substrate.The inhibitor may bind to the enzyme whether or not the substrate has already been
bound, but if it has a higher affinity for binding the enzyme in one state or the other, it is called a
mixed inhibitor.