Entropy
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Transcript of Entropy
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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
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Enthalpy
Enthalpy is a measure of the total energy or Heat content of a system.
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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
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Gibbs Free Energy & Entropy
Change in entropy (S) of surroundings, proportional to amount ofheat (H) transferred from system, & inversely proportional to thetemperature (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
-TStotal = Hsystem - TSsystem (4)
-TS has energy units, referred to as, Gibbs free energy
G = Hsystem - TSsystem (5)
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Gibbs Free Energy & Entropy, cont.
-TS has energy units, referred to as, Gibbs free energy G = Hsystem - TSsystem (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, TSsystem > Hsystem
Therefore, entropy will increase only if,G = Hsystem - TSsystem < 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.
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Free-energy change: Spontaneity not Rate
G tells us if the reaction can occur spontaneously:
1. If G is negative, reaction spontaneous, exergonic
2. If G is zero, no net change, system at equilibrium
3. 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.
1. 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
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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 andproducts as well as their concentrations
Consider this reactionA + 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)
and so Go’ = - RTln([C][D]/[A][B]) (3)
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Go’ relation to K’eq, cont.
Equilibrium constant under standard conditions, K’eq, is defined asK’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
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Free energy (G) & equilibrium constant (K)
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Example: Isomerization of DHAP to GAP A reaction in glycolysis
Go’ = - 2.303RTlog10K’eq
= - 2.303x1.987x10-3x298xlog10(0.0475)
= +1.8 kcal mol-1 (not spontaneous)
K’eq = 0.0475 (at equilibrium)
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)
M ratio = 1.5x10-2 (DHAP,2x10-4M; GAP,3x10-6M)
(initial concentrations)
G is concentration dependent
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.
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
Reversible Inhibitors
Competitive inhibitors: Structurally resemble the substrate and bind at the normal active site of enzyme Example dihydrofolate to Tetrahydrofolate
Noncompetitive inhibitors: 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.
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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
Reversible, Competitive Inhibitors
Competitive inhibition
16http://www.elmhurst.edu/~chm/vchembook/images/573compinhibit.gif
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
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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
Mixed inhibition
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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.
Regulation of Enzyme Activity
One of the major ways that enzymes differ from nonbiological catalysts is in the regulation of biological catalysts by cellsSome methods that organisms use to regulate enzyme activity are:
1. Produce the enzyme only when the substrate is present – common in bacteria
2. Allosteric enzymes3. Feedback inhibition4. Zymogens5. Protein modification
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
Allosteric Enzymes in MetabolismThe 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
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
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