Enzymes are proteins that act as biological catalysts and are necessary for most chemical reactions in living cells. They help reactions occur by lowering their activation energy. Enzymes have a unique three-dimensional shape with active sites that allow them to specifically bind to substrates and rearrange the molecules involved in chemical steps. While they are not consumed in these reactions, enzymes increase the rates of reactions and can be reused, making them highly efficient biological catalysts.

1. Enzymes

2. Enzymes
♦ Enzymes are substances that are necessary
for most of the chemical reactions that
occur in living cells.
♦ Chemical reactions that occur in the body
are done in small steps. In some of these
small steps, the enzyme works on the
reaction to help it take place, rearranging
the molecules involved.

3. Enzymes Are Biological Catalysts
• Enzymes are proteins that carry out most
catalysis in living organisms.
– Unique three-dimensional shape enables an
enzyme to react with specific substrates.
– Because the enzyme itself is not changed or
consumed in the reaction, only a small amount
is needed, and can then be reused.

4. Enzyme names
♦ Usually end in “-ase” like isomerase
♦ Most enzymes are used within the cell, but
some are used outside of the cell. Example:
Digestive enzymes

5. Free Energy
• Free energy refers to the amount of energy
actually available to break and subsequently form
other chemical bonds.
– in a cell - amt of energy contained in a molecules
chemical bonds
• Energy involved in chemical reactions:
• endergonic - any reaction that absorbs free
energy
• exergonic - any reaction that releases free
energy

7. Activation Energy
• Activation energy refers to the extra energy
required to destabilize existing chemical bonds
and initiate a chemical reaction.
• catalyst - substance that lowers the
activation energy
• Enzymes are catalysts.

8. Activation Energy and Catalysis

9. How do enzymes work?
• An enzyme reacts only with specific
substrates (Another word for reactants)
– Active site- the part of an enzyme that actually
interacts with the reactants

10. Enzyme Active Site
• Amino acid side chains interact, metal ions,
• Various types of polar, non-polar, ionic interactions

11. Enzymes
• Most enzymes are globular proteins with
one or more active sites.
– Substrates bind to the enzyme at these active
sites, forming an enzyme-substrate complex.

12. How Does It Fit?
♦ 2 Models
♦ Old – Lock and Key Model – the active site
only fits certain substrates.
♦ Newer – Induced Fit Model – Enzyme
changes shape in reaction to being exposed
to the substrate.

13. • Lock and Key Analogy: lock = enzyme, key = substrate.

15. Induced Fit Model

16. Control of enzyme activity:
• Concentration of enzyme and substrate molecules
• Temperature
• pH

17. Enzymes - Activity
• Enzyme and substrate concentrations

18. Enzymes - Activity
• Temperature and pH effect enzyme action

19. Enzymes - Activity
• Temperature and pH effect enzyme action