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Unit 2 Notes: Part 5 – Enzymes and Introduction to Metabolism
AP Biology, Ms. Ottolini
Metabolism= all the chemical reactions in an organism
Catabolic Pathway (Catabolism)
• release of energy by the breakdown of complex molecules to simpler compounds
EX: digestive enzymes break down food
Anabolic Pathway (Anabolism)
1) How are dehydration
synthesis and hydrolysis
related to catabolism and
anabolism?
• consumes energy to build complicated molecules from simpler ones
EX: linking amino acids to form proteins
Organisms transfer Energy (a characteristic of living organisms)

Energy: capacity to do work

Kinetic Energy: energy of moving objects

Potential Energy: energy stored as a result of position or structure

Chemical Energy- form of potential energy stored in chemical bonds in
2) Describe the forms of
energy found in an apple as
it grows on a tree, then
falls and is digested by
someone who eats it.
molecules
Thermodynamics: study of energy transformations that occur in matter
1st Law of Thermodynamics = Conservation of energy
3) How does the second law
• energy of universe is constant; energy CAN BE transferred and transformed, but
of thermodynamics help
NEVER created or destroyed
explain the diffusion of a
2nd Law of Thermodynamics
substance across a
• every energy transfer or transformation increases the entropy (disorder or
membrane?
randomness) in universe
Equation that describes energy of system;
G= H-T S
- change in free energy is represented by  G
S = ENTROPY
G = FREE ENERGY of a system
(energy that is able to perform
work when the temperature is
uniform)
H = Total energy in system
T = Absolute temperature in °Kelvin
You don’t need to be able to
do G problems; just know
that there is an equation;
Endergonic Reaction- requires energy; absorbs free energy from system; not
spontaneous, Energy of products is higher than energy of reactants (positive G)
Exergonic Reaction- releases energy and occurs spontaneously
Energy of products is lower than energy of reactants (negative G)
Spontaneous Reactions:
4) Draw graphs to show the
processes of endergonic
and exergonic reations.
x-axis: time
y-axis: energy
- can occur without outside help
- can be harnessed to do work (objects moving down their power gradient)
Cells manage their energy resources and do work by Energy Coupling
(use energy from exergonic reactions to drive endergonic ones)
Key role of ATP = Energy Coupling
Adenosine Triphosphate = ATP
= primary source of energy in all living things
ADP (adenosine diphosphate) + Pi → ATP;
-adding phosphate group stores energy;
-removing it releases energy
Activation Energy = amount of energy required to get chemical reaction started
Catalyst- substance that changes the rate of a chemical reaction without being
altered
Enzymes = biological catalysts; most enzymes are Proteins (Ch 17 & 26: RNA
enzymes = RIBOZYMES)
ENZYMES work by Lowering Activation Energy; Don’t change the FREE ENERGY of
reaction
Substrate= Reactant enzyme acts on
Active Site = region on enzyme that binds to substrate
Substrate held in active site by WEAK interactions (ie. hydrogen and ionic bonds)
SUBSTRATE(S) + enzyme → Enzyme-substrate complex → enzyme + PRODUCT(S)
5) Draw a graph to show
how an enzyme-mediated
reaction is different from
a reaction that proceeds
without the help of an
enzyme.
ENZYMES are Unchanged and Reusable
Lock and Key Model: enzyme fits substrate like “lock and key”
-only specific substrate will fit
Induced Fit Model: once substrate binds to active site, enzyme changes shape
slightly to bind the substrate more firmly placing a strain on the existing bonds in
substrate lowering act energy
Enzymes have Optimal Temperature for
activity
Higher temperatures = more collisions among
the molecules so increase rate of a reaction
BUT. . .
above a certain temperature, activity begins
to decline because the enzyme begins to
Denature
So rate of chemical reaction increases with temperature
up to optimum, then decreases.
Enzymes have own Optimal pH
Different enzymes have different pH
curves
Extremes in pH can denature enzymes
-causing them to unwind/lose their 3-D
Tertiary structure
-breaks hydrogen, ionic bonds; NOT
covalent peptide bonds
Many enzymes require Helpers
 Non-Protein Helper = Cofactor
Ex: metal ions (zinc, iron, and copper)
 Organic Helpers = Coenzymes
Ex: vitamins ; coenzyme A
Certain molecules can prevent enzymes from performing their
functions. This is called Inhibition
 Competitive Inhibitors: bind to the enzyme’s active
site and prevent the substrate from binding
(reversible)

Noncompetitive Inhibitors: bind to another spot on
the enzyme (not the active site), cause a shape change
6) Which temperature is
optimal for the enzyme
whose reaction rate is
pictured to the left?
7) Pepsin is an enzyme
found in the stomach,
whereas trypsin is found in
the small intestine. Both
are digestive enzymes.
Why is typsin not found in
the small intestine, like
pepsin?
Non Competitive Inhibition is a type of Allosteric Regulation of enzyme activity
 REGULATORS bind to
ALLOSTERIC site
 binding site on enzyme (not active
site)
binding changes shape of enzyme
 Activators can stimulate enzyme
activity
 Inhibitors inhibit enzyme activity
 Cooperativity: If an enzyme has
two or more subunits, a substrate
causing induced fit in one subunit
can trigger the same favorable
conformational change in all the
other subunits of the enzyme,
amplifies the response of enzymes
to substrates, one substrate
molecule primes an enzyme to
accept additional substrate
molecules more rapidly
Negative Feedback (feedback inhibition)
- switches off pathway when product is plentiful
- common in many enzyme reactions;
- saves energy; don’t make it if you don’t need it
8) Which of the following
characteristics is not
associated with allosteric
regulation of an enzyme’s
activity?
A. a mimic of the substrate
competes for the active site
B. a naturally occurring molecule
stabilizes a catalytically active
conformation
C. Regulatory molecules bind to a
site remote from the active site
D. Inhibitors and activators may
compete with one another.
E. The enzyme usually has a
quaternary structure.
Positive Feedback
– speeds up pathway
- Less common
EX: Chemicals released by platelets that accumulate at injury site,
attract MORE platelets to the site.
9) During labor, a uterine
contraction causes the
release of the hormone
oxytocin. This, in turn,
causes a nerve stimulus
that stimulates the brain’s
hypothalamus to produce
more oxytocin, which
causes more contractions.
This results in contractions
increasing in amplitude and
frequency. Is this an
example of positive or
negative feedback? Why?