principles of bioenergetics生物能量課件

1、Principles of Bioenergetics 1. Cells need energy to do all their work To generate and maintain its highly ordered structure (biosynthesis of macromolecules).To generate all kinds of movement.To generate concentration and electrical gradients across cell membranes.To maintain a body temperature.To ge

2、nerate light in some animals.The “energy industry”(production, storage and use) is central to the economy of the cell society! Bioenergetics (生物能學(xué)）: the quantitative study of energy transductions in living cells and the chemical nature underlying these processes.2. Cells have to use chemical energy

3、to do all their workAntoine Lavoisiers insight on animal respiration in the 18th century: it is nothing but a slow combustion of carbon and hydrogen (the same nature as a lighting candle).Living cells are generally held at constant temperature and pressure: chemical energy (free energy) has to be us

4、ed by living organisms, no thermal energy, neither mechanical energy is available to do work in cells.Biological energy transformation obey the two basic laws of thermodynamics revealed by physicists and chemists in the 19th century: energy can neither be created nor be destroyed (but conserved); en

5、ergy conversion is never 100% efficient (some will always be wasted in increasing the disorder or “entropy” of the universe).The free energy concept of thermodynamic is more important to biochemists than to chemists (who can always increase the temperature and pressure to make a reaction to occur!).

6、3. Application of the free energy (G) concept to biochemical reactionsFree energy (G): the amount of energy available to do work during a reaction at a constant temperature and pressure; change, not absolute value can be measured.Free energy change (G): The free energy difference between the product

7、s and the reactants.Gibbs observation: under constant temperature and pressure, all systems change in such a way that free energy is minimized (products should have less free energy than reactants for a reaction to occur spontaneously, i.e., G has a negative value ). Spontaneity has nothing to do wi

8、th rate!Standard free energy change in biochemistry (Go): value of the change in free energy under conditions of 298 K (25oC), 1 atm pressure, pH 7.0 (chemists use pH 0, i.e., the concentration of H+ they use is 1M, not 10-7 M as biochemists use here) and initial concentrations of 1 M for all reacta

9、nts and products (except H+).The actual free energy chang (G )depends on Go, temperature, ratio of product and reactant concentrations (Q): G = Go + RT ln QEnzymes only speed up thermodynamically favorable reactions (having a negative G) ! Go is related to the equilibrium constant Keq (the prime aga

10、in indicates its biochemical transformation): at equilibrium, G = 0, Q = Keq , thus G o = -RT ln Keq The G and Go values are additive when reactions are coupled (i.e., sharing common intermediates), thus a thermodynamically unfavorable reaction can be driven by a favorable one. The overall Keq is mu

11、ltiplicative (the product of two,兩值相乘), although Go is additive (the algebraic sum of two，兩值相加).4. ATP is the universal currency for biological energyThis was first perceived by Fritz Lipmann and Herman Kalckar in 1941 when studying glycolysis.Hydrolysis of the two phosphoanhydride (磷酸酐鍵) bonds in A

12、TP generate more stable products releasing large amount of free energy (Go is -30.5 kJ/mol; G in cells is -50 to -65 kJ/mol).The ATP molecule is kinetically stable at pH 7 (i.e., it has a high activation energy, G for hydrolysis) and enzyme catalysis is needed for its hydrolysis. ATP is not a long-t

13、erm storage form of free energy in living cells, being consumed within a minute following its formation (phosphocreatine, 磷酸肌酸, act as a energy storage form for longer term).A resting human consumes about 40 kg of ATP in 24 hours!ATP provides energy by group transfer (donating a Pi, PPi or AMP to fo

14、rm covalent intermediates), not by simple hydrolysis.ATP has an intermediate phosphoryl group transfer potential, thus ADP can accept and ATP can donate phosphoryl groups. In the lab, as little as a few picomoles (10-12 mol) of ATP can be measured using firefly luciferin and luciferase (熒光素酶), using

15、 spectroscopic methods.Inorganic polyphosphate could have served as an energy source in prebiotic and early cellular evolution.ATP provides energy usually through grouptransfer, thus Activatingthe substrateATP has an intermediate phosphoryl group transferpotentialATP can transfer a Pi, PPi or AMP to

16、 a reactantPicomoles (10-12 mol) of ATP can be measured Using leciferaseInorganic polyphosphate may act as an energy storage form5. Electron transfer via redox reactions generates biological energyWhen electrons flow from a low affinity carrier (reductant) to a high affinity carrier (oxidant), eithe

17、r in an electric battery or in a living cell, energy is released and work can be done.Oxidation of energy-rich biological fuels often means dehydrogenation (catalyzed by dehydrogenases, 脫氫酶) from carbons having various oxidation states.In the living cells, electrons are transferred directly as elect

18、rons (between metal ions), as hydrogen atoms (H+e-), or as a hydride ion (:H- or H+2e-). The affinity for electrons of a compound (in its oxidized form) is indicated by its reduction potential (E).Standard reduction potential (Eo) of each oxidant (a constant) is measured by connecting a half-cell ha

19、ving the oxidized and reduced species of the redox pair each at 1 M, or 1 atm for gases, pH 7 to a reference half-cell having 1 M H+ and 1 atm H2, whose E o is arbitrarily assigned as 0.00 V.A positive value of Eo indicates a tendency to acquire electron from the reference half cell (with 1M H+/1atm

20、 H2). The actual reduction potential (E) depends on , electrons transferred per molecule, temperature, ratio of electron acceptor/electron donor:Go of a redox reaction can be calculated from the Eo of the two redox pairs: Energy is “generated” via electron flow both in a battery and in a cell! Carbo

21、ns have various oxidation states, with hydrocarbon being the most reduced and CO2 being the most oxidizaed.The carbon atom may “own” different number of electrons in different compounds, thus having different oxidation statesThe standard reductionpotential (Eo) of a conjugate redox pairis measured b

22、y connecting the sample half cell to the H+/H2reference half cell.6. A few universal carriers (as coenzymes) collect electrons from the oxidation of various substratesNAD+, NADP+, FAD are the few commonly used such reversible electron carriers.NAD and NADP are dinucleotides able to accept/donate a h

23、ydride ion (thus with 2e-) for each round of reduction/oxidation.Reduction of NAD+ and NADP+ can be easily followed by spectroscopy (at 340 nm). In each specific NAD- or NADP-containing dehydrogenase, the hydride ion is added/taken stereospecifically from one side (A or B) of the nicotinamide (煙堿) r

24、ing.FAD is able to accept/donate one or two electrons (as hydrogen atom), with absorption maximum shifts from 570 nm to 450nm.NAD and NADP can easily diffuse out of the enzymes, but FMN and FAD are tightly bound to the enzymes.ADP is commonly present all these universal electron carriers (as well as

25、 in Coenzyme A and ATP), suggesting that RNA catalyzed these reactions in the early stages of life.They serve as cofactors of various enzymes catalyzing the oxidation of nutrients. NADH and FADH2 will be further oxidized via the respiratory chain on the inner membrane of mitochondria or plasma membr

26、ane of bacteria for energy generation (transduction).:H-Nicotinamide is derivedFrom niacin (a vitamin)NAD (Nicotinamide Adenine Dinucleotide) andNADP (Nicotinamide Adenine Dinucleotide Phosphate)NicotinamideReactive sitesFMN (Flavin MonoNucleotide) and FAD (Flavin AdenineDinucleotide)異咯嗪環(huán)SummaryBioenergy is