Bioenergetics MDSC1101 – Digestion & Metabolism Dr. J. Foster Biochemistry Unit, Dept. Preclinical...
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Transcript of Bioenergetics MDSC1101 – Digestion & Metabolism Dr. J. Foster Biochemistry Unit, Dept. Preclinical...
Bioenergetics
MDSC1101 – Digestion & Metabolism
Dr. J. FosterBiochemistry Unit, Dept. Preclinical Sciences
Faculty of Medical Sciences, U.W.I.
What do you think “bioenergetics” means?
bio = biologyenergetics = branch of physics that studies energy flow
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
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
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!
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
Garrett, Grisham. Biochemistry, 2nd ed © 2000
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
Which laws do you thinkimpact on thermodynamic
events within systems?
Can you recall any laws of physics that may apply?
clue : thermal heat energy
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?
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
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
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
Garrett, Grisham. Biochemistry, 2nd ed © 2000
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
•ΔG= ΔH – TΔS
°K = 273 + °C
ΔG= ΔH – TΔS
J/mol/K
J/mol
J/mol
as Δ S increases, ΔG becomes more -ve
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
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]
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
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
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)
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
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
Garrett, Grisham. Biochemistry, 2nd ed © 2000
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
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
Harper’s Biochemistry 26th ed, Appleton and Lange, USA
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
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
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
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+
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
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)
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
Further Reading
• Harper’s Biochemistry, 26th Ed. - Chapter 10