Transcript Metabolism: Energy and Enzymes

Metabolism: Energy and
Enzymes
Chapter Outline
• I) Cells and the Flow of Energy
• II) Metabolic Reactions and Energy
Transformations
• III) Metabolic Pathways and Enzymes
I) Cells and the Flow of Energy
• Energy is the ability to do work or bring
about a change
• Cells use acquired energy to maintain their
organization, grow, develop and reproduce
• There are two fundamental types of
energy
Potential Energy
• Potential = stored
energy waiting to do
work  its capacity to
do work is not being
used at this moment
(e.g. gravitational,
elastic, chemical
forms)
Kinetic Energy
• Kinetic = energy at
work or the energy of
movement (e.g. light,
heat, mechanical
forms)
1) Cells and the Flow of Energy
• Under the right conditions, energy can be
transformed from one type to another
(potential < kinetic) and from one form to
another (e.g. chemical> thermal)
• These energy transformations follow the
Laws of Thermodynamics
Energy Transformations
The First Law of Thermodynamics
• The First Law of Thermodynamics (sometimes
called the Law of Conservation of Energy) states
the amount of energy in any process is constant
• Energy can not be created nor destroyed by
ordinary processes, only transformed from one
form to another form, and between potential
and kinetic types
• With each transformation within any system,
some energy is released to the surroundings as
“useless” heat energy (which can not do useful
work)
“useless”
Heat
energy
“useless”
Heat
energy
The Second Law of Thermodynamics
• The Second Law of Thermodynamics
(sometimes called the Entropy Law) states that
the amount of “useful” energy decreases when
energy transformations occur (because
“useless” heat energy is released)
• Therefore, there is a tendency for all systems to
reach the lowest possible “useful” energy level
• Entropy is the term for the measure of the
amount of disorder (loss of higher “useful”
energy) in a system
• Therefore, there is a tendency for all systems to
reach the highest possible entropy level
“Useful” Energy = Chemical Energy
High
Low
“Useless” Energy = Heat Energy
Low
High
Entropy = State of Disorder
Low
High
• As an example, plants
trap light energy from the
sun in the process of
photosynthesis
(anabolism) to produce
carbohydrates
• Plants use the
carbohydrates to provide
energy to maintain
themselves (i.e. growth,
reproduction,
homeostasis, etc)
High useful
energy
Low useful
energy
High useful energy
Low useful energy
• Plants lose their energy when heterotrophic organisms
feed on them through digestion (catabolism)
• Plants become disordered (increase in entropy) as they
pass through the digestive system of an organism as the
latter processes the plants for nutrients in order to
maintain the organization of its cells
What is the Source of all Energy on
Earth?
• Energy must always be put into a system
in order to sustain it, because all of the
chemical reactions that occur in cells are
decreasing the amount of “useful” energy
and increasing the amount of “useless”
energy
• Where does this input of energy come
from?
The Sun
• The Sun constantly
loses useful light
energy and increases
in entropy, in order to
maintain life on Earth
II) Metabolic Reactions and Energy
Transformations
• Metabolism = the sum of all of the
chemical reactions in a cell
• Anabolism = reactions that synthesize
complex molecules from simple molecules
• Catabolism = reactions that breakdown
complex molecules into simple molecules
Chemical Reactions
• Reactants = Substrate = the substances
that start a chemical reaction
• Products = the substances that form as a
result of a chemical reaction
Reactants  Products
• Energy is released or absorbed in
chemical reactions
Chemical Reactions and Energy
Reactants  Products
• Chemical reactions proceed
spontaneously if there is an increase in
entropy (= decrease in “useful” energy) as
the reaction proceeds to products
• The “useful” energy of the products < the
“useful” energy of the reactants
Chemical Reactions and Energy
• Gibb’s Free Energy (Δ G)
= amount of energy available after a
chemical reaction to do work
= “useful” energy of products –
“useful” energy of the reactants
Exergonic Reactions
• Chemical reactions that release energy
• The amount of “useful” energy in the
products is less than that of the reactants
• Gibbs Free Energy (Δ G) is negative
• e.g. cellular respiration Δ G = -686 kcal
: Cellular Respiration
(CH2O)n
Endergonic Reactions
• Chemical reactions that require an input of
energy to proceed to products
• The amount of “useful” energy in the
products is greater than that of the
reactants
• Gibbs Free Energy (Δ G) is positive
• e.g. photosynthesis Δ G = +686 kcal
: Photosynthesis
+ O2
complex
simple
Coupled Reactions
• Where do endergonic reactions get there
input energy from ?
From exergonic reactions that they are
coupled to.
Coupled Reactions
• How is the energy transferred from the
exergonic reaction to the endergonic
reaction?
By the energy carrier molecule ATP.
Adenosine Triphosphate (ATP)
• Cells do not directly use forms of energy : the
energy of chemical fuel molecules (e.g. glucose)
must be transformed into ATP in an exergonic
chemical reaction
• ATP is then used to provide the energy to
complete an endergonic chemical reaction.
• There are a number of additional energy carrier
molecules involved in cell metabolic processes
such as photosynthesis and cell respiration:
NAD+, NADP+, FAD and the Cytochromes.
Adenosine Triphosphate (ATP)
• The second and third phosphate bonds of ATP
are unstable. When this phosphate bond is
broken by hydrolysis, energy is released (an
exergonic reaction). This released energy is just
perfect for the amount of energy needed for
many cell reactions. This is why we call ATP an
energy carrier. It "carries" the energy needed to
do the cell work.
• The product of the hydrolysis of ATP is a
molecule of ADP and a free phosphate molecule
(Pi ). Much of the energy released when ATP is
broken is in the form of less useful heat energy.
III) Metabolic Pathways and
Enzymes
• Metabolic Pathway = series of linked chemical
reactions that begins with a particular reactant
and terminates with an end product
Controlling Metabolic Pathways
• Metabolic pathways must be tightly regulated for
our cells to function:
1) Cells couple endergonic chemical reactions
with exergonic chemical reactions
2) Cells have energy-carrier molecules that
capture energy released in exergonic reactions
and transport it to endergonic reactions
3) Cells regulate chemical reactions using
enzymes, which are protein catalysts
Chemical Reactions, Activation
Energy and Enzymes
• All reactant molecules are in constant
motion
• For any chemical reaction to get proceed,
the reactants must come together at the
right bonding place at the right time
• No matter how "spontaneous" a chemical
reaction is, some energy is needed to get
the reaction started
• This energy is called the activation energy
: Cellular Respiration
(CH2O)n
Chemical Reactions, Activation
Energy and Enzymes
• Reactants molecules will come together
randomly, but far too slowly at normal earth
temperatures
• Catalysts speed up the rate of chemical
reactions by lowering the activation energy
• The catalyst is not part of the chemical reaction.
It is neither a reactant nor a product
• A catalyst facilitates the reaction and is never
consumed in the reaction
• In living organisms, we use a special class of
catalysts, called enzymes
Enzymes
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•
•
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Enzymes are globular proteins with tertiary structure
There is a place on the surface of the enzyme where reactant molecules
can bind this "notch" is the active site
The active site has a precise size, shape, and electrical charge that exactly
complements the reactant molecules enzymes are highly specific to their
reactant molecules
Therefore, each chemical reaction that occurs in cells has its own enzyme
Mechanism of Enzyme Action
• When a reactant molecule binds to the enzyme, it "fits"
into the active site of the enzyme  induced fit
• This binding temporarily distorts the reactant molecules
 transition state
• In the transition state, the bonds of the reactant
molecules are more easily broken (lowered activation
energy), thereby promoting the reaction
• Once the reaction occurs, the active site is altered,
releasing the product
• The enzyme is unaffected by the reaction.
Factors Affecting Reaction Rate
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1) Substrate Concentration
2) Enzyme Concentration
3) Temperature
4) pH
5) Presence of Competitive Inhibitor
6) Presence of Allosteric Inhibitor
7) Presence of Enzyme Cofactors
8) Presence of Metal Ions
1) Effect of Substrate
Concentration on Reaction Rate
• 1) At lower substrate
concentrations, the active sites
on most of the enzyme
molecules are not filled
• 2) At higher substrate
concentrations, there are more
collisions between the
substrate and enzyme
molecules  faster reaction
rate
• 3) The maximum reaction rate
is reached when the active
sites are almost continuously
filled
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1
2) Effect of Enzyme Concentration
on Reaction Rate
• 1) If there is insufficient
enzyme present, the reaction
will not proceed as fast as it
otherwise would because there
is not enough enzyme for all of
the reactant molecules
• 2) As the amount of enzyme is
increased, the rate of reaction
increases
• 3) If there are more enzyme
molecules than are needed,
adding additional enzyme will
not increase the rate
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2
1
3) Effect of Temperature on
Enzyme Activity
4) Effect of pH on Enzyme Activity
5) Effect of Competitive Inhibitor on
Reaction Rate
Types of Competitive Inhibitors
• e.g. cyanide competes with oxygen for the
active site of the enzyme cytochrome
oxidase
• e.g. penicillin competes for the active site
of an enzyme unique to bacteria
6) Effect of Allosteric Inhibitor on
Reaction Rate
Allosteric Inhibition
7) Effect of Enzyme Cofactors on
Reaction Rate
• Cofactors = inorganic molecule required by
enzyme for proper functioning of enzyme
• e.g. copper (Cu+), zinc (Zn++), iron (Fe++),
magnesium (Mg++), potassium (K+), and
calcium (Ca++) ions
• The cofactors bind to the enzyme and participate
in the reaction by removing electrons, protons ,
or chemical groups from the substrate.
7) Effect of Enzyme Coenzymes on
Reaction Rate
• Coenzymes = organic (non- protein) molecule
required for proper functioning of enzyme
• e.g. NAD, FAD, vitamin complexes
• Coenzymes often remove electrons from the
substrate and pass them to other molecules
• Often the electron is added to a proton to form a
hydrogen atom before it is passed
• In this way, coenzymes serve to carry energy in
the form of electrons (or hydrogen atoms) from
one compound to another
8)Presence of Metal Ions
• The addition of heavy metal ions such as
mercury, lead and arsenic to an enzymemediated reaction decreases the reaction
rate
• The heavy metal ions denature the
enzyme by destroying the 3-dimensional
shape of the active site