Chapter 6:
Energy and Metabolism
Biological Work Requires Energy
• Remember to study the terms
• Energy Concepts Video
Fig. 8-2
A diver has more potential
energy on the platform
than in the water.
Diving converts
potential energy to
kinetic energy.
Climbing up converts the kinetic
energy of muscle movement
to potential energy.
A diver has less potential
energy in the water
than on the platform.
2 Laws of Thermodynamics
• 1.) Energy cannot be created or destroyed, but
it can be transferred or changed from 1 form
to another
• Photosynthesis:
– Sun’s energy  chemical energy in bonds of carbs
– Later
• Chemical energy  cellular work OR mechanical energy
• 2.) energy is converted from 1 from to
another; some usable energy is converted to
heat (disperses into surroundings)
• SO – total amount of energy available to do
work is decreasing over time
• Total amount of energy overall remains
constant
Fig. 8-3
Heat
Chemical
energy
(a) First law of thermodynamics
CO2
+
H2O
(b) Second law of thermodynamics
Entropy
•
•
•
•
Measure of disorder or randomness
Less-usable energy is more disorganized
Organized = low entropy
Disorganized = high entropy
– Ex: heat
• Entropy – continuously increasing in universe
in all natural processes
– As more heat is released, our universe becomes
more disorganized
Enthalpy
• Total potential energy of the system
Free Energy
• Amount of energy available to do work under
the conditions of a biochemical reaction
H = G + TS
•
•
•
•
•
H = enthalpy
G = free energy
T = absolute temperature in K
S = entropy
Can’t effectively measure total free energy but
can measure CHANGES, so
ΔG = ΔH - TΔS
Reactions
•
•
•
•
Exergonic – release energy HL
Endergonic – gain energy from surroundings
Activation energy – needed to start a reaction
Coupled reaction – endergonic + exergonic
– exergonic reaction provides energy required to
drive endergonic reaction
Fig. 8-6
Free energy
Reactants
Energy
Products
Amount of
energy
released
(∆G < 0)
Progress of the reaction
(a) Exergonic reaction: energy released
Free energy
Products
Energy
Reactants
Amount of
energy
required
(∆G > 0)
Progress of the reaction
(b) Endergonic reaction: energy required
Enzymes – How Enzymes Work Video
• Enzyme – protein catalyst
– Lower activation energy
• Catalyst
– Speed up reaction
• Substrate – substance that enzyme acts on
• Enzyme-Substrate complex –
– Enzymes orders structure of substrate
– Gets reaction going
– When breaks  product + original enzyme
Enzymes
• Active site – on enzyme, where substrate
binds
• Induced Fit – substrate binds, changes shape
of enzyme
– Enzyme and substrate not exactly complementary
Fig. 8-17
Substrates
enter active site; enzyme
1
changes shape such that its active site
enfolds the substrates (induced fit).
Substrates
2 Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Enzyme-substrate
complex
3 Active site can lower E
and speed up a reaction.
6
Active
site is
available
for two new
substrate
molecules.
Enzyme
5
Products are
released.
4
Products
Substrates are
converted to
products.
Enzymes
• Some – all protein
• Some – 2 parts – work together to function
– 1) protein = apoenzyme
– 2) chemical component = cofactor ( C or no C)
– Coenzyme – C, nonpolypeptide
• Binds to apoenzyme as cofactor
Enzymes are most Effective At Certain
Conditions
• Temperature
– Most – temp increases, reaction rate increases
– Low temp = slow
– Too high temp. – denatures enzymes
• pH – enzyme active in narrow range
• Charge – affect ionic bonds for tertiary and
quaternary structure
Fig. 8-18
Rate of reaction
Optimal temperature for
typical human enzyme
Optimal temperature for
enzyme of thermophilic
(heat-tolerant)
bacteria
40
60
80
Temperature (ºC)
(a) Optimal temperature for two enzymes
0
20
Optimal pH for pepsin
(stomach enzyme)
100
Optimal pH
for trypsin
Rate of reaction
(intestinal
enzyme)
4
5
pH
(b) Optimal pH for two enzymes
0
1
2
3
6
7
8
9
10
Concentrations of Enzyme and
Substrate
• Lots substrate – enzyme concentration limits
• Less substrate than enzyme – substrate
concentration limits
Enzyme Activity
• Feedback Inhibition
– Formation of a product inhibits an earlier reaction
in the sequence of reactions
–ABCDE
– When E is low – sequence proceeds rapidly
– E is high – E1 slows and can stop entire sequence
Fig. 8-UN1
Enzyme 1
Enzyme 2
B
A
Reaction 1
Starting
molecule
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
• Allosteric Regulation
– Substance binds to enzyme’s allosteric site,
changing shape of active site and modifying
enzyme’s activity
– Allosteric site = receptor site on enzyme, not
active site
– Allosteric regulators – affect enzyme activity by
binding to allosteric sites
• Keep inactive
• activate
Fig. 8-20
Allosteric enyzme Active site
with four subunits (one of four)
Regulatory
site (one
of four)
Activator
Active form
Stabilized active form
Oscillation
NonInhibitor
functional Inactive form
active
site
Stabilized inactive
form
(a) Allosteric activators and inhibitors
Substrate
Inactive form
Stabilized active
form
(b) Cooperativity: another type of allosteric activation
Enzyme Inhibition – can be inhibited or destroyed
by certain chemicals
• Reversible Inhibition – inhibitor forms weak
chemical bonds w/ enzyme
– Competitive – inhibitor competes w/ normal
substrate for active site
• No permanent damage
– Noncompetitive – inhibitor binds w/ enzyme but
not at active site
• e enzymes by altering shape
Fig. 8-19
Substrate
Active site
Competitive
inhibitor
Enzyme
Noncompetitive inhibitor
(a) Normal binding
(b) Competitive inhibition
(c) Noncompetitive inhibition
• Irreversible Inhibition – inhibitor permanently
inactivates or destroys an enzyme when it
combines w/ enzyme at active site or
elsewhere
– Ex: poison
• Mercury, lead, nerve gas, cyanide