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Entropy

Entropy. Entropy is the quantitative measure of disorder in a system. or Entropy is a thermodynamic property that can be used to determine the energy not available for work in a thermodynamic process. Enthalpy.

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Entropy

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  1. Entropy • Entropy is the quantitative measure of disorder in a system. • or • Entropy is a thermodynamic property that can be used to determine the energy not available for work in a thermodynamic process

  2. Enthalpy The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements, because it simplifies certain descriptions of energy transfer. The total enthalpy, H, of a system cannot be measured directly. Thus, change in enthalpy, ΔH, is a more useful quantity than its absolute value • Enthalpy is a measure of the total energy or Heat content of a system.

  3. Gibbs Free Energy & Entropy Change in entropy (S) of surroundings, proportional to amount of heat (H) transferred from system,& inversely proportional to the temperature (T) of surroundings (heat content ‘H’ is enthalpy) Ssurroundings = - Hsystem/T (1) Total entropy change expression: Stotal = Ssystem + Ssurroundings (2) Substituting eq. 1 into eq. 2 yields Stotal = Ssystem - Hsystem/T (3) Multiplying by -T gives -TStotal = Hsystem - TSsystem (4) -TS has energy units, referred to as, Gibbs free energy G = Hsystem - TSsystem (5)

  4. Gibbs Free Energy & Entropy, cont. -TS has energy units, referred to as, Gibbs free energy G = Hsystem - TSsystem (5) G used to describe energetics of biochemical reactions Equation (3) shows that total entropy will increase only if, Ssystem > Hsystem/T (6) {2nd Law} Multiplying by ‘T’ gives, TSsystem > Hsystem Therefore, entropy will increase only if, G = Hsystem - TSsystem < 0 (7) This means, free-energy change must be negative for a reaction to be spontaneous, with increase in overall entropy of the universe. Therefore, free-energy of the system is the only term we need consider, Any effects of changes within the system on the surroundings are automatically taken into account.

  5. Free-energy change: Spontaneity not Rate G tells us if the reaction can occur spontaneously: If G is negative, reaction spontaneous, exergonic If G is zero, no net change, system at equilibrium If G is positive, free energy input required, endergonic G of a reaction depends only on free-energy of products minus free-energy of reactants. G of a reaction is independent of path (or molecular mechanism) of the transformation 2.G provides no information about the rate of a reaction

  6. Go’ of a reaction is related to K’eq Go’ is standard free-energy change, K’eq is equilibrium constant To determine G, must consider nature of both reactants and products as well as their concentrations Consider this reaction A + B  C + D G is given by G = Go + RTln([C][D]/[A][B]) (1) Standard conditions: concentrations of reactants = 1.0 M (Go’) Convention for biochemical reactions: standard state has pH of 7, (if H+ is a reactant, its activity value = 1). H2O activity value = 1 (Go’) Relation between Go’ & K’eq expresses energetic relation between products and reactants in concentration terms. At equilibrium, G = 0. Equation 1 becomes, 0 = Go’ + RTln([C][D]/[A][B]) (2) Go’ = - RTln([C][D]/[A][B]) (3) and so

  7. Go’ relation to K’eq, cont. Equilibrium constant under standard conditions, K’eq,is defined as K’eq =[C][D]/[A][B] (4) Substituting equation 4 into equation 3 gives Go’ = - RTlnK’eq (5) Go’ = - 2.303RTlog10K’eq (6) = - 2.303x1.987x10-3x298xlog10K’eq = - 1.36xlog10K’eq R = 1.987x10-3 kcal mol-1 deg-1 & T = 298K (=250C) For example, if K’eq = 10, Go’ = -1.36 kal mol-1 Note, for each 10-fold change in K’eq , Go’ changes by 1.36 kcal mol-1

  8. Free energy (G) & equilibrium constant (K)

  9. Example: Isomerization of DHAP to GAP A reaction in glycolysis K’eq = 0.0475 (at equilibrium) Go’ = - 2.303RTlog10K’eq = - 2.303x1.987x10-3x298xlog10(0.0475) = +1.8 kcal mol-1(not spontaneous) M ratio= 1.5x10-2 (DHAP,2x10-4M; GAP,3x10-6M) (initial concentrations) G = Go’ + 2.303RTlog10 (1.5x10-2) =1.8 + 2.303x1.987x10-3x298xlog10(1.5x10-2) =1.8 - 2.49 = -0.69 kcal mol-1(spontaneous) G is concentration dependent

  10. Inhibition of Enzyme Activity • Chemicals that can bind to enzymes and eliminate or drastically reduce catalytic activity called inhibitors. • Enzyme inhibitors can be classifes on the basis of reversibility and competition • Irreversible inhibitors bind tightly to the enzyme and thereby prevent formation of the E-S complex • Reversible inhibitors structurally resemble the substrate and bind at the normal active site as well as other sites.

  11. Irreversible Inhibitors • Irreversible enzyme inhibitors bind very tightly to the enzyme • Binding of the inhibitor to one of the R groups of a amino acid in the active site • This binding may block the active site binding groups so that the enzyme-substrate complex cannot form • Alternatively, an inhibitor may interfere with the catalytic group of the active site eliminating catalysis • Irreversible inhibitors include: • Arsenic • Snake venom • Nerve gas

  12. Reversible Inhibitors • Competitive inhibitors: Structurally resemble the substrate and bind at the normal active site of enzyme Example dihydrofolate to Tetrahydrofolate • Noncompetitiveinhibitors: usually bind at someplace other than the active site binding is weak and thus, inhibition is reversible. • Uncompetitive inhibitiors: Inhibitor can not bind to the free enzyme, but only to the ES-complex.The EIS-complex thus formed is enzymatically inactive. This type of inhibition is rare, but may occur. • Mixed Inhibitors: This type of inhibition resembles the non-competitive, except that the EIS-complex has residual enzymatic activity.

  13. Reversible, Competitive Inhibitors • Reversible, competitive enzyme inhibitors are also called structural analogs • Molecules that resemble the structure and charge distribution of a natural substance for an enzyme • Resemblance permits the inhibitor to occupy the enzyme active site • Once inhibitor is at the active site, no reaction can occur and the enzyme activity is inhibited • Inhibition is competitive because the inhibitor and the substrate compete for binding to the active site • Degree of inhibition depends on the relative concentrations of enzyme and inhibitor

  14. Reversible, Competitive Inhibitors

  15. Competitive inhibition http://www.elmhurst.edu/~chm/vchembook/images/573compinhibit.gif

  16. Reversible, Noncompetitive Inhibitors • Reversible, noncompetitive enzyme inhibitors Non-competitive inhibitors can bind to the enzyme at the same time as the substrate, i.e. they never bind to the active site. Both the EI and EIS complexes are enzymatically inactive • This binding is weak • Enzyme activity is restored when the inhibitor dissociates from the enzyme-inhibitor complex • These inhibitors: • Do not bind to the active site • Do modify the shape of the active site once bound elsewhere in the structure

  17. Allosteric sites In allosteric site, inhibitor is not reacted, but causes a shape change in the protein. The substrate no longer fits in the active site, so it is not chemically changed either. ghs.gresham.k12.or.us/.../ noncompetitive.htm

  18. Mixed inhibition Mixed inhibition This type of inhibition resembles the non-competitive, except that the EIS-complex has residual enzymatic activity. The coenzyme folic acid (left) and the anti-cancer drug methotrexate (right) are very similar in structure.

  19. Four Types of inhibition. • Types of inhibition.

  20. Regulation of Enzyme Activity • One of the major ways that enzymes differ from nonbiological catalysts is in the regulation of biological catalysts by cells • Some methods that organisms use to regulate enzyme activity are: • Produce the enzyme only when the substrate is present – common in bacteria • Allosteric enzymes • Feedback inhibition • Zymogens • Protein modification

  21. Allosteric Enzymes • Effector molecules change the activity of an enzyme by binding at a second site • Some effectors speed up enzyme action (positive allosterism) • Some effectors slow enzyme action (negative allosterism) 19.9 Regulation of Enzyme Activity

  22. Allosteric Enzymes in Metabolism • The third reaction of glycolysis places a second phosphate on fructose-6-phosphate • ATP is a negative effector and AMP is a positive effector of the enzyme phosphofructokinase 19.9 Regulation of Enzyme Activity

  23. Feedback Inhibition • Allosteric enzymes are the basis for feedback inhibition • With feedback inhibition, a product late in a series of enzyme-catalyzed reactions serves as an inhibitor for a previous allosteric enzyme earlier in the series • In this example, product F serves to inhibit the activity of enzyme E1 • Product F acts as a negative allosteric effector on one of the early enzymes in the pathway

  24. Proenzymes or Zymogen • A proenzyme, an enzyme made in an inactive form • It is converted to its active form • By proteolysis (hydrolysis of the enzyme) • When needed at the active site in the cell • Pepsinogen is synthesized and transported to the stomach where it is converted to pepsin

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