AP Bio
Energetic of
Chemical Reactions
& The Role of
Enzymes
Lecture
Campbell & Reece, Biology 7th Edition
pp. 150-157
Chapter 8
Energy and Chemical Reactions
• Exergonic rxn’ = release energy (products have less
chemical energy than reactants)
ex: AB + CD  AC + DB + energy
- ΔG
• Endergonic rxn’s = absorb energy (products have
more chemical energy than reactants)
ex: AB + CD + energy  AC + DB
+ ΔG
• Activation energy: energy added to reactants to
"jumpstart" the rxn
• Catalysts: reduce the amount of activation energy that
is needed to start the rxn.
2
The energetics of chemical reactions
•
•
Think about rolling a boulder up a hill…it takes
energy input, right? And the boulder at the
top of the hill now has that energy stored in it
(potential). So the product ends up having
MORE energy than the reactant.
Think about a boulder rolling down a hill…it has
lots of energy. And the boulder at the bottom
of the hill now has less (potential) energy. So
the product ends up having LESS energy than
the reactant.
3
Self-Quiz
• Which of these biological processes is (overall)
endergonic? Exergonic? –ΔG ? +ΔG ?
4
Chemical Reactions
• The activation energy, EA
– Is the initial amount of energy needed to start a
chemical reaction
– Is often supplied in the form of heat from the
surroundings in a system
The effect of enzymes on reaction rate
• An enzyme catalyzes reactions
– By lowering the EA barrier
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
∆G is unaffected
by enzyme
Products
Figure 8.15
Progress of the reaction
Enzymes
– Are a type of protein that acts as a catalyst, lowering
the EA and speeding up chemical reactions
1 Active site is available for
a molecule of substrate, the
reactant on which the enzyme acts.
Substrate
(sucrose)
2 Substrate binds to
enzyme.
Glucose
OH
Enzyme
(sucrase)
H2O
Fructose
H O
4 Products are released.
Figure 5.16
3 Substrate is converted
to products.
Substrate Specificity of Enzymes
• The substrate (A)
– Is the reactant an enzyme acts on
• The enzyme
– Binds to its substrate, forming an enzyme-substrate
complex (C)
• The Active Site
– Is the region on the enzyme where the substrate
binds
• Their shape
(conformation) makes each
enzyme substrate-specific
• “Lock-and-key” fit enables
reaction
• The active site can
lower an EA barrier by
– Orienting substrates
correctly
– Straining substrate
bonds
– Providing a favorable
microenvironment
– Covalently bonding to
the substrate
Effects of Local Conditions on Enzyme
Activity
• The activity of an enzyme
– Is affected by general environmental factors
•
•
•
•
•
•
temperature
pH
salinity
enzyme concentration
substrate concentration
presence of any inhibitors or activators.
• Denaturation is a process in which proteins or
nucleic acids lose their tertiary structure and
secondary structure by application of some
external stress or compound, such as a strong
acid or base, a concentrated inorganic salt, an
organic solvent (e.g., alcohol or chloroform), or
heat.
• If proteins in a living cell are denatured, this
results in disruption of cell activity and possibly
cell death.
Effects of Temperature and pH
• Each enzyme
– Has an optimal temperature in which it can
function
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
Rate of
reaction
Optimal temperature for
typical human enzyme
0
20
40
80
Temperature (Cº)
(a) Optimal temperature for two enzymes
100
Optimal pH for two enzymes
Optimal pH for trypsin
(intestinal enzyme)
Rate of
reaction
Optimal pH for pepsin
(stomach enzyme)
0
1
2
3
4
5
6
7
8
9
– Each enzyme has an optimal pH in which it can
function
Cofactors & Coenzymes
• Cofactors
– Are nonprotein enzyme helpers
– Most often minerals (metal ions)
– Zn 2+, Mg2+
• Coenzymes
– Are organic cofactors
– Usually vitamins
– B2, B6, B12, etc
Enzymes Regulate reactions
• Concept 8.5: Regulation of enzyme activity
helps control metabolism
• A cell’s metabolic pathways
– Must be tightly regulated
Enzyme 1
A
Enzyme 2
D
C
B
Reaction 1
Enzyme 3
Reaction 2
Reaction 3
Starting
molecule
Product
Biochemical pathway
Feedback inhibition of a biochemical pathway
Enzyme action and the hydrolysis of sucrose
Allosteric Activation and Inhibition
• Many enzymes are allosterically regulated
• Allosteric regulation
– Is the term used to describe any case in which a protein’s
function at one site is affected by binding of a regulatory
molecule at another site
– Enzymes can be “turned on” or “turned off” this way
Enzyme Inhibitors
• Competitive inhibitors
– Bind to the active site of an enzyme, competing with the
substrate
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme
(a) Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Figure 8.19
(b) Competitive inhibition
Competitive
inhibitor
Noncompetitive inhibitors
• Noncompetitive inhibitors
– Bind to another part of an enzyme, changing the
function
A noncompetitive
inhibitor binds to the
enzyme away from
the active site, altering
the conformation of
the enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition
Figure 8.19
Feedback inhibition
• In feedback
inhibition
Active site
available
– The end product
of a metabolic
pathway shuts
down the
pathway
Initial substrate
(threonine)
Threonine
in active site
Enzyme 1
(threonine
deaminase)
Isoleucine
used up by
cell
Intermediate A
Feedback
inhibition
Active site of
enzyme 1 no
longer binds
threonine;
pathway is
switched off
Enzyme 2
Intermediate B
Enzyme 3
Intermediate C
Isoleucine
binds to
allosteric
site
Enzyme 4
Intermediate D
Enzyme 5
Figure 8.21
End product
(isoleucine)