Transcript Chapter 6 Metabolism: Energy and Enzymes

Chapter 6 Metabolism:
Energy and Enzymes
Metabolism
• The totality of an organism's chemical
reactions, consisting of catabolic and
anabolic pathways
• Catabolic pathway: A metabolic
pathway that releases energy by
breaking down complex molecules to
simpler compounds. (process?)
• Anabolic pathway: A metabolic
pathway that synthesizes a complex
molecule from simpler compounds.
(process?)
Energy
• The capacity to do work; Cells must
continually use energy to do
biological work.
Types of energy
• Kinetic Energy: The energy of motion,
which is directly related to the speed
of that motion. Moving matter does
work by imparting motion to other
matter.
• Potential Energy: The energy stored
by matter as a result of its location or
spatial arrangement.
Transformations
between kinetic
and potential
energy.
Energy as explained by the Laws
of Thermodynamics
• First Law: (conservation of energy)
– Energy cannot be created nor destroyed; it can
only change from one from to another.
– Energy is found in covalent bonds b/t atoms
• Second Law: Every energy transfer or
transformation increases the entropy of the
universe.
– Entropy: A quantitative measure of disorder or
randomness.
– Energy cannot be changed from one form to
another with out a loss of usable energy (heat =
the most random form of energy)
Chemical energy: Energy stored in the chemical bonds of
molecules; a form of potential energy.
•Energy is trapped inside molecules; the more complex the
molecule the more energy it has, visa versa.
QOD
How does the second law of
thermodynamics help explain the
diffusion of a substance across a
membrane?
QOD answer
The second law is the trend toward
randomness. Equal concentrations of
a substance on both sides of a
membrane is a more random
distribution than unequal
concentrations.
Diffusion of a substance to a region
where it is initially less concentrated
increases entropy, as mandated by
the second law.
Energy Transformations
In a reaction:
A+BC+D
A and B are the reactants
C and D are the products
• Free Energy (G): The amount of energy
that is free to do work after a
chemical reaction takes place.
• Change in Free Energy (DG)
Free Energy
• Exergonic Reaction: the free energy of
reactants are greater than free energy of products
(-DG) happens spontaneously.
• Endergonic Reaction: the free energy of the
reactants are less than the free energy of products
(+DG) requires input of energy.
- Because systems at
equilibrium are at a minimum of
G and can do no work, a cell
that has reached metabolic
equilibrium is dead! The fact
that metabolism as a whole is
never at equilibrium is one of
the defining features of life.
- Cells use the products from
one reaction as the reactants in
a second reaction, which pulls
the first reaction in one
direction.
- Energy is utilized more
efficiently in small increments
QOD
• Cellular respiration uses glucose,
which has a high level of free
energy, and releases CO2 and water,
which have low levels of free energy.
Is respiration spontaneous or not? Is
it exergonic or endergonic? What
happens to the energy released from
glucose?
QOD Answer
• Cellular respiration is a spontaneous
and exergonic process. The energy
released from glucose is used to do
work in the cell, or is lost as heat.
Coupling Reactions
When the energy released by exergonic
reactions is used to drive an endergonic
reaction.
• ATP  ADP + P
– Highly Exergonic
– The breakdown of ATP is coupled to all of a
cells endergonic reactions
ATP: The Energy Currency of Cells
• When cells require energy they “spend”
(breakdown) ATP.
• Cells are constantly producing ATP because it is
in high demand.
Function of ATP
ATP is constantly recycled in
cells (more efficiency)
Respiration
Photosynthesis
6.3 Metabolic
Pathways and
Enzymes
Reactions in cells are orderly
• Metabolic Pathways: orderly sequence of chemical
reactions, where each one is catalyzed by a
specific enzyme.
• Enzymes: Proteins that act as catalysts that speed
up chemical reactions.
– Not part of the reaction, they only aid in the process.
– Enzyme does not change itself only the reactants of the
reaction.
– Specific to reaction (only catalyze one reaction)
• Substrate: a reactant in an enzymatic reaction
Energy of Activation (EA)
• The amount of free energy that must
be added to cause molecules to react
– Heating
Example:
AB + CD  AC + BD
• Enzymes speed up reactions by
lowering the energy of activation (EA)
Enzyme-Substrate Complexes
Enzymes lower the energy of activation by
forming a complex with their substrate.
• Active Site: small region on surface of
enzyme where substrate binds.
• Induced-fit model: when substrate binds
to enzyme, the active site changes
shape to facilitate the reaction.
Enzyme-Substrate Complexes
• Enzymes do not get used up in a reaction
(substrate does), so only a small amount
of enzyme is required.
• Every cell reaction requires a specific
enzyme; therefore enzymes are named
after their substrates by adding the
ending “ase”.
Speed of Enzyme Activity
Four factors that affect enzyme
activity.
1. Substrate Concentration
2. Temperature
3. pH
4. Enzyme Inhibition
Substrate Concentration
Enzyme activity increases as substrate
concentration increases.
• Due to higher probability of collisions
between substrate molecules and
enzyme.
• As the enzyme’s active sites are
filled the enzyme activity levels off
(reaches a max rate).
Temperature and pH
• As temperature rises, enzyme
activity increases b/c there are more
molecular collisions.
• If temperature rises beyond a certain
point, the activity of the enzyme
declines rapidly ( enzyme [protein] is
denatured ).
• Each enzyme has an optimal pH that
maintains its normal shape.
• A change in pH causes denaturation
of enzyme which decreases activity.
Enzyme Inhibition
When an enzyme is prevented from
binding with its substrate. 2 types
1. Competitive Inhibition: another
similar molecule competes with the
substrate for the enzyme’s active
site. Decreases product formation
2. Noncompetitive Inhibition: a
molecule binds to the Allosteric site
which changes the shape of the
active site and thus it’s ability to
bind to substrate.
QOD
Malonate is a competitive inhibitor of
the enzyme succinate
dehydrogenase. Describe how
malonate would prevent the enzyme
succinate dehydrogenase from acting
on its normal substrate succinate.
QOD Answer
As a competitive inhibitor, malonate
binds to the active site of succinate
dehydrogenase and so prevents the
normal substrate, succinate, from
binding.
Enzyme Inhibition
Feedback Inhibition: A method of metabolic
control in which the end product of a
metabolic pathway acts as an inhibitor of
an enzyme within that pathway.
Inhibitor can be either Competitive or
Noncompetitive
• When product is abundant, there is more
inhibition and enzyme activity drops.
• When product is used up, there is less
inhibition and enzyme activity increases.
Fig 6.8
An example of feed back inhibition that directs
different metabolic pathways.
What would happen if there was high levels of Q in the
cytoplasm?
What would happen if there was high levels of O in the
cytoplasm?
Metabolic Pathways & Redox
reactions
Oxidation: The loss of electrons from a substance
involved in a redox reaction.
Reduction: The addition of electrons to a substance
involved in a redox reaction.
• In oxidation-reduction reactions, electrons pass
from one molecule to another
– Happens at the same time
– Photosynthesis and respiration are examples
• In living things H+ ions accompany e-, so
oxidation is a loss of H atoms and reduction is a
gain of H atoms
Photosynthesis
6CO2 + 6H2O  C6H12O6 + 6O2
• H atoms are transferred from water to
carbon dioxide. (water is oxidized, carbon
dioxide is reduced)
• The coenzyme NADP+ is needed as an
electron acceptor to remove hydrogen
from water:
NADP+ + 2e- + H+  NADPH
Respiration
C6H12O6 + 6O2  6CO2 + 6H2O
• Opposite of photosynthesis (glucose is
oxidized and oxygen is reduced)
• Respiration requires the coenzyme NAD+ as
the electron acceptor to remove Hydrogen
atoms from glucose:
NAD+ + 2e- + H+  NADH