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Chapter 6—An Introduction to
Metabolism
YOU are an energy transformer!
I. Energy, Metabolism, & Life
• Metabolism—
– All of an organism’s chemical reactions
• Emergent property arising from interactions
between molecules
• Manages the material and energy resources of
cells (supply and demand)
The Complexity of Metabolism
Catabolic vs. Anabolic
• Catabolic pathways— – Release energy by breaking down complex molecules
to simpler ones • Ex.—Cellular respiration
» C6H12O6 + 6O2 → 6H2O + 6CO2 + energy (ATP)
• Anabolic pathways— – Consume energy to build complex molecules from
simpler ones • Ex.—amino acids → protein
• Bioenergetics— – How organisms manage their energy resources
Let’s Review… Energy!!
• Energy = ability to rearrange matter
• Kinetic vs. Potential
• Chemical energy—potential or kinetic?
• All organisms are energy transformers
Yes, the Laws of Thermodynamics
Apply
• Thermodynamics—
– Energy transformations happening in a collection of matter (such as an organism)
– Organisms—closed or open systems?
• 1st Law of Thermodynamics— – Energy is transferred or transformed, but not created or
destroyed
» Examples? Cars? Plants?
• 2nd Law of Thermodynamics— – Energy transfers and transformations increase entropy
(disorder, randomness) of the universe
» Examples? Cars? People?
What is the ultimate fate of all
energy?
• HEAT—
– energy in its most random state, its lowest
grade
– All chemical energy eventually becomes heat
• The _________ of energy in the universe
is constant, but its _________ is not.
How can we reconcile the unstoppable
increase in entropy of the universe (2nd
Law)—with the orderliness of life?
• Organisms are islands of low entropy in an
increasingly random universe (because of
constant energy input!)
We owe it all to free energy…
• Free Energy—
– Portion of a system’s energy that can perform
work when temp. is uniform throughout the
system (i.e. a living cell)
• Organisms live at the EXPENSE of free energy
acquired from the surroundings
Relationship of Free Energy to Stability,
Work Capacity, & Spontaneous Change
Figure 6.5
Free Energy = Spontaneous
Change
• Free Energy (G)
– Measure of instability, the tendency to change
to a more stable state
– G = H – TS
• H = total energy of the system
• S = entropy of the system
• T = absolute temperature in Kelvin (K = °C + 273)
• Free energy < Total energy due to entropy
In any spontaneous process, the
free energy of a system decreases
• ∆G = ∆H – T∆S
– For a process to occur spontaneously, ∆G < 0
– At equilibrium, ∆G = 0 (no work can be done, death in living things)
• The greater the decrease in free energy, the more work can be performed
– Nature runs ―downhill‖
Exergonic vs. Endergonic
Photosynthesis = +686 kcal/mole
Respiration = -686 kcal/mole
ATP Powers Cellular Work
• Cellular Work = mechanical, transport,
chemical
• Energy coupling—
– ATP uses exergonic processes to drive
endergonic ones
ATP + H2O → ADP + Pi
∆G = -7.3 kcal/mol (standard conditions)
Exergonic
Loaded spring
How ATP Performs Work
ATP transfers its
phosphate group to
other molecules
making them unstable,
and more likely to react
chemically
Endergonic +
Exergonic = Overall
reaction is
spontaneous (∆G
The Regeneration of ATP
ATP couples the cell’s energy-yielding processes to the energy-
consuming ones.
ADP + Pi → ATP + H2O
∆G = +7.3 kcal/mol (endergonic)
ATP is recyclable
10 million/per
second/per cell
II. Enzymes speed up metabolic
reactions by lowering energy barriers
• Enzyme—
– Catalytic protein that changes the rate of a
reaction w/o being consumed by it
Enzyme for this reaction?
Energy profile of an exergonic
reaction
Activation Energy
(EA)—
Investment of
energy for starting
a reaction
(required to break
bonds in
reactants)
Usually comes
from heat in
surroundings
Why is EA important?
Enzymes lower the barrier of
activation energy
Enzymes speed up
reactions by
lowering EA
(transition
state can
occur at a
lower temp,
safe for cells)
ΔG stays the
same!
Enzyme + Substrate =
• Substrate— – The reactant an enzyme acts on
Enzyme
• Substrate(s) → Product(s)
Sucrase
• Sucrose + H2O → Glucose + Fructose
• Enzymes are substrate specific—each type of enzyme catalyzes a particular reaction – Specificity results from 3-D shape of enzyme
Catalytic Cycle of an Enzyme
The Active Site—‖where the magic
happens‖
• Substrate binds to active site to form enzyme-substrate complex (induced fit)
– Held in place by weak bonds (hydrogen/ionic)
• R groups/side chains catalyze the change from substrate to product
• Enzyme emerges unchanged, ready to be used again (and again and again…)
– FAST—1,000 reactions/second (typical enzyme)
How do they do that?
• Enzymes mechanisms:
– Induced fit
– Microenvironment
– Direct participation
Environmental factors affect enzyme activity
• i.e. pH, temp, chemicals
• Enzyme Helpers— – Cofactors
(inorganic) • Zn, Fe, Cu
– Coenzymes (organic)
• vitamins
Inhibition of Enzyme Activity
• Competive vs.
Noncompetive
Inhibitors
III. The Control of Metabolism
• Allosteric regulation
– Allosteric activators and inhibitors
Feedback Inhibition
• An end product
switches off a
metabolic pathway
• Allosterically inhibits
enzyme early in
pathway
Localizing enzymes within cells
orders metabolism