Some definitions and ground rules*
• Energy: the capacity to do work
• Work: transfer of energy from one place to another
• Chemical energy: energy contained in molecules and
released through chemical reactions.
• Sugar, glycogen and fat are the molecules in our
bodies that can store chemical energy.
• ATP is the molecule that can accept the energy and
transfer it from one chemical reaction to another.
• This energy allows us to move and grow. EX: muscle
contraction results from the interaction of proteins
that are fueled by ATP.
Definitions & ground rules*
• Chemical reaction: a process that forms or
breaks chemical bonds.
• These reactions change molecules from one
form to another:
• Reactants (name for molecules BEFORE reaction)
go through a chemical reaction. At the end of the
reaction, the reactants have become the products
(name for molecules AFTER the chemical
Reactants and End Products of Burning
Chemical Reactions can be EXERGONIC
• Exergonic: the result of the reaction is that
reactants have been changed to products AND
energy is RELEASED.
• Endergonic: the result of the chemical
reaction is that reactants have been changed
to products BUT ENERGY IS REQUIRED TO
MAKE THE REACTION HAPPEN.
An Exergonic Reaction
An Endergonic Reaction
HERE IS THE KEY*
• Most organisms are powered by the breakdown of sugar into CO2
• The energy that is released from this reaction does all of the work
that a cell needs to do: make proteins, move materials around,
muscle contraction, etc.
• But the energy from this reaction must be carried from the place
the glucose is broken down to the place that the energy is needed.
• ATP IS THE CARRIER MOLECULE OF CHEMICAL ENERGY IN OUR
BODIES. IT CANNOT STORE THE ENERGY FOR LONG PERIODS, ONLY
ENOUGH TIME TO CATCH THE ENERGY FROM AN EXERGONIC
REACTION AND MOVE IT TO A PLACE WHERE ENERGY IS NEEDED IN
ANOTHER PART OF THE CELL. (LATER IT CAN PICK UP MORE ENERGY
AND DELIVER IT TO OTHER AREAS OF THE CELL.)
• AND FINALLY, THE ENERGY FROM EXERGONIC REACTIONS OFTEN
FUELS ENDERGONIC REACTIONS. (called coupled reactions)
• Recall that ATP is a nucleotide composed of a
nitrogen-containing base, a sugar, and three
• ADP (the base, sugar and TWO phosphates) is a stable
molecule. It gets unstable when energy from a reaction
causes another Phosphate to be added to ADP and ATP
is formed. ATP will then deliver the ENERGY
CONTAINED IN THE LAST PHOSPHATE BOND to a place
where it is needed.
• When it “drops off” the energy, ATP is converted back
to ADP BECAUSE THE ENERGY IS IN THE PHOSPHATE
The Interconversion of ADP and ATP
(a) ATP synthesis: Energy is stored in ATP
(b) ATP breakdown: Energy is released
P P P
P P P
+P P P
P P P+
Other types of energy carriers
• ATP is not the only molecule that carries
energy within its bonds:
– Electron carriers: some energy in exergonic
reactions is transferred to electrons within special
molecules called electron carriers. When the
carriers reach their destination, the electrons (and
the energy they carry) can be released.
• NADH & FADH2 are electron carriers
Reactions may need enzymes to make
• Some reactions (exergonic and endergonic) may
never actually happen unless an catalyst is
• A catalyst is a molecule that speeds up a reaction
without the catalyst itself being changed.
• ENZYMES are catalysts that make reactions in
the body happen. They are proteins.
• Each enzyme can only catalyze ONE (or a small
number of) reactions.
• This protein is an enzyme that makes these
• ADP + Phosphate ATP
• ATP ADP + Phosphate
Structure of an enzyme*
• The function of an enzyme is determined by its structure.
• Every enzyme has an active site (created as a byproduct of
it’s quartenary structure)
• The molecule to be changed (the reactant, called the
substrate in the case of enzymes) in the reaction enters the
• Active site is altered once the substrate is attached (aa’s
within the active site may bond with atoms of the
• The reaction occurs, and the products of the reaction are
released from the active site along with the enzyme.
The Cycle of Enzyme-Substrate Interactions
The substrates, bonded
together, leave the enzyme;
the enzyme is ready for a
new set of substrates
The substrates and
active site change shape,
promoting a reaction
between the substrates
the active site in a
Regulation of reactions
• In the human body, reactions are SOMETIMES
linked in sequences called metabolic
pathways: an initial reactant molecule is
modified by one enzyme, that product is then
modified by another enzyme and so on.
• EX: Glycolysis is the initial stage of the
breakdown of glucose. Photosynthesis is a
metabolic pathway that plants use to change
H20 and CO2 into glucose.
Simplified Metabolic Pathways
Initial reactant Intermediates Final products
enzyme 1 enzyme 2 enzyme 3 enzyme 4
enzyme 5 enzyme 6
A B D E
Speed of reactions depends on
amount of substrate, enzymes
• The more substrate you have, the faster the given reaction will
occur until all available enzymes are being used.
• HOWEVER, our bodies can regulate the speed of a reaction (it’s not
just substrate in, reaction happens)…HOW? By changing the rate at
which enzymes are produced. (if our body doesn’t make enzymes,
all of the substrate in the world doesn’t matter. The enzymes are
required for the reaction to occur.)
• WE MAKE ENZYMES (PROTEINS) VIA PROTEIN SYNTHESIS. MORE
• So, our cells can actually “turn on” the production of enzymes when
there is a lot of substrate, and “turn off” the production of enzymes
when there is less substrate around.
• Our bodies can also synthesize enzymes in an inactive form, so they
will only “turn on” when they are activated.
• We can also inhibit enzymes from catalyzing reactions.
Inhibition/Regulation of enzymes
• Competitive Inhibition: a substance that is not the normal
substrate for an enzyme can “sit” in the active site, thus
inhibiting the normal substrate to enter
• Noncompetitive inhibition: a substance binds to the
enzyme (BUT NOT IN THE ACTIVE SITE) and the presence of
this substance will cause the enzyme to not bind its normal
• Allosteric regulation: an enzyme can have 2 different
configurations: an active and an inactive configuration
depending on the presence of molecules that activate or
inhibit the enzyme. These activators/inhibitors are often
either the end product in the reaction OR intermediate
products in a metabolic pathway.
(b) Competitive inhibition
A competitive inhibitor
molecule occupies the
active site and blocks
entry of the substrate
(c) Noncompetitive inhibition
causes the active site
to change shape, so the
substrate no longer fits
Allosteric Regulation of an Enzyme by
enzyme 1 enzyme 2 enzyme 3 enzyme 4 enzyme 5
A B C D
As levels of isoleucine rise,
it binds to the regulatory site
on enzyme 1, inhibiting it
Example of allosteric
• Metabolic pathway that makes isoleucine
from threonine. 5 enzymes needed for this
pathway, with 4 intermediate products. As
amount of isoleucine increases, it will actually
inhibit the first enzyme in the pathway.
• EX #2: ATP can inhibit enzymes in the
metabolic pathways that create it. (if too
much ATP, no need to make more at the