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