Download ch. 8 An Introduction to Metabolism

An Introduction to
Metabolism
Ch. 8
AP Biology
Ms. Haut
Metabolic Pathways

Catabolic Pathways
 Release energy by breaking down complex
molecules into simpler ones
 Cellular respiration provides energy for
cellular work
C6H12O6 + 6O2  6CO2 + 6H2O + energy

Energy released drives anabolic reactions
Metabolic Pathways

Anabolic Pathways
 Consume energy by building molecules
 Photosynthesis uses energy
6CO2 + 6H2O
energy
C6H12O6 + 6O2
Organisms Transform Energy
Solar
Energy
(EK)
Plants (glucose)
Stored in
chemical bonds
(EP)
Animals
Break down
Sugars;
Some used (EK),
some stored in
chemical bonds
(EP)
Energy

Kinetic energy is energy
associated with motion


Diving converts
potential
energy to
kinetic energy.
Climbing up converts
kinetic energy of
muscle movement to
potential energy.
In the water, the
diver has less
potential energy.
Heat (thermal energy) is
kinetic energy associated
with random movement of
atoms or molecules
Potential energy is energy that
matter possesses because of
its location or structure


On the platform,
the diver has
more potential
energy.
Chemical energy is potential
energy available for release
in a chemical reaction
Energy can be converted from
one form to another
Laws of Thermodynamics


First Law—Energy can be transferred, but never
created or destroyed
Second Law—Every energy transfer results in
increased entropy (randomness in the universe)


Some of the energy is converted to heat
Reactions occur spontaneously
Heat
Chemical
energy
First law of thermodynamics
CO2
H2O
Second law of thermodynamics
Free Energy


Organisms live at the expense of free
energy (portion of a system’s energy
available for work) acquired from the
surroundings
Free energy is needed for spontaneous
changes to occur
Gibbs-Helmholtz Equation
G = H - TS
Free Total Temp
energy energy (K)
entropy
enthalpy


Can be used to determine if a reaction is
spontaneous
Spontaneous reactions occur in systems
moving from instability to stability
High
energy
Low
energy
Gibbs-Helmholtz Equation
G =  H - T  S
Measure
of heat
in the
reaction


In chemical reactions, reactions absorb
energy to break bonds
Energy is then released when bonds form
between rearranged atoms of the product
Key Importance of G





Indicates amount of energy available for work
Indicates whether a reaction will occur
spontaneously (low G)
G decreases as reaction approaches equilibrium
G increases as reaction moves away equilibrium
G = 0 when a reaction is in equilibrium
Chemical Reactions
Exergonic
Endergonic
Chemical products have
lower G than reactants
Products store more G than
reactants
Reaction releases energy
Reaction requires energy
input (absorbs)
G = negative value
G = positive value
Spontaneous
Non spontaneous
In cellular metabolism, exergonic reactions drive
endergonic reactions
Rate of Reactions


G indicates spontaneity not speed of reaction
Spontaneous reactions will occur if it releases free
energy (- G ), but may occur too slowly to be
effective in living cells


Can leave sucrose in sterile water for yrs. with hydrolysis
occuring; add sucrase and reaction will hydrolyze in
seconds
Biochemical reactions require enzymes to speed up
and control reaction rates
ATP couples exergonic reactions to
endergonic reactions

A cell does three main kinds of work:




Mechanical
Transport
Chemical
To do work, cells manage energy resources by
energy coupling, the use of an exergonic
process to drive an endergonic one
ATP Powers Cellular Work
Unstable
Bonds—can release
energy when
broken
Energy transferred to
another molecule (phosphorylated intermediate)
with the phosphate
Less stable
More stable
LE 8-11
Pi
P
Motor protein
Protein moved
Mechanical work: ATP phosphorylates motor proteins
Membrane
protein
ADP
+
Pi
ATP
Pi
P
Solute transported
Solute
Transport work: ATP phosphorylates transport proteins
P
NH2
Glu
+
NH3
+
Pi
Glu
Reactants: Glutamic acid
and ammonia
Product (glutamine)
made
Chemical work: ATP phosphorylates key reactants
The Regeneration of ATP

•
•
ATP is a renewable resource that is regenerated by
addition of a phosphate group to ADP
The energy to phosphorylate ADP comes from
catabolic reactions in the cell
The chemical potential energy temporarily stored in
ATP drives most cellular work
ATP
Energy for cellular work
(endergonic, energyconsuming processes)
Energy from catabolism
(energonic, energyyielding processes)
ADP +
P
i
Enzymes


Catalyst—chemical agent that speeds up a chemical
reaction without being consumed by the reaction
Hydrolysis of sucrose by the enzyme sucrase is an
example of an enzyme-catalyzed reaction
Sucrose
C12H22O11
Glucose
C6H12O6
Fructose
C6H12O6
The Activation Energy Barrier


Every chemical reaction
between molecules
involves bond breaking
and bond forming
The initial energy needed
to start a chemical
reaction is called the free
energy of activation, or
activation energy (EA)
Activation energy is often
supplied in the form of
heat from the
surroundings
A
B
C
D
Transition state
Free energy

A
B
C
D
EA
Reactants
A
B
G < O
C
D
Products
Progress of the reaction
Enzymes

Catalytic proteins that speed up metabolic
reactions by lowering energy barriers
1. Reactants must absorb
energy to reach transition
state (unstable)
2. Rxn occurs and energy is
released as new bonds
form to make products
3. G for overall rxn is
difference b/w G of
products and G of
reactants
Substrate Specificity of Enzymes



Substrate—reactant that an enzyme acts
Substrate binds to the active site on the enzyme
Induced fit of a substrate brings chemical groups of
the active site into positions that enhance their
ability to catalyze the reaction
Induced Fit Model of Enzymatic
Reactions
Substrates enter active site; enzyme
changes shape so its active site
embraces the substrates (induced fit).
Substrates held in
active site by weak
interactions, such as
hydrogen bonds and
ionic bonds.
Substrates
Enzyme-substrate
complex
Active
site is
available
for two new
substrate
molecules.
Enzyme
Products are
released.
Substrates are
converted into
products.
Products
Active site (and R groups of
its amino acids) can lower EA
and speed up a reaction by
• acting as a template for
substrate orientation,
• stressing the substrates
and stabilizing the
transition state,
• providing a favorable
microenvironment,
• participating directly in the
catalytic reaction.
How do Enzymes Work?




Active site holds 2 or more reactants in the
proper position to react
Induced fit may distort chemical bonds so
less thermal energy is needed to break
them
Active site may provide micro-environment
that aids a reaction (localized pH)
Side chains of amino acids in active site
may participate in reaction
Enzyme Activity

A cell’s physical and chemical environment
affects enzyme activity

Each enzyme has optimal environmental
conditions that favor the most active enzyme
conformation
Effects of Temperature


Optimal temp. allows greatest number of molecular
collisions without denaturing the enzyme
Reaction rate  when temperature 
 Kinetic energy increases and collisions increases
 Beyond optimal temperature, reaction rate slows
 Too low, collisions b/w substrate and active site
don’t occur fast enough
 Too high, agitation disrupts weak
bonds of the tertiary structure of
enzyme (enzyme unfolds)
Effects of pH


Optimal pH range for most enzymes is pH 6
–8
Beyond optimal pH, reaction rate slows
 Too low (acidic) H+ ions interact with
amino acid side-chains and disrupt weak
bonds of the tertiary structure of enzyme
 Too high (basic) OH- ions interact with
amino acid side-chains and disrupt weak
bonds of the tertiary structure of enzyme
Cofactors

Small non-protein molecules that are required
for proper enzyme catalysis


Inorganic—Zn, Fe, Cu
Coenzymes—vitamins
Effects of Substrate Concentration


The higher the
[substrate], the faster
the rate (up to a limit)
If [substrate] high
enough, enzyme is
saturated with substrate


Reaction rate depends
on how fast the active
site can convert
substrate to product
When reaction is
saturated with substrate,
you can speed up
reaction rate by adding
more enzyme
Effects of Enzyme Inhibitors
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Active site
Enzyme

Competitive inhibitors—
chemicals that resemble an
enzyme’s normal substrate
and compete with it for the
active site


Blocks active site from
substrate
If reversible, can be
overcome by
increasing substrate
concentration
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Normal binding
Active site
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Enzyme
Competitive
inhibitor
Normal binding
A competitive
inhibitor mimics the
substrate, competing
for the active site.
Competitive inhibition
Competitive
inhibitor
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
Competitive inhibition
active site no longer
Noncompetitive inhibitor
functions.
Noncompetitive inhibition
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
Competitive Inhibitor
A substrate can
bind normally to the
active site of an
enzyme.
Substrate
Effects of Enzyme Inhibitors
Active site
Enzyme

Noncompetitive
inhibitors—chemicals
that bind to another
part (allosteric site)of
an enzyme


Causes enzyme to change
shape and prevents
substrate from fitting in
active site
Essential mechanism in
cell’s regulating
metabolic reactions
A substrate can
bind normally to the
A competitive
active
site of an
inhibitor mimics
the
enzyme.
substrate, competing
for the active site.
Normal binding
Substrate
Active site
Competitive
inhibitor
Enzyme
Normal binding
Competitive inhibition
A competitive
inhibitor mimics the
substrate, competing
for
the active site.
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Competitive
inhibitor
Allosteric site
Competitive inhibition
Noncompetitive inhibitor
Noncompetitive inhibition
A noncompetitive
inhibitor binds to the
enzyme away from the
active site, altering the
conformation of the
enzyme so that its
active site no longer
functions.
Noncompetitive inhibitor
Noncompetitive inhibition
Negative Feedback
Metabolic Control often Depends on Allosteric
Regulation



Allosteric enzymes have 2 conformations,
catalytically active and inactive
Binding of an activator to the allosteric site
stabilizes active conformation
Binding of an inhibitor (noncompetitive) to the
allosteric site stabilizes inactive conformation
Control of Metabolism
Initial substrate
(threonine)
Active site
available
Isoleucine
used up by
cell
Threonine
in active site

Enzyme 1
(threonine
deaminase)
Intermediate A
Feedback
inhibition
Enzyme 2
Active site of
enzyme 1 can’t
bind
Intermediate B
theonine
pathway off
Enzyme 3
Isoleucine
binds to
allosteric
site
Intermediate C
Enzyme 4
Intermediate D
Enzyme 5
End product
(isoleucine)

In feedback inhibition,
the end product of a
metabolic pathway
shuts down the
pathway
Feedback inhibition
prevents a cell from
wasting chemical
resources by
synthesizing more
product than is needed
Specific Localization of Enzymes
Within the Cell



Structures within the cell help bring order to
metabolic pathways
Some enzymes act as structural components
of membranes
Some enzymes reside in specific organelles,
such as enzymes for cellular respiration being
located in mitochondria
Mitochondria,
sites of cellular respiration
1 µm