9/14/16 Bio-Energetics : Biochemistry the quantitative study of energy transductions in living cells and the 4. Bio-Energetics physical-chemical nature underlying these processes. 4.1) Introduction Prof. Dr. Klaus Heese Cells Need Energy Several Forms of Energy in Biological Systems • ----> to generate and maintain its highly ordered structure - Kinetic energy (including heat or thermal energy, the energy of the motions of molecules (biosynthesis of macromolecules). - Mechanical Energy (e.g. changes in lengths of cytoskeletal filaments generates forces that push or pull on membranes • ----> to generate motion (mechanical work). --- > see lecture and organelles) --- > see lecture about neuron-energy about neuron-energy - Potential energy (chemical potential energy (stored in the • ----> to generate concentration and electrical gradients bonds connecting atoms in molecules), concentration gradient (across membranes), electric potential (across across cell membranes (active transport). plasma membranes)). • ----> to generate heat and light. Cells have to use chemical energy A typical (bio-) chemical reaction can be described as: aA + bB + cC + … ---> zZ + yY + xX + … • Living cells are generally held at constant temperature and pressure: chemical energy (free (Gibbs) energy, D G ) has to <--- be used by living organisms. With the equilibration constant: • Biological energy transformation obey the two basic laws of thermodynamics. K eq = [X] x [Y] y [Z] z • The free energy concept of thermodynamic is more [A] a [B] b [C] c important to biochemists than to chemists = k f /k r ; where k f or k r are the rate constants for the forward and reverse reactions, respectively 1
9/14/16 Thermodynamic quantities describe energy changes occurring in a chemical reaction • If D G <0, the forward reaction (from left to right as usually written) will tend to occur spontaneously. • D G = G products - G reactants = D H - T D S = (Gibbs Free Energy [J/mol]; Enthalpy [KJ/mol], Entropy [J/mol K]) • If D G >0, the reverse reaction will tend to occur. D G' o + RT ln Q (Q = [products]/[reactants]) • D G' o = - RT ln K ' eq ( K ' eq : equilibrium constant) • If D G =0, both forward and reverse reactions occur at equal • The actual free energy change ( D G ) determines whether a rates; the reaction is at equilibrium. reaction occurs spontaneously. • The standard free energy change in biochemistry ( D G' o ) is a constant (measured under a standard set of conditions). • D G for a reaction can be larger, smaller, or the same as D G' o , depending on the concentrations of the reactants and products. - In an exothermic reaction, the products contain less bond energy than the reactants, the liberated energy is usually converted to heat (the energy of molecular motions), and D H is negative. • The D G and D G' o values are additive when reactions are - in an endothermic reaction, the products contain more bond coupled , thus a thermodynamically unfavorable reaction can be driven by a favorable one. energy than the reactants, heat is absorbed, and D H is positive. • The overall K ` eq is multiplicative (the product of two), although D G' o is additive (the algebraic sum of two). - the combined effects of the changes in the enthalpy and entropy determine if D G for a reaction is positive or negative. • Note: the rate of a chemical reaction has nothing to do with its An exothermic reaction ( D H<0) in which entropy increases D G or D G' o , but is determined by its activation energy ( D G ‡ )! ( D S>0) occurs spontaneously ( D G<0). An endothermic (see ---> enzymes as catalysts to lower E A ) reaction ( D H>0) will occur spontaneously if D S increases enough so that the T D S term can overcome the positive D H. - many biological reactions lead to an increase in order and thus a decrease in entropy ( D S<0). The D G and D G' o values are additive when reactions are ATP is the universal ‘ currency ’ coupled : for biological energy D G' o of a reaction can be calculated from its K eq D G' o = -2.3RT log K eq = - 1362 log K eq • This was first perceived by Fritz Lipmann and Herman Kalckar in 1941 when studying glycolysis. (under standard conditions) • Hydrolysis of the two phosphoanhydride bonds in ATP generates K eq = 10 -( D G ’ o /2.3RT) more stable products releasing large amount of free energy • ( D G' o is about -30.5 kJ/mol; D G p in cells is -50 to -65 kJ/mol ). An unfavorable chemical reaction can proceed if it is coupled with an energetically favorable reaction: A ---> B + X D G' o = + 5 kcal/mol • The ATP molecule is kinetically stable at pH 7 and enzyme <--- catalysis is needed for its hydrolysis. X ---> Y + Z D G' o = - 10 kcal/mol • ATP actually exists as a sum of various species in cells (e.g., ATP 4- <--- and MgATP 2- ). ----------------------------------------------------------------- D G' o = - 5 kcal/mol Sum: A ---> B + Y + Z <--- 2
9/14/16 Hydrolysis of ATP releases Substantial Free Energy ATP + H 2 O ---> ADP + P i + H + and Drives Many Cellular Processes ( D G' o is about -30.5 kJ/mol; D G p in cells is -50 to -65 kJ/mol ). Adenine ATP + H 2 O ---> AMP + PP i + H + The ATP molecule is kinetically stable at pH 7 and enzyme catalysis is needed for its hydrolysis. ADP + H 2 O ---> AMP + P i + H + D G' o is about 7.3 kcal/mol for the hydrolysis of one bond and about 3x more than D G' o for: Phosphoester bond Ribose Example: ATP provides energy usually through group transfer (protein could also be such acceptors) 2. 1. Nucleophilic attacks Not phosphate Gln synthetase 3. ATP usually provides energy by group transfer of See: Neuron-Glia interaction ! phosphoryl groups , not phosphate groups; forming covalent intermediates), not by simple hydrolysis. ATP is not a long-term storage form of free energy in ATP has an intermediate phosphoryl group transfer living cells, but phosphocreatine is one such phosphoryl potential, thus ADP can accept and ATP can donate reservoir, or so-called phosphagen. Therefore, PC can phosphoryl groups ( forming the ATP-ADP cycle and transfer a P i on ADP to form ATP. acting as an energy currency) 3
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