BIOENERGETICS AND METABOLISM

Objectives:At the end of today’s lecture the student should be

able to:• know the basic principles governing energy

transduction in the living cell.• define catabolism and anabolism.• understand the concept of energy

metabolism.• understand the different stages in which

energy is extracted from different complex macromolecules.

LECTURE TOPICS:

Bioenergetics

Laws of thermodynamics

Free energy concept

ATP energy currency of the cellIntroduction to metabolismStrategy of central catabolic pathways

Organisms within the biosphere exchange molecules and energy

(e.g. some bacteria, animals, humanshumans)

complex carbon, glucose, amino acids

CO2, H2O

Autotrophs:Phototrophs

& chemotrophsHeterotrophs

Chemical oxidations(via iron & sulfur

bacteria)

Light (via Light (via plantsplants))

Need 9 amino acids & 15 vitamins from

outside sources

Energy of sunlight

Useful chemical bond energy

Bioenergetics and Thermodynamics

BIOENERGETICS

The quantitative study of: the energy transductions in the living cell the nature and function of the chemical

processes underlying these transductions.

BIOLOGICAL ENERGY TRANSFORMATIONS FOLLOW THE FIRST & SECOND LAWS OF THERMODYNAMICS

Laws of Thermodynamics

First law – amount of energy in universe is constant

Energy can change form but cannot be created or destroyed

Second law – disorder in the universe is continuously increasing

Energy transformations proceed spontaneously toward more disordered states.

SYSTEM – the collection of matter that is undergoing a particular chemical or physical processes.

UNIVERSE – reacting system + its surrounding

CHANGE IN FREE ENERGY OF A REACTION

∆G = ∆H - T∆S

where :

∆G is the change in free energy of a reaction

∆H is the heat content (enthalpy factor)

T is temperature (°K)

∆S is entropy

Entropy : a measure of the degree of randomness (chaos)

: temperature dependent (°K)

QUALITATIVE EVALUATION OF G

G < 0 : (-) EXERGONIC

FAVORABLE

G > 0 : (+) ENDERGONIC

NOT FAVORABLE

G = 0 : AT EQUILIBRIUM

Unit of free energy : kcal, kjoules

The free energy change from chemical reactions can be calculated from the equilibrium constants or from redox potentials.

Free energy & Equilibrium constant:∆G = ∆GO + RTln Kequilibrium A B reaction

∆G : observed free energy change ∆Go : standard free energy change (reflects energy of A and B)R : gas constantT : temperature (°K)

Consider what happens at equilibrium: ∆G = 0 = ∆Go + RTln Kequilibriumor ∆Go = - RTln Kequilibrium

Bioenergetics

Oxidation/Reduction (OIL RIG: Oxidation Is Loss, Reduction Is Gain): oxidation = electron removal = dehydrogenation (loss of H+): reduction = electron gain = hydrogenation (gain of H+)

Bioorganic systems : 2 electron transfer (or the equivalent eg. 2 H+)Consider : AH2 + B A + BH2

electron or electron or becomes becomesH+ donor H+ acceptor oxidized reduced

Every oxidation is accompanied by a reduction.

Simple transfer of electrons.

Right to Left : B can give up electrons, provided that A can accept them.

The direction a reaction goes depends upon affinity of A or B for electrons.

Bioenergetics

Redox Potential (Eo): measure of a system’s affinity for electrons: electrons flow from lower to higher redox potential

AH2 + B A + BH2

Eo Eo

: left to right B has higher affinity for electrons

∆Eo : difference in standard electrode potential between the two interacting systems : Ee-acceptor – Ee-donor

∆Go = -nF ∆Eo Where:n = no. of electronsF = Faraday’s constant (96500 J/volts.mol)

Free energies (G) are additive

A B C+ Go’ = +5 kcal/mol(requires energy)(requires energy)

B D Go’ = -8 kcal/mol(yields energy)(yields energy)

+

A C D+ Go’ = -3 kcal/mol(net: yields energy)(net: yields energy)

Reaction couplingReaction couplingThermodynamically Thermodynamically favorablefavorable reactions drive reactions drive unfavorableunfavorable ones. ones.

– – G is G is favorablefavorable (spontaneous) (spontaneous)+ + G is G is unfavorableunfavorable (requires energy (requires energy

input)input)

Many metabolic processes, e.g. glucose breakdown, proceed due to

reaction couplingreaction coupling.Go’ = +4 kcal/mol

(requires energy)(requires energy)

Go’ = -7 kcal/mol(yields energy)(yields energy)

+

Go’ = -3 kcal/mol(net: yields energy)(net: yields energy)

glucose + PO4 glucose-6-PO4

ATP + H2O ADP + Pi + H+

glucose + ATP glucose-6-PO4 + ADP

HexokinaseHexokinase(This enzyme couples

the two reactions)

ATP: the universal currency of free energy;“high energy” phosphate compound

ATP + H2O ADP Pi H+ Go’ = -7.3 kcal/mol+ +

Go’ = -7.3 kcal/molPi H+ADP H2O+ + +AMP

(G in cells = -12 kcal/mol)

ATP

ADP

How are catabolism and anabolism coupled?

Heterotrophic metabolism: Interconversion of material and

energy

CatabolismCatabolism (breakdown):Yields energy,

precursors

AnabolismAnabolism (synthesis):

Requires energy, precursors

coupledcoupled

ATPATP couples energy between catabolism and anabolism

catabolismcatabolism

anabolismanabolism

ADPATP + Pi

Energy from food (fuel molecules) or from

photosynthesis

Energy available for work & chemical synthesis (e.g.

movement, signal amplification, etc.

ATPATP is the principal carrier of chemical energy in the cell!

Major activities promoted by ATP:

-locomotion-membrane transport-signal transduction-keeping materials

in the cell-nucleotide synthesis

Another source of energy is the coupling of Oxidation & Reduction reactions

anabolismanabolism

catabolismcatabolismReduced fuel

Reduced Products

Oxidized Fuel

Oxidized Precursors

NADH(reduced)NAD+(oxidized)

NADNAD++ (and NADP (and NADP++) carry high-energy electrons and hydrogen atoms.) carry high-energy electrons and hydrogen atoms.

Bioenergetics

Metabolism: the sum or totality of the reactions going on within a cell: high molecular weight

CATABOLISM comes down in energy (exergonic) in a series of steps

low molecular weight

high molecular weight

ANABOLISM must pump energy (endergonic) into system

: low molecular weight

E

E

E

E

EE

E

E

Energy metabolism is the part of intermediary metabolism concerned with the generation and storage of metabolic fuels.

The strategy of the central catabolic pathways is to:

1. form ATP for the energy-dependent activities of the cell.

2. provide reducing power NADPH

3. provide building blocks for biosynthesis.

Cellular Metabolism

Part 1:Breakdown of large macromolecules to

simple subunits

Part 2:Breakdown of

simple subunits to acetyl CoA

accompanied by production of

limited amounts of ATP and NADH

Part 3:Complete oxidation

of acetyl CoA to H2O and CO2

accompanied by production of large amounts of NADH

and ATP in mitochondrion

fats

fatty acids and glycerol

polysaccharides

simple sugars

proteins

amino acids

Acetyl CoA

glucose

Citric acid cycle

CoA

2 CO2

8 e- (Reducing power as NADH)

oxidative phosphorylatio

n

O2

H2O

ATPATP

glyc

olys

is

pyruvate

ATPATP

NADH

END…..