OBJECTIVES:Energy transformation- the process by which energy is converted from one form to anotherBioenergetics- the study of energy conversion in biological systemsA + B C + DExergonic reactions lead to a decrease in free energy and an increase in entropy.Chemical equilibria.Keq = (product of concentrations of products at equilibrium)Equilibria and spontaneity.Energy coupling- the use of an exergonic process to drive an endergonic processATP ADP + inorganic phosphate (Pi)Fig. 6.10- the ATP hydrolysis/regeneration cycle in cellsRates of reactions.Enzyme terminology;Factors influencing enzyme velocityTopic 7: METABOLISM: THERMODYNAMICS, CHEMICAL EQUILIBRIA, ENERGYCOUPLING and CATALYSIS (lectures 9-10)OBJECTIVES:1. Understand the concepts of kinetic vs. potential energy.2. Understand the concepts of free energy and entropy; use these concepts andthermodynamic principles to show whether a particular reaction is going bespontaneous or not.3. Be able to define equilibrium constant and how this relates to degree of spontaneityof a given reaction.4. Understand the process by which an endergonic reaction is coupled to a highlyexergonic reaction and the role of ATP in biological systems.5. Understand the principle of mass action.6. Draw a free energy diagram to explain the concept of activation energy (Ea) andthen show the impact of enzymatic catalysis on Ea.7. Understand the concepts of enzyme velocity, maximal velocity (Vmax) and “affinity”as well as the factors (substrate concentration, pH, temperature etc.) which impactthe rate of enzyme catalyzed reactions.Energy- physico-chemical term for the capacity to do work ( work = moving a forceover a distance); units are in calorie or more commonly in Joule. (note: force = mass xacceleration). There are two forms of energy:(1) kinetic- energy that is actively engaged in doing work(2) potential- energy that is not actively engaged in doing work but has the potential todo so.Energy transformation- the process by which energy is converted from one form toanother- chemical energy into mechanical energy as would take place during muscle contraction- chemical energy into covalent bonds as would take place during the biosynthesis of macromoleculesBioenergetics- the study of energy conversion in biological systemsMetabolism- the sum total of all the chemical reactions taking place in an organism;consists of a network of chemical reactions often called pathways. Two general types ofpathways:(1) catabolic- breakdown complex molecules into simpler molecules(2) anabolic - form complex molecules from simpler molecules; biosynthesisrequires energy inputThermodynamics- the study of energy transformations as applied to all physico-chemical systems including biological.1Consider the following model chemical reaction: A Bwe ask the simple question, what determines whether this reaction takes placespontaneously or not ? The principles of thermodynamics help us to understand thisquestion. First of all we need to define yet another term-free energy- as applied to molecular reactions, it is the energy available to do work;often denoted by the symbol G (for Gibb’s free energy)first law of thermodynamics- energy transformations do not create nor destroy energybut simply result in the interconversion from one form to the othersecond law of thermodynamics- all energy transformations result in an increase indisorder; entropy is a term which is a measure of the extent of disorder in a system.Thus, the second law can be restated by saying that all energy transformations result inan increase in entropy in the system.Now lets apply the above two laws to defining whether a reaction is spontaneous or not- A + B C + D Gi Gf Si Sfwhere Gi = free energy at initial state; Gf = free energy at final state and G = Gf.-Gi and Si = entropy at initial state; Sf = entropy at final state and S = Sf -Si . Thus, whenG = negative value, reaction is spontaneous; it is said to be exergonic; spontaneousreactions lead to a decrease in free energy.S = positive value, reaction is spontaneous; spontaneous reactions lead to an increasein entropy.Exergonic reactions lead to a decrease in free energy and an increase in entropy.Endergonic reactions- are not spontaneous; movement in this direction would lead toan increase in free energy and a decrease in entropy. A good example is biosynthesis oflarge molecules. We’ll see in a few minutes how it is possible to drive endergonicreactions by coupling them with exergonic reactions.Fig. 6.5- relationship of free energy to stability, work capacity and spontaneous changeFig. 6.6 – exergonic vs. endergonic reactionsChemical equilibria.2Suppose you mix A and B together; they will react to form C and D which willaccumulate. C + D will begin to react to form A + B. Eventually,A + B C + D reaction rate = C +D A + B reaction rate; at this point we can say thatthe reaction has reached chemical equilibrium. Each kind of reaction has its ownunique chemical equilibria which can be defined by the equilibrium constant (Keq)-Keq = (product of concentrations of products at equilibrium) (product of concentrations of reactants at equilibrium)in our example above Keq = [C] x [D] [A] x [B]Equilibria and spontaneity.(1) reactions which have Keq >>>> 1 are highly exergonic(2) reactions which have Keq <<<< 1 are highly endergonicHowever you can make an endergonic reaction go in a non-spontaneous direction bycoupling it with an exergonic reaction.Energy coupling- the use of an exergonic process to drive an endergonic processsuppose A B, G A-> B = positive valueX Z, G X-> Z = negative valueGnet = G A->B + G X-> Z ; if Gnet is negative, then the A B reaction willproceed. Energy coupling is extremely common in biological systems. By far, the most commoncoupling reaction is the reaction which involves the hydrolysis of a compound known asATP, adenosine triphosphate:ATP ADP + inorganic phosphate (Pi)fig. 6.8- ATP is very unstable and is spontaneously hydrolyzed by water; this reaction,however, can be coupled to another reaction as shown in fig. 6.9 ( glutamine formation).ATP is often referred to as the energy currency of cells; it is
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