Enzymes and Introduction to Metabolism
Chapter 8: Cell Metabolism Introduction
Catabolic vs. Anabolic Pathways
• Catabolic pathways break large molecules down into simpler compounds in order to RELEASE
ENERGY!
o These are downhill reactions.
• Anabolic pathways combine small molecules using energy to make more complex compounds.
o These are uphill reactions.
o Called Biosynthesis.
• Energy released from the downhill reactions are used to power the uphill reactions.
o Catabolic reactions drive Anabolic reactions.
Laws of Energy Transformation
• 1st Law of Thermodynamics
o The energy of the universe is constant.
You can turn the energy in a hamburger into ATP inside your body, but that energy
is transferred or transformed, not created and not destroyed.
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• 2 Law of Thermodynamics
o Every energy transfer or transformation increases the entropy (disorder) of the universe.
When you turn that hamburger into ATP in your mitochondria, some of the energy
contained in the hamburger is lost as heat in the transfer.
Then when you convert that ATP into motion by using it to power your muscles,
even more energy is lost as heat.
• Entropy = Disorder.
Spontaneous Reactions
• Any reaction that increases the entropy of the universe is spontaneous.
o Spontaneous does NOT mean the reaction will occur quickly, only that given enough time it
will occur without an input of energy.
• So:
o Catabolic Reactions = Spontaneous. (Don’t need energy.)
o Anabolic Reactions = Not Spontaneous. (Do need energy.)
Free-Energy Change ∆G
• Free-Energy is the amount of energy available in a system (reaction) available to make that
reaction occur.
• The change in free-energy is ∆G, or the amount of energy available AFTER a reaction.
o ∆G is calculated (simply) by ∆G = Gfinal - Ginital
o If ∆G = positive then:
The amount of energy available in a reaction is MORE after the reaction is over.
Anabolic Reaction, not spontaneous, we stored energy in a larger molecule.
o If ∆G = negative then:
The amount of energy available in a reaction is LESS after the reaction is over.
Catabolic Reaction, spontaneous, we broke down molecules and free energy for use.
• Equilibrium
o If no work is being done, no catabolic nor anabolic reaction, then a system is at
equilibrium.
o ∆G = 0, and the system if living, is now dead.
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Free Energy and Metabolism
• Exergonic Reactions
o These reactions release energy, have a ∆G = negative, and are catabolic.
• Endergonic Reactions
o These reactions require energy, have a ∆G = positive, and are anabolic.
• For instance:
C6H12O6 + O2 → 6 CO2 + 6 H2O
•
o Cellular Respiration has a ∆G = -686 kcal / mol. This means for every mole of glucose we
get 686 kcal of energy available for work.
o Is this reaction:
Exergonic or Endergonic?
Catabolic or Anabolic?
Spontaneous or Not Spontaneous?
Why?
For instance:
6 CO2 + 6 H2O → C6H12O6 + 6 O2
o Photosynthesis has a ∆G = +686 kcal / mol. This means for every mole of glucose we make
we need to input 686 kcal of energy. Where does photosynthesis get this energy?
o Is this reaction:
Exergonic or Endergonic?
Catabolic or Anabolic?
Spontaneous or Not Spontaneous?
Why?
The Power of ATP
• We use ATP to couple
exergonic reactions to
endergonic reactions.
• A Cell does 3 Types of Work:
o Mechanical Work: like
the contraction of
muscles cells and
beating of cilia and
flagella.
o Transport Work: like
the pumping of
substances across a membrane (active transport).
o Chemical Work: the pushing of endergonic reactions (making polymers from monomers).
• ATP is the go-between for exergonic and endergonic reactions.
o Exergonic reactions release energy which is transferred to ATP as in Cellular Respiration.
o Endergonic reactions then use the energy in ATP to drive their processes as in protein
synthesis.
• ATP performs work by phosphorylation.
o ATP has 3 high energy phosphate bonds.
o By transferring the 3rd phosphate group to another molecule, some energy is transferred as
well.
o This process turns ATP into ADP, which can be recycled back into ATP for use again.
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Enzymes
• An enzyme is a protein catalyst that lowers the EA, or activation energy, of a reaction.
o As a catalyst it is never
consumed in the reaction and
can be used.
o The enzyme ONLY lowers
the EA.
o It can NOT change the ∆G.
An endergonic reaction can
not be turned exergonic by
an enzyme.
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•
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Substrate Specificity
o Every enzyme has an active site, a location where a molecule (substrate) in the reaction
binds.
o This active site is specific, meaning the enzyme binds with only ONE specific substrate.
Think of a glove made only for your hand.
Temperature and pH
o Because enzymes are proteins,
environmental conditions can
change the shape and therefore
the functionality of the protein.
Enzyme Inhibitors
o Competitive Inhibitors
These compete for the
active site and prevent the
proper substrate from
binding.
This can be overcome by
increasing the
concentration of the
enzyme.
o Noncompetitive Inhibitors
These bind to another
location other than the
active site and cause the
enzyme to no longer
function.
Very often toxins and
poisons work in this way.
Antibiotics often will
disable enzymes in bacteria
necessary for them to
survive.
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