BIOENERGETICS

1
BIOENERGETICS

• Reading
• Harpers Biochemistry pp. 123-129
• Lehninger Principles of Biochemistry 3rd Ed.
  pp. 485-522

2
OBJECTIVES

• To gain an understanding of concepts used to deal
  with energy flow in living organisms.
• To understand the following terms and concepts
• 1. Enthalpy
• 2. Entropy
• 3. Free Energy
• 4. Bioenergetic coupling of chemical reactions
• 5. Additivity of free energy changes
• 6. Relationship between standard free energy
  and equilibrium constant
• 7. Role of ATP as energy currency of cell

3
Bioenergetics- Biochemical Thermodynamics

• Quantitative study of the energy transductions
  that occur in living cells, and of the nature and
  function of the chemical processes underlying
  these transductions
• Provides underlying principles to explain why
  some reactions may occur while other do not
• Non-biological systems may use heat energy to
  perform work, whereas biological systems are
  essentially isothermic and use chemical energy to
  power living processes

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Biomedical Importance of Bioenergetics

• Fuel is required to provide energy for normal
  processes, so understanding energy production and
  utilization is fundamental to understanding
  normal nutrition and metabolism
• Starvation - occurs when available energy
  reserves are depleted
• Certain forms of malnutrition are associated with
  energy imbalance e.g. marasmus- wasting disease
  due to insufficient energy and protein intake
• Excess storage of surplus energy results in
  obesity which can have negative effects on health

5
Gibbs Free Energy Change (?G)

• ?G is that portion of the total energy change in
  a system that is available for doing work - it is
  the useful energy
• When a reaction proceeds with a release of free
  energy (i.e. the system changes so as to possess
  less free energy), the free energy change, ?G,
  has a negative value and the reaction is said to
  be exergonic
• In endogonic reactions, the system gains energy
  and ?G is positive
• Units of ?G - joules/mole (J/mol)
• - calories/mole (cal/mol)

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Enthalpy, H

• Enthalpy is the heat content of the reacting
  system
• It reflects the number and kinds of chemical
  bonds in the reactants and products
• When a chemical reaction releases heat, it is
  said to be exothermic - the heat content of the
  products is less than that of the reactants and
  by convention, ?H has a negative value
• Reacting systems that take up heat from their
  surroundings are endothermic and have positive
  value of ?H
• Units of ?H - joules/mole (J/mol)
• - calories/mole (cal/mol)

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Entropy, S

• Entropy is a quantitative expression for the
  randomness or disorder of a system
• When the products of a reaction are less complex
  and more disordered than the reactants, the
  reaction is said to proceed with a gain in
  entropy
• Units of ?S - J/molK
• - cal/molK
• K units of absolute temperature (25?C 298K)

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Entropy, S

• Example
• Oxidation of glucose C6H12O66 O2?6 CO2 6 H2O
• Increase in number of molecules, or when a solid
  is converted to liquid or gas, generates
  molecular disorder. Entropy increases.

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Relationship between ?G, ?H, and ?S

• Under conditions existing in biological systems
  (constant temperature and pressure), changes in
  free energy (?G), enthalpy (?H), and entropy (?S)
  are related to each other quantitatively
• ?G ?H - T ?S, where T absolute temp (K)
• ?H has a negative sign when heat is released by
  the system to the surroundings
• ?S has a positive sign when entropy increases

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Relationship between ?G, ?H, and ?S

• In a favorable exergonic process which releases
  heat and increases entropy
• e.g. Oxidation of glucose
• ?G (negative value of ?H) - (T ? positive
  value ?S)
• ?G negative value
• For favorable or spontaneous processes, ?G has a
  negative value

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Free Energy

• We must subtract the energy lost to increasing
  entropy of the system from the total enthalpy
  change to figure the amount of energy left over
  available for useful work
• ?G ?H - T?S
• At equilibrium in a closed system no net change
  in free energy can occur, ?G 0 and ?H T?S

12
Free Energy

• Example
• Heat water in tea kettle - steam is produced and
  potentially capable of doing work
• Allow to cool, no work is done, temp of
  surroundings increases by infinitesimal amount
  until equilibrium is reached. Kettle and
  surroundings are at the same temp, the free
  energy that was once in the kettle has
  disappeared.
• ?H (change in heat) T?S (change in entropy)
• ?G ?H - T?S, ?G 0, no free energy
• available to do work
• Irreversible

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• For general reaction aAbB cCdD, where
  a,b,c,d number of molecules of A,B,C,D
• Equilibrium constant, Keq CcDd
• AaBb
• When a reacting system is not at equilibrium, the
  tendency to move toward equilibrium represents a
  driving force, the magnitude of which can be
  expressed as the free energy change for the
  reaction, ?G
• At 25?C (298K) and at 1M for participants,
  driving force ?G ? standard free energy
  change
• If H involved, 1M H pH 0

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• For biochemical reactions, at pH 7, define
• ?Go standard transformed free energy change
• ?Go - RT ln Keq -2.303 RT log Keq

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• Indicates how much free energy is available from
  the indicated reaction under standard conditions

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Actual free energy changes depend on reactant and
product concentrations

• The standard free-energy change tells us which
  direction and how far a given reaction will go to
  reach equilibrium when the initial concentration
  of each component is 1.0M, the pH is 7.0, the
  temp is 25?C, and the pressure is 1 atm.

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Actual free energy changes depend on reactant and
product concentrations

• However, actual free-energy change, ?G, is a
  function of the reactant and product
  concentrations and the prevailing conditions
• ?G and ?Go are related for AB CD by
• ?G ?Go RT ln CD
• AB
• At equilibrium, ?G 0
• 0 ?Go RT ln CD
• AB
• ?Go -RT ln Keq

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Example

• Oxaloacetate acetyl-CoA H2O?citrate CoA
  H
• At pH 7 and 25C in rat heart mitochondria -
  oxaloacetate 1?M acetyl-CoA 1?M citrate
  220 ?M CoA 65 ?M
• ?G -32.2 kJ/mol
• RT 2.48 kJ/mol
• What is direction of metabolite flow?
• Solution - calculate ?G, positive or negative?
• ?G ?G RT ln PP
• RR
• ?G -32.2 2.48 ln 22065x10-12
• 11 x10-12
• ?G -32.2 2.48 x ln14300
• -32.2 23.7
• -8.5 kJ/mol
• ?G is negative, reaction proceeds to the right

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Standard Free-Energy Changes are Additive

• For sequential reactions,
• A B and B C, the ?Go values are
  additive
• ?Go Total ?G1o ?G2o
• A thermodynamically unfavorable reaction
  (endergonic) can be driven in the forward
  direction by coupling it to an exergonic reaction
  

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• Vital processes- e.g. synthetic reactions, muscle
  contractions, active transport, obtain energy by
  chemical linkage, or coupling, to oxidative
  reactions
• One way of coupling an exergonic to an endogonic
  process is to synthesize a compound of high
  energy potential in the exergonic reaction and to
  incorporate the new compound into the endergonic
  reaction

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• Can theoretically serve as a transducer of
  energy for a wide range of reactions

E
22

• Example
• Synthesis of glucose 6-phosphate
• Glucose Pi?glucose 6-phosphate H2O
• ?Go 13.8 kJ/mol
• (will not proceed spontaneously in this
  direction)
• ATP H2O?ADP Pi ?Go - 30.5 kJ/mol
• These reactions share the common intermediates Pi
  and H2O and may be expressed as
• (1) Glucose Pi?glucose 6-phosphate H2O
• (2) ATP H2O?ADP Pi
• Glucose ATP?ADP glucose 6-phosphate
• ?Go 13.8 kJ/mol (-30.5 kJ/mol) -16.7
  kJ/mol
• Overall reaction is exergonic
• The actual pathway of glucose 6P formation is
  different from these reactions, but net result in
  energetic terms is the same.

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ATP has a special role as energy currency

• ATP is the shared chemical intermediate linking
  energy-releasing to energy-requiring cell
  processes. Its role in the cell is analogous to
  that of money in an economy It is
  earned/produced in exergonic reactions and
  spent/consumed in endergonic reactions.

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Chemical basis for the large free-energy change
associated with ATP hydrolysis

• Hydrolysis causes charge separation, relieving
  electrostatic repulsion among the four negative
  charges on ATP
• Inorganic phosphate released is stabilized by
  formation of a resonance hybrid
• ADP2- produced ionizes
• Greater degree of solvation of ADP and Pi than ATP

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ATP has two high-energy phosphate groups

• Standard free-energy of hydrolysis of ATP is
  intermediate in list of organophosphates

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• ATP can act as a donor of high-energy phosphate
  to compounds below it in the table
• ADP can accept high-energy phosphate to form ATP
  from those compounds above it in the table
• This forms ATP/ADP cycle

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Adenylyl Kinase Interconverts Adenine Nucleotides

• Adenylyl Kinase (or myokinase) is present in most
  cells and catalyzes the interconversion of ATP
  and AMP to ADP and vice versa
• ATP AMP 2 ADP
• Allows high-energy phosphate in ADP to be used in
  synthesis of ATP
• Allows AMP (formed as a consequence of several
  activating reactions involving ATP) to be
  recovered by rephosphorylation to ADP

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ATP can Donate Phosphoryl, Pyrophosphoryl, or
Adenylyl Groups

• The transfer of these groups couples the energy
  of ATP breakdown to endergonic transformation of
  substrates

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Activation of a fatty acid

• Involves attachment of the carrier coenzyme A
• Direct condensation of a fatty acid with coenzyme
  A is endergonic, but process is made exergonic by
  stepwise removal of two phosphoryl groups from
  ATP
• Hydrolysis of PPi to 2Pi by inorganic
  pyrophosphatase releases additional energy

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Other Nucleoside Triphosphates Participate in the
Transfer of High-Energy Phosphate

• By means of the enzyme nucleoside diphosphate
  kinase (NDK), nucleoside triphosphates similar to
  ATP but containing different bases (U,G,C) can be
  synthesized
• ATP UDP ADP UTP
• ATP GDP ADP GTP
• ATP CDP ADP CTP
• Similarly, specific nucleoside monophosphate
  kinases (NMK) exist
• ATP nucleoside ? ADP nucleoside
  ??

NDK
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Summary

• Biological systems are isothermic and use
  chemical energy to power living processes
• Chemical reactions are influenced by two forces
• (1) The tendency to achieve the most stable
  bonding state (enthalpy, H)
• (2) The tendency to achieve the highest degree
  of randomness (entropy, S)
• The net driving force of a reaction, ?G, the
  free-energy charge, represents the net effect of
  those two factors ?G ?H - T?S.

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Summary

• The standard free-energy change, ?Go, is a
  physical constant for a given reaction and is
  related to the equilibrium constant
• ?Go - RT ln Keq
• The actual free energy change, ?G, is a variable
  which depends on ?Go and the actual conditions
• ?G ?Go RT ln products
• reactants
• ?G large, negative - reactions go in forward
  direction
• ?G large, positive - reactions go in reverse
• ?G is zero - system is at equilibrium

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Summary

• Endergonic processes occur only when coupled to
  exergonic processes. Free-energy changes are
  additive for successive reactions sharing a
  common intermediate.
• ATP acts as the energy currency of the cell and
  is the chemical link between catabolism and
  anabolism. Its exergonic conversion to ADP and
  Pi or to AMP and PPi is coupled to a large number
  of endergonic reactions.