8:
An Chapter Introduction
to Metabolism
Important Point:
Metabolism (Overview)
Metabolism = Catabolism + Anabolism
Catabolic reactions are energy yielding
They are involved in the breakdown of
more-complex molecules into simpler
ones
Anabolic reactions are energy requiring
They are involved in the building up of
simpler molecules into more-complex
ones
We can consider these bioenergetics in
terms of the physical laws of
thermodynamics
1st & 2nd Laws of Thermodynamics
Energy can be
transferred or
transformed but neither
created nor destroyed.
p. 143, Campbell & Reece (2005)
Every energy transfer or
transformation increases the
disorder (entropy) of the
universe. p. 143, Campbell & Reece (2005)
Note especially the waste heat
Organisms are Energy
Transducers
Organisms take in energy & transduce it to new
forms (1st law)
As energy transducers, organisms are less than
100% efficient (2nd law)
Organisms employ this energy to:
Grow
Protect Themselves
Repair Themselves
Compete with other Organisms
Make new Organisms (I.e., babies)
In the process, organisms generate waste
chemicals & heat
Organisms create local regions of order at the
expense of the total energy found in the
Universe!!! We are Energy Parasites!
Water Fall Analogy
Get it?
Laws of Thermodynamics
First Law of Thermodynamics:
Energy can be neither created nor destroyed
Therefore, energy generated in any system is
energy that has been transformed from one state
to another (e.g., chemically stored energy
transformed to heat)
Second Law of Thermodynamics:
Efficiencies of energy transformation never equal
100%
Therefore, all processes lose energy, typically as
heat, and are not reversible unless the system is
open & the lost energy is resupplied from the
environment
Conversion to heat is the ultimate fate of
chemical energy
Movements Toward Equilibrium
Increase
stability
Downhill
G < 0
Greater
entropy
Entropy!
Free Energy & Spontaneity
What is the name
of this molecule?
Movement Toward Equilibrium
Work
Potential
energy
Equilibrium
Spontaneous
Forward
reaction
Movement Toward Equilibrium
Viable organisms exist in a
chemical disequilibrium that
is maintained via the
harnessing of energy
obtained from the
organisms environment
(e.g., you eat to live)
Waterfall Analogy
Potential Energy
R
y
all
e
Kinetic Energy
Stayring of a turbine
generator, Priest Rapids
Dam, 1958
Gravity (center Earth)
Waste Heat
(once reaches
Bottom)
Movement Toward Equilibrium
Food
Potential
energy
Forward
reaction
Spontaneous
Waste
heat
Work
Movement Toward Equilibrium in Steps
Note that Spontaneity is
not a measure of speed of a
process, only its direction
Exergonic Reactions
Food
Energy
released
Movement toward
equilibrium
Endergonic Reactions
Work
Energy
required
Exergonic Reaction (Spontaneous)
Decrease in Gibbs free energy (- G)
Increase in stability
Spontaneous (gives off net energy upon going forward)
Downhill (toward center of gravity well, e.g., of Earth)
Movement towards equilibrium
Coupled to ATP production (ADP phosphorylation)
Catabolism
Endergonic Rxn (Non-Spontaneous)
Increase in Gibbs free energy (+ G)
Decrease in stability
Not Spontaneous (requires net input of energy to go forward)
Uphill (away from center of gravity well, e.g., of Earth)
Movement away from equilibrium
Coupled to ATP utilization (ATP dephosphorylation)
Anabolism
Coupling Reactions
Minus the
cut for the
2nd law
Exergonic
reactions can
supply energy
for endergonic
reactions
Energy Coupling in Metabolism
Catabolic reactions provide the energy
that drives anabolic reactions forward
Catabolic
reaction
Anabolic
reaction
Adenosine Triphosphate (ATP)
Call this A
Energy Coupling via ATP
Hydrolysis of ATP
Movement
toward
equilibrium
Coupled Reactions
How that really
works
Various reaction Pi Transfers
Summary of Metabolic Coupling
Endergonic
reaction
Exergonic
reaction
Exergonic
reaction
Endergonic
reaction
Get it? Exergonic processes drive Endergonic processes
Movement Toward Equilibrium
Food
Exergonic
Endergonic
Coupling the Biosphere
Anabolic
process
Catabolic
process
Chemically
stored energy
Enzyme Catalyzed Reaction
Question: Is
this reaction
endergonic or
is it exergonic?
Enzyme
Activation Energy (EA)
Anything that
doesnt require
an input of
energy to get
started has
already
happened!
Low- (i.e., body-) Temp. Stability
Why don't energy-rich molecules, e.g., glucose,
spontaneously degrade into CO2 and Water?
To be unstable, something must have the
potential to change into something else,
typically something that possesses less free
energy (e.g., rocks)
To be unstable, releasing somethings ability to
change into something else must also be
relatively easy (i.e., little input energy)
Therefore, stability = already low free energy
Alternatively, stability = high activation energy
Things, therefore, can be high in free energy
but still quite stable, e.g., glucose
Catalysis
Lowering of
activation
energy
Catalysis
At a given
temperature,
catalyzed reactions
can run faster
because less energy
is required to achieve
the transition state
This is instead of
adding heat; heat is
an inefficient means
of speeding up
reactions since it
simply is a means of
increasing the
random jostlings of
molecules
Enzyme-mediated Catalysis
= Subtle
application
of energy
Mechanisms of Catalysis
Active sites can hold two or more
substrates in proper orientations so that
new bonds between substrates can form
Active sites can stress the substrate into
the transition state
Active sites can maintain conducive
physical environments (e.g., pH)
Active sites can participate directly in the
reaction (e.g., forming transient covalent
bonds with substrates)
Active sites can carry out a sequence of
manipulations in a defined temporal order
(e.g., step A step B step C)
Catalysis as Viewed in 3D
Active site is
site of
catalysis
The rest of an
enzyme is involved in
supporting active
site, controlling
reaction rates,
attaching to other
things, etc.
Induced Fit (Active Site)
Induced fit not only allows the enzyme to
bind the substrate(s), but also provides a
subtle application of energy (e.g., bending
chemical bonds) that causes the substrate(s)
to destabilize into the transition state
Enzyme Saturation
Su
st
b
ate
r
Pr
Enzyme Activity at Saturation is a
Function of Enzyme Turnover Rate
od
uc
t
Enzyme Saturation
Turnover rate
Non-Specific Inhibition of Enzyme Activity
Reduced rate
of chemical
reaction
Reduced
enzyme fluidity
Turnover
rate
Change in
R group
ionization
Instability
& shape
change
(too fluid)
Denatured?
Change in
R group
ionization
Even at saturation, rates
of enzymatic reactions
can be modified
Activators of Catalysis
Metal Ion or =
Organic Molecule
Dont worry about
apoenzyme and
holoenzyme
= Organic
Cofactor
Polypeptide
Specific Inhibition
Competitive
inhibitors can
be competed
off by
supplying
sufficient
substrate
densities
Non-competitive
inhibitors cannot
be competed off
by substrate
Allosteric Interactions
Reversible
interactions,
sometimes on,
sometimes off,
dependent on
binding
constant and
density of
effector
Cooperativity
Cooperativity
is when the
activity of
other
subunits are
increased by
substrate
binding to
one subunits
active site
Feedback Inhibition
Energy-Metabolism Regulation
Enzyme Localization
Organization of
Electron
Transport
Chain of
Cellular
Respiration:
Substrate
Enzyme
Product
Enzyme chains
are co-localized
Enzymes in single pathway
may be co-localized so that
the product of one enzyme
increases the local
concentration of the
substrate for another
The End