Download Energy, enzymes and metabolism

BIOL 303
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Energy, enzymes and metabolism
The ultimate goals of cellular metabolism
are to:
• Increase biomass through growth and
reproduction
• Preserve and reproduce genetic
information
• Maintain optimal temperature
(homeothermic animals)
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The second law of thermodynamics dictates
that every energy transformation ∆H
between a system and its surroundings must
carry a net increase in entropy.
The amount of energy lost in the process
equals T∆S. The remaining energy that can
be used to do work is called the Gibbs free
energy:
∆G = ∆H-T∆S
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∆G represents the difference in the ability to
produce work between the products and the
reactants.
∆G = ∆G°products - ∆G°reactants
Spontaneous reactions always have a
negative ∆G because the products have less
energy available for work than the reactants.
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In the cell, even thermodynamically unfavorable
reactions are performed by coupling them to an
exergonic reaction
Glutamic acid + NH4+ → Glutamine
∆G°°= +3.4 kcal/mole
Unfavorable
ATP + H20 → ADP + Pi
∆G°°= -7.3 kcal/moll
Favorable
Glutamic + NH4++ ATP + H20 → Glutamine + ADP + Pi
∆G°°= +3.4 -7.3 = -3.9 kcal/moll Favorable
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The rate and specificity of a reaction is enhanced by
enzymatic catalysis
Glutamine synthetase
Glutamic acid + ATP + H20 → Glutamyl 5'-phosphate +
ADP
Glutamyl 5'-phosphate + NH4+ → Glutamine + Pi
Glutamic + NH4++ ATP + H20 → Glutamine + ADP + Pi
∆G°°= +3.4 -7.3 = -3.9 kcal/mole Favorable
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Substrates
aligned
Enzyme
Enzymes can catalyze
chemical reactions by
lowering the energy of
activation.
(a)
Substrate
acquires
charged
region
Enzyme
(b)
Substrate
stressed
Enzyme
(c)
Enzymes can also change
the reaction pathway by
holding the reactants in a
specific configuration, or
by altering the charge
distribution or shape of the
substrate.
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Metabolism
• Collection of biochemical reactions that occur
within a cell
• Organized into distinct pathways
• Pathway - a series of sequential reactions, each
catalyzed by a different enzyme, that consists of
one or more intermediates and an end-product
• Pathways interconnect by sharing a substrate, endproduct or intermediate (e.g. E. coli pathways)
• Catabolic pathways ⇒ disassembly
• Anabolic pathways ⇒ assembly
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Pathways for the capture and utilization of
chemical energy
1. Photosynthesis produces molecular oxygen as a
by-product
2. The Earth possesses an eminently oxidizing
atmosphere
1. In such an environment, highly reduced forms of
carbon, such as fats and carbohydrates, release large
amounts of energy (∆G) when their electrons are
transferred to oxygen to form CO2 and water
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H
H
H
C
H
Methane
(CH 4 )
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H
H
C
H
Methanol
(CH 3 OH)
H
OH
H
C
O
HO
O
Formaldehyde
(CH 2O)
Most
reduced
state
H
C
O
Formic acid
(HCOOH)
C
O
Carbon dioxide
(CO 2 )
Most
oxidized
state
Covalent bond in which carbon atom has greater share of electron pair
Covalent bond in which oxygen atom has greater share of electron pair
4. Redox reactions involve a transfer of electrons
between a donor (less affinity) and an acceptor
(more affinity)
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5. Glucose oxidation (∆G°'= -686 kcal/moll) is
carried out in small, incremental steps, allowing the
released energy to be captured efficiently as ATP
(–686 kcal/mol)
Glucose
Pyruvate
ATP
ATP
ATP
∆G'
Acetyl CoA
ATP
ATP
ATP
ATP
ATP
ATP
CO
(0 cal)
2
+ H 2O
ATP
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6. The main pathway for glucose degradation and
ATP biosynthesis is called glycolysis
(Figure 3.23)
7. Two types of high energy molecules are formed
during glycolysis: ATP and NADH
8. ATP formation occurs by:
• Substrate level phosphorylation of ADP by 1,3biphospho-glycerate and phosphoenolpyruvate
• NADH → high energy e- → electron transport
chain → oxidative phosphorylation of ADP