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 cell  Introduction to metabolism  Strategy of central catabolic pathways

Organisms within the biosphere exchange molecules and energy humans (e.g. some bacteria, animals, humans) complex carbon, glucose, amino acids CO 2, H 2 O Autotrophs : Phototrophs & chemotrophs Heterotrophs Chemical oxidations (via iron & sulfur bacteria) Light (via plants) 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 = ∆G O + RT ln Kequilibrium A  B reaction ∆G : observed free energy change ∆G o : standard free energy change (reflects energy of A and B) R : gas constant T : temperature (°K) Consider what happens at equilibrium: ∆G = 0 = ∆G o + RT ln Kequilibrium or ∆G o = - RT ln 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 :AH 2 + B A + BH 2 electron orelectron orbecomesbecomes H + donorH + acceptoroxidizedreduced 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 (E o ) : measure of a system’s affinity for electrons : electrons flow from lower to higher redox potential AH 2 + BA + BH 2 E o E o : left to right  B has higher affinity for electrons ∆E o : difference in standard electrode potential between the two interacting systems : E e-acceptor – E e-donor ∆G o = -nF ∆E o Where: n = no. of electrons F = Faraday’s constant (96500 J/volts.mol)

Free energies (  G) are additive ABC+  G o’ = +5 kcal/mol (requires energy) BD  G o’ = -8 kcal/mol (yields energy) + ACD+  G o’ = -3 kcal/mol (net: yields energy) Reaction coupling Thermodynamically favorable reactions drive unfavorable ones. –  G is favorable (spontaneous) +  G is unfavorable (requires energy input)

reaction coupling Many metabolic processes, e.g. glucose breakdown, proceed due to reaction coupling.  G o’ = +4 kcal/mol (requires energy)  G o’ = -7 kcal/mol (yields energy) +  G o’ = -3 kcal/mol (net: yields energy) glucose + PO 4 glucose-6-PO 4 ATP + H 2 OADP + P i + H + glucose + ATPglucose-6-PO 4 + ADP Hexokinase (This enzyme couples the two reactions)

ATP: the universal currency of free energy; “high energy” phosphate compound ATP+H2OH2OADPPiPi H+H+  G o’ = -7.3 kcal/mol ++ PiPi H+H+ ADPH2OH2O+++AMP (  G in cells = -12 kcal/mol) ATP ADP

How are catabolism and anabolism coupled? Heterotrophic metabolism: Interconversion of material and energy Catabolism Catabolism (breakdown): Yields energy, precursors Anabolism Anabolism (synthesis): Requires energy, precursors coupled

ATP ATP couples energy between catabolism and anabolism catabolism anabolism ADPATP + P i Energy from food (fuel molecules) or from photosynthesis Energy available for work & chemical synthesis (e.g. movement, signal amplification, etc. ATP ATP 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 anabolism catabolism Reduced fuel Reduced Products Oxidized Fuel Oxidized Precursors NADH (reduced) NAD + (oxidized) NAD + (and NADP + ) carry high-energy electrons and hydrogen atoms.

Bioenergetics Metabolism : the sum or totality of the reactions going on within a cell : high molecular weight CATABOLISMcomes down in energy (exergonic)in a series of steps low molecular weight high molecular weight ANABOLISMmust pump energy (endergonic)into system : low molecular weight E E E E E E 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 H 2 O and CO 2 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 CO 2 8 e - (Reducing power as NADH) oxidative phosphorylation O2O2 H2OH2O ATP glycolysis pyruvate ATP NADH

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