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Chapter 8: Introduction to
Metabolism: all the chemical processes of an
I) Anabolic pathways – consume energy to build
complicated molecules from simpler
II) Catabolic pathways – release energy by
breaking down complex molecules to simpler
Metabolic reactions may be coupled
• energy released from a catabolic reaction can
be used to drive an anabolic reaction.
Kinetic vs. Potential Energy
Kinetic energy – the energy of motion
 Heat (thermal energy) is KE expressed in random
movement of molecules
 light
Potential energy – energy that matter possesses because
of its location or arrangement.
 Chemical energy is PE sorted in molecules because of
the arrangement of nuclei and electrons in atoms
* An object on a hill or water behind a dam
Thermodynamics – study of energy transformation
First Law of Thermodynamics
 Energy can be transferred and transformed, but it
cannot be created or destroyed.
Second Law of Thermodynamics
 Every energy transfer or transformation makes the
universe more disordered (increases entropy)
 Entropy is a quantitative measure of disorder that is
proportional to randomness
• Free Energy
Free energy (G) is the portion of a system’s energy
available to do work
Free energy equation:
 G = H - TS G = change in free energy
H = change in total energy
S = change in entropy
T = temperature(Kelvin)
What’s the significance of free energy?
• It indicates whether a reaction will occur
A spontaneous reaction will occur without
additional energy.
• Exergonic reactions – a reaction that
proceeds with a net loss of free energy
• Endergonic reaction – an energy-requiring
reaction that absorbs free energy from its
surrounding [REQUIRES ENERGY]
• Exergonic ReactionChem. - products have less free
energy than reactants(decomposition)
Spontaneous rxn and G is negative
• Endergonic Reaction -products store more free energy
than reactants(synthesis)
Non-spontaneous rxnG is positive
In cellular metabolism, endergonic reactions are driven
by coupling them to exergonic reactions.
 ATP plays a critical role in this energy coupling
ATP (Adenosine Triphosphate):
ATP is the immediate source of energy that
drives most cellular work, which includes:
• Mechanical work – muscle contraction,
cytoplasmic flow, chromosome movement
during mitosis
• Transport work – pumping substances across
• Chemical work – endergonic processes of
• What type of macromolecule is ATP?
ATP is a nucleotide with unstable phosphate
bonds that the cell hydrolyzes for energy to drive
endergonic reactions.
Consists of:
 Adenine, a nitrogenous base
 Ribose, a five-carbon sugar
 Chain of 3 phosphate groups
the terminal phosphate bond is hydrolyzed
producing ADP (adenosine diphosphate).
• ATP + H2O  ADP + Phosphate + ENERGY!!
• The hydrolysis of the phosphate bonds is
exergonic as the system shifts to a more
stable state.
• Regeneration of ATP:
 The reaction is endergonic
 driven by the exergonic process of
cellular respiration
 A chemical reaction will occur
spontaneously if it releases free energy (-G),
but it may occur too slowly to be effective in
living cells.
 Biochemical reactions require enzymes to
speed up and control reaction rates.
• Catalysts are chemical agents that accelerate
a reaction w/o being permanently changed in
the process; they are recycled
• Enzymes are biological catalysts, which are
usually proteins.
Before a reaction can occur, the reactants must absorb
activation energy.
 The transition state is the unstable condition of
reactant molecules that have absorbed sufficient free
energy to react.
Molecules react very slowly at cellular temperatures
Enzymes allow reactions to occur at cellular temps by
lowering the activation energy.
Substrate – The substance an enzyme acts on and makes
more reactive
 An enzyme binds to the substrate and catalyzes its
conversion to product. The enzyme is released in
original form.
Active site – A region of the enzyme which binds to the
 Usually a pocket or groove on the protein’s surface
What determines the specificity of an enzyme?
 Three-dimensional shape of enzyme
Steps in the catalytic cycle of enzymes:
1-Substrate binds to the active site forming an enzyme-substrate
complex. The substrate is held in the active site by chemical
interactions (e.g. H-bonds and ionic bonds).
2-Side chains of a few amino acids in the active site catalyze the
conversion of substrate to product.
3-Product departs active site and the enzyme emerges in its original
Since enzymes are used over and over, they can be effective in very
small amounts.
 Active site can hold two or more reactants in the
proper position so they may react
 Induced fit of the enzyme’s active site may distort the
substrate’s chemical bonds, so less thermal energy
(lower G) is needed to break them during a reaction.
 Active site might provide a micro-environment
conductive to a particular type of reaction (e.g. low pH)
Temperature and pH
Enzyme reaction rate increases with increasing
temp.(to a point)
Optimal pH range for most enzymes is pH 6-8.
Cofactors – small nonprotein molecules that are required for proper
enzyme catalysis- called coenzymes if organic-may be active site
residents, or just bind temporarily with substrate
Enzyme Inhibitors
• Competitive inhibitors – chemicals that resemble an enzyme’s
normal substrate and compete with it for the active site
• Noncompetitive inhibitors – enzyme inhibitors that do not enter
the enzyme’s active site, but bind to another part of the enzyme
 Causes the enzyme to change shape so the active site
cannot bind the substrate
Allosteric regulation
Requires enzymes with allosteric sites- remote
from the active site
- A product of a pathway can shut down an
enzyme in the pathway – this is feedback
inhibition – ATP can act as a noncompetitive
yet AMP can allosterically activate the same
enzyme as an activator
One substrate’s binding to an active site can
prime the enzyme for additional substrate
So are inhibitors bad or good?
 DDT – inhibits key enzymes in the nervous
 Penicillin – block active site of an enzyme
that many bacteria use to build cell walls.