Enzymes are protein catalysts that speed up biochemical reactions. They work by lowering the activation energy of reactions through their specific three-dimensional structure. This structure complements that of substrates, facilitating the formation of enzyme-substrate complexes. Factors like temperature, pH, enzyme and substrate concentrations can affect reaction rates by impacting enzyme shapes and complex formation. Inhibitors also regulate enzymes by competing for or altering their active sites.

1. Enzyme
‫الطالبة‬ ‫اسم‬:‫الجامعي‬ ‫الرقم‬ ‫الشريدة‬ ‫أحمد‬ ‫رياض‬ ‫أماني‬:435203244532zoo

2. Content
• Enzyme structure .
• How do enzyme work ?
• Factors that Affect the Rate of Enzyme Reactions .

3. Introduction
• Enzymes are biological catalysts.
• There are about 40,000 different enzymes in human cells,
each controlling a different chemical reaction.
• They increase the rate of reactions, allowing the chemical
reactions that make life possible to take place at normal
temperatures.

4. Enzyme structure
• Enzymes are proteins and their function
is determined by their complex structure.
• The reaction takes place in a small part
of the enzyme called the active site,
while the rest of the protein acts as
"scaffolding".
• This makes the enzyme specific for one
reaction only, as other molecules won't fit
into the active site – their shape is
wrong.

5. Cofactor ( coenzyme )
• Many enzymes need cofactors (or coenzymes)
to work properly.
• Cofactor type:
• These can be metal ions (such as Fe2+,
Mg2+, Cu2+) or cofactor
• organic molecules (such as haem, biotin,
FAD, NAD or coenzyme A) act as electron
carrier ,Many of these are derived from
dietary vitamins.
• The complete active enzyme with its cofactor
is called a holoenzyme, while just the protein
part without its cofactor is called the
apoenzyme.

6. Who do enzyme work ?
• There are three parts about enzyme catalysis. They each describe different
aspects of the same process .
1. Mechanism reaction .
2. Molecular geometry .
3. Energy change .

7. 1- Reaction mechanism for enzyme
• In any chemical reaction, a substrate (S) is converted into a product (P)
• In an enzyme-catalysed reaction, the substrate first binds to the active site of the
enzyme to form an enzyme-substrate (ES) complex, then the substrate is converted into
product whilst attached to the enzyme, and finally the product is released, thus allowing
the enzyme to start all over again .

8. Example

9. 2- molecular geometry
• The substrate molecule is complementary in shape to that of the active site. like a
key fitting into a lock .
• the substrate and the active site both change shape when the enzyme-substrate
complex is formed, bending (and thus weakening) the target bonds.
• single enzyme can act on different substrate having similar molecular geometry and
hence here specificity is very less .
Example , alcohol dehydrogenase can oxidize both ethanol and methanol to yield
corresponding aldehydes since both these alcohol have similar molecular geometry .

11. 3 – energy change
• Most of the reactions critical to a living cell happen too
slowly at normal temperatures to be of any use to the
cell. Without enzymes to speed up these reactions, life
could not persist. Enzymes do this by binding to the
reactant molecules and holding them in such a way as
to make the chemical bond-breaking and -forming
processes take place more easily.
• Because they do not change the free energy of the
reactants or products. They only reduce the activation
energy required for the reaction to go forward
• In addition, an enzyme itself is unchanged by the
reaction it catalyzes.

12. Factors that Affect the Rate of Enzyme Reactions
1. Temperature .
2. PH.
3. Enzyme concentration .
4. Substrate concentration .
5. Covalent modification .
6. Inhibitor .
7. Feed back inhibition .

13. 1- temperature
• Enzymes have an optimum temperature at
which they work fastest.
• As the temperature increases, so does the
rate of reaction. But very high temperatures
denature enzymes.

14. 2- PH
• Changes in pH also alter an enzyme’s shape.
• Different enzymes work best at different pH
values. The optimum pH for an enzyme
depends on where it normally works. For
example, intestinal enzymes have an
optimum pH of about 7.5.
• Enzymes in the stomach have an optimum
pH of about 2

15. 3 – enzyme concentration
• As the enzyme concentration
increases the rate of the reaction
also increases, because there are
more enzyme molecules (and so
more active sites), available to
catalyse the reaction therefore more
enzyme-substrate complexes form.

16. 4- substrate concentration
• when substrate concentration increases,
the rate increases because more substrate
molecules can collide with active sites, so
more enzyme-substrate complexes form
• At higher concentrations the enzyme
molecules become saturated with substrate,
and there are few free active sites, so
adding more substrate doesn't make much
difference

17. 5 – covalent modification
• The activity of some enzymes is controlled by other enzymes, which modify
the protein chain by cutting it, or adding a phosphate or methyl group.
• This modification can turn an inactive enzyme into an active enzyme ,
• this is used to control many metabolic enzymes and to switch on enzymes in
the gut e.g. HCl in stomach → activates pepsin → activates rennin.

18. 6 – inhibitors
• Inhibitors inhibit the activity of enzymes,
reducing the rate of their reactions. They are
found naturally .
1- competitive inhibitor :
• molecule has a similar structure to the
substrate molecule, and so it can fit into the
active site of the enzyme. It therefore
competes with the substrate for the active site,
so the reaction is slower. Increasing the
concentration of substrate restores the reaction
rate and the inhibition is usually temporary and
reversible.

19. Cont ….
2- non-competitive inhibitor
• molecule is quite different in structure
from the substrate and does not fit
into the active site. It binds to
another part of the enzyme molecule,
changing the shape of the whole
enzyme, including the active site, so
that it can no longer bind substrate
molecules. Non-competitive inhibitors
therefore simply reduce the amount
of active enzyme.

20. 7- feed back inhibition

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