1 Principles of Bioenergetics Lehninger 3 rd ed. Chapter 14
2 Metabolism A highly coordinated cellular activity aimed at achieving the following goals: Obtain chemical energy. Convert nutrient molecules into the cell s own characteristic molecules. Degrade biomolecules.
Carbon Flow 3
Nitrogen Flow 4
Catabolism & Anabolism 5
Divergence & Convergence 6
7 Synthesis versus Degradation Most cells posses the enzymes to both synthesize and degrade a particular molecule. Is this not wasteful? No, since the cell: Regulates each process. Segregates their location.
Antoine Lavoisier 8 respiration is nothing but a slow combustion of carbon and hydrogen (A.L. Lavoisier 1743-1794)
9 Bioenergetics The quantitative study of cellular energy transductions and the chemical reactions underlying these transductions. Obviously, biological energy transductions obey the laws of Thermodynamics. ΔG = ΔH TΔS
10 ΔG = ΔH TΔS G: Gibbs free energy at constant temperature and pressure; Units are Joule per mole. H: Enthalpy; Units are Joule per mole. Τ: Temperature; Units in Kelvins. S: Entropy; Units are Joule per mole times temperature in Kelvins.
11 ΔG: Free Energy at constant temperature and pressure (Joules per mole) If ΔG < 0 then the reaction will be spontaneous. The value of ΔG is directly related to the equilibrium constant ΔG 0 = RT lnk eq Actual free energy depends on the reactant and product concentrations: aa + bb cc + dd ΔG = ΔG 0 + RT ln [C]c [D] d [A] a [B] b
Free energies are additive, thus a 12 favorable reaction (ΔG 1 < 0) can drive an unfavorable reaction (ΔG 2 > 0), when ΔG 1 + ΔG 2 <0
13 S: entropy According to Boltzmann: S = k ln W where W is the number of states in the system. Thus any reaction such as aa + bb cc + dd in which a+b < c+d, can be said to be driven by entropy.
C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O 14
15 Phosphoryl groups and ATP ATP: Adenosine triphosphate, a ribonucleotide, is the energy currency of the cell.
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17 Why is the hydrolysis of ATP Relieves electrostatic repulsion between the negatively charges phosphates. Inorganic phosphate can be stabilized by resonance hybrid. ADP 2- can ionize. highly exergonic? The products are better solvated than the reactants.
ATP 4 + H 2 O ADP 3 + Pi 2 + H + 18 Under standard conditions: ΔG '0 = 30.5kJ /mol But in the cell the phosphorylation potential ΔGp is: ΔGp = ΔG '0 + RT ln [ADP][P i ] [ATP] = 51.8kJ /mol
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High Energy Phosphorylated Compounds 20
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Thioesters hydrolysis is also highly exergonic 22
23 Making use of ATP Since the ATP hydrolysis is very favorable (i.e. ΔG << 0) it can drive unfavorable reactions, but how? It does so not by harnessing the energy of hydrolysis, but rather through the coupling of group transfer.
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26 Biological Oxidation-Reduction The flow of electrons can do work. Electrons flow from a reducing agent to an oxidizing agent due to their different electron affinities. This difference in affinities is called the electromotive force (emf). The reducing agent undergoes oxidation and the oxidized undergoes reduction.
27 Redox reactions can be described as Half-reactions: Fe 2+ + Cu 2+ Fe 3+ + Cu + (1) Fe 2+ Fe 3+ + e - (2) Cu 2+ + e - Cu +
28 Redox reactions in bio-chemicals R C O H + R C O 4OH - + 2Cu 2+ + Cu OH 2 O + 2H 2 O R C O H + R C O 2OH - + OH 2e - + H 2 O + + 2Cu 2+ 2e - 2OH - Cu 2 O H 2 O +
Electronegativity series: O > N > S > C > H 29
30 Dehydrogenation = oxidation Carbon is less electronegative than all atoms it is bound to, except hydrogen. Thus all atoms that bind to carbon oxidize it except hydrogen. Thus removing a hydrogen and replacing that bond with any other atom (including carbon) is synonymous with oxidation.
31 Electron transfer modes Directly as electrons: Fe 2+ + Cu 2+ Fe 3+ + Cu + As hydrogen atoms: AH 2 A + 2e - + H + As a hydride ion (H - ): AH 2 + B + A + BH + H + Direct combination with oxygen: R-CH 3 + 1 / 2 O 2 R-CH 2 -OH
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33 Reduction potentials = e - affinity E = E 0 + RT ni = E 0 + 0.026V ln[electron acceptor] [electron donor] n ln [electron acceptor] [electron donor] ΔG = niδe, or ΔG '0 = niδe '0 I = 96,480 J/V mol R = 8.315 J/mol K
34 Glucose oxidation is highly exergonic The complete oxidation of glucose is our major source of energy. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O The process involves many steps each catalyzed by a specific enzyme. ΔG 0 = -2,840 kj/mol
NAD +, NADP +, FAD & FMN: 35 universal electron carriers NAD + (nicotinamide adenine dinucleotide) and NADP + (phosphorylated form of NAD + ) are reversal redox cofactors in which. In their capacity as reducing agents, the substrate undergoes a double dehydrogenation (oxidation) and NAD + (or NADP + ) accepts a hydride ion (H - ), with a release of a H + to the environment. NAD + + 2e - + 2H + NADH + H + CH 3 CH 2 OH + NAD + Ethanol CH 3 CHO + NADH + H + Acetaldehyde
36 Nicotinic acid Nicotine COOH N N N CH 3
37 FMN, FAD and flavoproteins Flavoproteins are enzymes that use FMN or FAD cofactors in redox reactions. The cofactor is derived from riboflavin (vitamin B 2 ). FAD and FMN can accept either 1 or 2 hydrogens, thereby accepting 1 or 2 electrons, and are therefore more versatile than NAD + or NADP +. The fully reduced forms are written as FADH 2 and FMNH 2
38 Riboflavin (B 2 ) O H 3 C N NH H 3 C N N O HO HO HO CH 2 CH CH CH CH 2 OH
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