Enzymes
Enzymes
A.
Are Proteins (usually) that speed up
metabolic reactions by lowering the
activation energy.
A.
Some chemical reactions will occur
spontaneously, but it may occur too slowly to be
effective in living cells. Example:
___________________
B.
Biochemical reactions require enzymes to speed
up and control reaction rates. (Catalyst)
A.
A catalyst is a substance that speeds up
the rate of a chemical reaction without
being changed or used up.
B.
An enzyme may accelerate a reaction by
a factor of 1010 – that is, making it
happen 10,000,000,000 times faster. A
reaction that might normally take 1500
years would only take 5 seconds with an
enzyme.
Before a reaction can occur, the reactants must absorb
energy to break chemical bonds. This initial energy
investment is the activation energy (Ea).
(the graph on the right)
This is how enzymes speed up the rate of a reaction
B. Substrate specific
1. Enzymes are specific for a particular
substrate.
a. Substrate – The substance an
enzyme acts on and makes more reactive.
b. The enzyme is released in original
form after the reaction.

Maltose is the Substrate
2. Active Site – Restricted region of an enzyme molecule
which binds to the substrate.
a. Is usually a pocket or groove on the protein’s surface.
b. Determines enzymes specificity.
c. Changes its shape in response to the substrate.
d. Induced fit – change in the shape of an enzyme’s
active site, which is induced by the substrate.
Induced Fit
C. Active Site
1. The enzymatic cycle is very fast.
Step 1 – substrate binds to the active site forming an
enzyme-substrate complex. (Held together by weak
interactions)
Step 2 – Induced fit of the active site around the
substrate (Conversion of substrate to product)
Step 3 – Product departs active site and the enzyme
emerges in its original form.
1. Enzymes lower activation energy and speed up
reactions.
a. Active site can hold two or more reactants in the
proper position so they may react.
b. Induced fit of the enzyme’s active site may distort the
substrate’s chemical bonds so less energy is needed to
break them during the reaction.
c. Active site might provide a micro-environment
conducive to a particular type of reaction.
2. Initial substrate concentration partly determines the
rate of an enzyme controlled reaction.
a. The higher the substrate concentration, the faster the
reaction – up to a limit.
b. If high enough (substrate concentration), the enzyme
becomes saturated with substrate.
c. If saturated, the reaction rate depends upon how fast
the active sites can convert substrate to product.
d. Add more enzymes to increase reaction rate.
D. Physical and Chemical Environments
1. Each enzyme has optimal environmental conditions that
favor the most active enzyme conformation.
2. Temperature and pH
a. Optimal temperature allows the greatest number of
molecular collisions without denaturing enzymes.
35 – 40 degrees Celsius
b. Optimal pH range for most enzymes is pH 6 – 8
3. Cofactors – Small non-protein molecules that are
required for proper enzyme catalysis.
a. May bind tightly to active site.
b. May bind loosely to both active site and
substrate.
c. Some or inorganic (Zn, Fe, or Cu)
d. Some are organic and are called coenzymes
(vitamins)
4. Enzyme Inhibitors
a. Certain chemicals can selectively inhibit enzyme activity.
1. Irreversible – if the inhibitor attaches by covalent
bonds
2. Reversible – if the inhibitor attaches by weak
bonds.
b. Competitive inhibitors – chemicals that resemble an
enzyme’s normal substrate and compete with it for the
active site.
1. Blocks active site from the substrate.
c. Noncompetitive inhibitors – Enzyme inhibitors that do
not enter the enzyme’s active site, but bind to another
part of the enzyme molecule.
1. Cause enzyme to change its shape so the active
site cannot bind substrate.
2. May act as a metabolic poison (DDT and
antibiotics)
3. Selective inhibition is an essential mechanism
in the cell for regulating metabolic reactions.