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

Enthalpy is a measure of the total energy or Heat content of a system.

3

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

4

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)

5

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.

6

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

7

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)

8

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

9

Free energy (G) & equilibrium constant (K)

10

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.

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

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

18

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

19

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.

Types of inhibition.

Four Types of inhibition.

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