bioenergetics mdsc1101 – digestion & metabolism dr. j. foster biochemistry unit, dept....
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Bioenergetics
MDSC1101 – Digestion & Metabolism
Dr. J. FosterBiochemistry Unit, Dept. Preclinical Sciences
Faculty of Medical Sciences, U.W.I.
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What do you think “bioenergetics” means?
bio = biologyenergetics = branch of physics that studies energy flow
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What then is the significance of “bioenergetics”?
Processes by which the body meets energy demands
e.g. digestion & other metabolism
Bioenergetics – study of how such energy flows, transforms & is harnessed
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Bioenergetics
• The study of transformation and flow of energy within
biological systems, and with their environment
• Concerned with the initial and final energy states of
reactants and not the mechanism or kinetics
• Simply put - biochemical thermodynamics
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Bioenergetics
• Thermodynamics - laws & principles describing the flow
and interchanges of heat, energy, & matter in systems
• Concepts very applicable to biological systems
• System – portion of the universe we are concerned with
• Surroundings – everything else!
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Systems
• Three types
• Isolated - cannot exchange matter or energy with its surroundings
• Closed - may exchange energy, but not matter, with the
surroundings
• Open - may exchange matter, energy, or both with the
surroundings
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Garrett, Grisham. Biochemistry, 2nd ed © 2000
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What kind of system would you classify the human body as?
Justify your choice.
Exchanges matter (food in & waste out)
Exchanges heat (homeostasis)
The human body is an open system
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Which laws do you thinkimpact on thermodynamic
events within systems?
Can you recall any laws of physics that may apply?
clue : thermal heat energy
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Laws of Thermodynamics
• 1st Law - the total energy of a system (including surroundings) remains constant• energy cannot be gained or lost
• it can be transferred from part to part
• It can be converted from one form to another
• What exactly determines how energy flows and whether reactions occur?
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Laws of Thermodynamics
• Gibbs free energy (G)
• energy available to reactants/products in rxn
• determines the feasibility of chemical reactions i.e. direction &
extent (predictive)
• Two forms of G used in defining chemical reactions• ΔG, the change in G of rxn
• ΔG°, the standard ΔG (reactants/products@1 mol/L)
• ΔG° useful only under standard conditions
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Laws of Thermodynamics
• 1st Law - the total energy of a system (including surroundings) remains constant
• 2nd law - total entropy of a system must increase for a process to occur spontaneously
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Free Energy
• Given a reaction where A ⇆ B, if ΔG • is negative – rxn is exergonic*, energy is lost from system, spontaneous
from A → B
• is positive – rxn is endergonic*, energy is required by system from surroundings for rxn to occur
• equals zero – rxn is at equilibrium; no direction favoured
• also Δ G A→B = - Δ G B→A
• Spontaneous reactions equilibrium
*differ from exothermic/endothermic which relate to only heat
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Garrett, Grisham. Biochemistry, 2nd ed © 2000
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Free Energy
• ΔG is determined by two factors• Enthalpy (ΔH) – change in heat of reactants and products of a
rxn (e.g. chemical bonds)
• Entropy (ΔS) – change in randomness/disorder of reactants & products
• Neither ΔH or ΔS can predict rxn feasibility alone
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•ΔG= ΔH – TΔS
°K = 273 + °C
ΔG= ΔH – TΔS
J/mol/K
J/mol
J/mol
as Δ S increases, ΔG becomes more -ve
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Free Energy
• ΔG can also be defined by the concentrations of A & B:
ΔG = ΔG° + RT ln [B]/[A]
At constant P (pressure) & T (absolute ) – thermal equilibrium R = gas constant (8.315 J/mol/K) In = natural logarithm [B] = concentration of product [A] = concentration of reactant
• Note that ΔG and ΔG° can have different signs
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Free Energy
• Under standard conditions [A]=[B]= 1 mol/L
• ΔG = ΔG° + RT ln [B]/[A]
• ΔG = ΔG° + RT ln 1 (ln 1 = 0)
• ΔG = ΔG°
• ΔG° is predictive only under standard conditions
• ΔG and ΔG° can differ greatly depending on [A], [B]
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Free Energy
• At equilibrium (steady-state) [A] / [B] = constant = Keq
Thus, ΔG = ΔG° + RT ln [B]/[A]
becomes ΔG = ΔG° + RT lnKeq
• At equilibrium ΔG = 0
0 = ΔG° + RT lnKeq
ΔG° = -RT lnKeq
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Bioenergetics of pathways
• Biochemical pathway - series of rxns each with
characteristic ΔG
• Thermodynamically for a pathway
• ΔGpathway can be considered additive
• feasibility depends on sum of individual ΔG’s
• As long as the sum of ΔG is –ve the pathway is feasible
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A rxn can still occur even if ΔG is +ve if it is kinetically favoured…
how??
Enzymes – they reduce the activation energy needed for a
rxn (kinectics)
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Bioenergetics of pathways
• Many biological systems have rxns that have +ve ΔG
• How do biological systems overcome +ve ΔG’s?
• Exergonic reactions are usually coupled with endergonic
ones
• Such coupling of rxns involves using a common
intermediate – an energy coupler
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E.g. First step of Glycolysis
• The individual half-reactions in aqueous solution:
ATP + H2O ADP + Pi Go' = 31 kJ/mol (exg)
Pi + glucose glucose-6-P + H2O Go' = +14 kJ/mol (end)
• Hexokinase catalyses rxn (active site excludes H2O & promotes coupled
over individual rxns)
• ATP + glucose ADP + glucose-6-P Go' = 17 kJ/mol
• ATP is thus the coupler for the reaction
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Garrett, Grisham. Biochemistry, 2nd ed © 2000
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ATP as an energy coupler
• The energy currency of cells – universal coupler
• Intermediate in the rank of high-energy phosphates
• Allows it to accept and donate energy in numerous rxns
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ATP as an energy coupler
• Terminal phosphate bonds are “high energy” (~) i.e.
release a large amount of energy on hydrolysis
• Phosphate bonds allow for
• ATP to release energy for metabolic processes when hydrolysed
to ADP + Pi
• ADP to store energy from catabolic processes as chemical
potential energy in the form of ATP
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Harper’s Biochemistry 26th ed, Appleton and Lange, USA
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Lehninger, Biochemistry, 4nd edition © 2005
Anabolic – complex
molecules from simple ones
(endergonic)
Catabolic – simple
molecules from complex
ones (exergonic)
ATP bridges the 2 types
of metabolism
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Thermodynamics vs Kinetics
• A high activation energy barrier usually causes hydrolysis
of a “high energy” bond to be very slow in the absence of
an enzyme catalyst.
• Such kinetic stability is essential to the role of ATP and
other compounds with ~ bonds
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Why is this kinetic stability for ATP hydrolysis a good thing??
• Rapid hydrolysis (due to low barriers) would hinder ATP’s role in metabolism
• enzymes lower these barriers, and most importantly couple the rxn with other useful ones
• prevents free energy released from ATP hydrolysis being wasted
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Redox Reactions
• Redox (oxidation-reduction) rxns are inherently coupled & involve both• donating of e- (oxidation)
• accepting of e- (reduction)
• The two halves of a redox rxn are considered separately:
Fe2+ + Cu2+ ↔Fe3+ + Cu+
Can be rewritten in half-reactions as
Fe2+ → Fe3+ = e-
Cu2++ e- → Cu+
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Redox Reactions
• Free energy is also transferred via the movement of these
electrons
• The transfer of e- can be measured as reduction potentials
(E)
• This is the tendency of a chemical species to acquire
electrons and thereby be reduced
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Redox Reactions• For a rxn, E can be used to calculate G:
ΔG = -nFΔE likewise, ΔGo = -nFΔEo
n = number of electrons transferred, F = Faraday’s constant (96,480 J/V/mol), E = reduction potential Eo = std reduction potential
• +ve E favours a –ve G (forward rxn)• –ve E favours a +ve G (backward rxn)
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Summary
• Organisms are open systems that utilise free energy (in the
form of chemical energy) to live
• Use chemical coupling of spontaneous exergonic rxns to
overcome the energy demand of endergonic ones
• ATP is the most important coupler and acts as energy
currency of cells
• Redox reactions also determined by free energy
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Further Reading
• Harper’s Biochemistry, 26th Ed. - Chapter 10