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Chemical Reactions in Cells
Energetics, Enzymes and
Metabolic Reactions
Energy
• Energy is the capacity for work or
change.
• Kinetic Energy = energy of movement
• Potential Energy = stored energy
• 1st Law of Thermodynamics
– Energy can be transferred and
transformed from one form to another but
it cannot be created or destroyed.
• 2nd Law of Thermodynamics
Energy
– Energy transfer or transformation
increases the entropy of the universe
– Increase in entropy = randomness
– Energy conversions result in a loss of
useful energy
usable
usable
usable
usable
Free Energy = Energy Useful for Change
•Spontaneity of a reaction depends on free
energy change
G
Greaction = Gproducts – Greactants
•If G is negative, free energy is
released and the reaction is spontaneous
•If
G is positive, free energy is consumed
Free Energy = Energy Useful for Change
•Free Energy change depends on changes in
–total energy
–entropy
S
H (enthalpy)
(unusable energy, disorder)
•In living systems, entropy changes have
substantial influence
–when S is positive, and the term T S is large,
a negative G value predicts a spontaneous
reaction
G =
H –T S
Chemical Reactions
• Involve the breaking and formation of
chemical bonds
– Reactants are converted to products.
– Two types of reactions based on
energy use:
• Exergonic– free energy released
• Endergonic – free energy consumed
Exergonic Reactions
Burning glucose (sugar):
an exergonic reaction
high
Reactants changed to
transition-state species
Activation energy needed
to ignite glucose
Energy Glucose + O2
content
of
Energy released by
molecules
burning glucose
G
C O2 + H2O
low
Progress of reaction
Endergonic Reactions
Photosynthesis:
an endergonic reaction
high
Glucose
Energy
content
of
molecules
Net energy
Activation
captured by
energy from
light captured synthesizing
glucose
by photosynthesis
CO2 + H2O
low
Progress of reaction
G
Applying Your Knowledge
1. Endergonic Reaction
2. Exergonic Reaction
A. Which type of reaction would be spontaneous?
B. For which type of reaction will the products
have a higher energy than the reactants?
C. Which type of reaction releases energy?
D. Which type of reaction would have a positive
value for G?
ATP Provides Energy for Cellular Reactions
G  7.3 kcal/mol
+ H2O
+ Pi
+ free energy
Short-Term Energy Storage
• Chemical Energy is stored in the bonds
of ATP
– ATP = adenosine triphosphate
– ADP = adenosine diphosphate
– to store energy
• ADP + Phosphate + Energy ATP
– to release energy
• ATP  ADP + Phosphate + Energy
Coupled Reactions
Pairing of an Exergonic reaction, often
involving ATP, with an Endergonic reaction
Note that overall free energy change is negative
Metabolic Reactions
• Anabolic
– link simple molecules to produce complex
molecules (eg. dehydration synthesis of starch)
– require energy
• Catabolic
– break down complex molecules to release
simple ones (eg. hydrolysis of starch sugars)
– release energy stored in chemical bonds
Metabolic Pathways
Initial
Reactants
A
Intermediates
B
Enzyme 1
Pathway 1
C
Enzyme 2
Pathway 2
Final
Products
D
Enzyme 3
Enzyme 4
F
Enzyme 5
E
Enzyme 6
G
Enzymes Assist in Biological Reactions
Enzymes are biological catalysts.
biological: composed of protein or, rarely,
RNA
catalyst: speeds up a reaction without
being changed by the reaction
Properties of Enzymes
• Enzymes speed up biological reactions
by lowering the activation energy for the
reaction.
– Enzymes provide a surface where the
catalysis takes place
– The reaction reaches equilibrium more
rapidly
– The value of G and the ratio of
reactants and products at equilibrium is
the same as for an uncatalyzed reaction
Activation Energy: Controls Rate of Reaction
high
Energy
content
of
molecules
transition state
Amount of energy
required for
reaction to occur
Activation
energy without
catalyst
Activation
energy with
catalyst
low
Progress of reaction
G
Properties of Enzymes
• Enzymes are SPECIFIC for the
reactants (substrates) in the reactions
that they catalyze.
• Only substrates that fit the active site of
the enzyme can bind and complete the
reaction
– active site: region on enzyme where
substrates bind
Enzyme-Substrate Interactions
Substrate
1 Substrates
enter active
site
Substrate
Active
Site
Enzyme
2 Shape change
promotes reaction
3 Product released;
enzyme ready again
induced fit
Chemical Events at Active Sites
• Enzymes hold substrates in the proper
orientation for the reaction to take place
Chemical Events at Active Sites
• Enzymes induce strain in the substrate to
produce a transition state favorable to
reaction
• Active site provides a microenvironment
that favors the chemical reaction
Chemical Events at Active Sites
• Active site directly participates in the
reaction
– covalent bonding can occur between
enzyme and substrate
– R groups of the enzyme’s amino acids can
temporarily add chemical groups to the
substrates
Molecules that Assist Enzymes
• Cofactors: inorganic ions that bind to
enzymes, eg. zinc
• Coenzymes: small organic factors that
temporarily bind to enzymes, eg. biotin, NAD,
ATP
• Prosthetic groups: non-protein factors that
are permanently bound an enzyme, eg. heme
Factors Influencing Reaction Rate
• Substrate Concentration
Rate no longer increases since
the active sites of all enzymes
are saturated with substrate
Rate is more rapid
Rate is proportional
to substrate
concentration
Factors Influencing Reaction Rate
• Competitive Inhibitors: Bind at the active
site, compete for binding with substrate
– Irreversible: form covalent bond with amino
acids in the active site
DIPF
Factors Influencing Reaction Rate
• Competitive Inhibitors: Bind at the active
site, compete for binding with substrate
– Reversible: molecule similar to substrate
occupies active site but does not undergo
reaction
Factors Influencing Reaction Rate
• Non-Competitive Inhibitors: Bind to a
different site, cause a conformational
change in the enzyme that alters the
active site
– Reversible
Factors Influencing Reaction Rate
• Allosteric Regulation
– Conversion between
active and inactive forms
of an enzyme due to
binding of regulatory
molecules at an allosteric
site
• Activators stabilize the
active form
• Allosteric inhibitors
stabilize the inactive
form
Factors Influencing Reaction Rate
• Allosteric Regulation
– Cooperativity: a substrate causing
induced fit in one enzyme subunit can
cause a change to the active form in all
the other subunits
Enzyme Regulation: Feedback Inhibition
Commitment step
CH3
CH3
CH2
A
B
C
D
H C OH
H C CH3
Enz.
1
Enz.
2
Enz.
3
Enz.
4
Enz.
5
H C NH3
H C NH3
COOH
COOH
Threonine
Feedback Inhibition
(substrate)
Isoleucine allosterically
Isoleucine
inhibits enzyme 1
(end product)
Feedback Inhibition: The product of a pathway inhibits an
initial step in the pathway to decrease its own production
Properties of Enzymes
• Three dimensional structure of an
enzyme preserves its ACTIVE SITE
• Conditions that can affect three
dimensional structure include: heat, pH
(acid/base balance) and other chemicals
(salt, charged ions)
Effects of Temperature and pH on
Enzymatic Activity
fewer collisions
between enzyme
and substrate
enzyme unfolds
(denatures)
enzyme unfolds
(denatures)
Applying Your Knowledge
1.
2.
3.
4.
5.
Active Site
Activation Energy
Allosteric Site
Commitment step
Induced fit
A. Where can an inhibitor bind to stabilize the
inactive form of an enzyme?
B. Where do the substrates bind?
C. Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of a
reaction.
D. What is the model for a shape change
caused by substrate binding to the enzyme?