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CCatalysisatalysis Catalysis provides an additional mechanism by which reactants can be converted to products. The alternative mechanism has a lower energy than the reaction in the absence of a catalyst. v A 0 B v0 no catalyst vc -- catalyst present v c (v0 = -d[A]/dt with no catalyst) (vc = -d[A]/dt with a catalyst) DE NotNot affected Energy barrier without catalyst by catalyst by catalyst Ea,f y

g Ea,f and Ea,r r e Ea,f n are lowloweredered E

l by a i t catalyst n e t E

o a,r P Ea,r

Ea,f ∆E Products Reactants Energy barrier with catalyst Reaction coordinate Generally a catalyst is defined as a substance which increases the rate of a reaction without itself being changed at the end of the reaction.

This is strictly speaking not a good definition because some things catalyze themselves, but we will use this definition for now.

Catalyst supplies a reaction path which has a lower than the reaction in the absence of a catalyst. CCaatatalylysissis bbyy EEnnzyzymmeses

Enzymes may be loosely defined as catalysts for biological systems. They increase the rate of reactions involving biologically important systems. are remarkable as catalysts because they are usually amazingly specific (work only for a particular kind of reaction.)

They are also generally very efficient, achieving substantial Rate increases at concentrations as low as 10-8 M!

Typical molecular weights are 104-106 gm/mole ( ) SumSummmaryary ofof EnzymEnzymee CCharacteristicsharacteristics

1) of large to moderate weight 104 - 106.

2) Extremely efficient (work at 10-8 M)

3) Very specific (work only on special types of reactions). General Behavior of Enzyme Catalyzed Reactions E S () æ æ Æ Products If the initial rate of the reaction is plotted versus the initial concentration of substrate S for a constant enzyme concentration, the following behavior is found: Vi Initial Rate

[S] concentration when Vi = VS / 2

Substrate Concentration Maximum initial rate

Vi Initial Rate

(Half maximum initial rate)

[S] concentration when Vi = VS / 2

Substrate Concentration Vs is found to be directly proportional to the total enzyme concentration (Eo): Vs ~ (Eo) (S) concentration required to reach half maximum initial velocity

(Vi = VS/2) found to be independent of (Eo). (S)1/2 = KM

Explanation:Explanation: MMichichaaeliselis--MMentenenten MMechanismechanism

E is free enzyme and ES is an enzyme-substrate complex

It may generally be assumed that (S) >> (E) since E are so efficient they catalyze reaction at very small concentration. (ES) d = k1 (E) (S) - k-1 (ES) - k2 (ES) First Order Process dt Step 1 Step 2 Step 3 First Ord(ESde)r Process Steady State assumption: 0 = dt

Second Order Process k1(E0 )(S) (ES) = = k1E0/[k1+(k-1+k2)/(S)] k -1 + k2 + k1(S) Mechanism

k1 E + S Æ ES Step 1

k-1 ES Æ E + S Step 2

k2 ES Æ P + E Step 3 All are elementary kinetic steps.

Divide top and bottom by k1 to get Æ Bonus * Bonus * Bonus * Bonus * Bonus * Bonus k2 k2 (E0 ) (E ) (S)1/2 is the substrate concentration when the 0 = K 2 1 + m initial rate reaches half its maximum value. (S) 1 VS/2 2

Km Case III : >> 1 dP k2(E0 ) (S) = K dt 1 + m (S)

Depends linearly on [S] in region of low substrate concentration. dP Vmax Invert this Mechanism = V = k dt Km equation to 1 1 + E + S Æ ES Step 1 (S) get--- k-1 ES Æ E + S Step 2

k2 ES Æ P + E Step 3

{Lineweaver-Burke Plot}

Km is rate at which ES decomposes by two mechanisms (k-1 or k2) divided by rate constant for formation of ES.

Large Km fi weak binding of E to S k-1 + k2 Km = k1 Small Km fi strong binding of E to S

Application of to ecological and toxicological problems

I. Application of : l Degradation of organophosphate pesticides

Enzymes can be used to catalyze degradation of pesticides While extremely beneficial for protection of crops, pesticides can have serious environmental impact Possible deleterious consequences include seepage of these otherwise helpful chemicals into soil and ground

Case study: organophosphate pesticides l abundant l highly toxic l “neutralized” via reaction The same enzyme catalysts which can neutralize these pesticides can also be used to detoxify chemical nerve agents. l nerve agents are structurally similar to organophosphate pesticides Nerve used General in Japanese formula for subway attack. pesticides

l both contain organophosphate

Degradation paraoxon produced in body from parathion Recent studies have shown that enzymes, which effectively degrade organophosphates, can be incorporated into -- specifically, foams -- in order to aid in their practical application.

One enzyme under current investigation is organophosphorus (OPH) a.k.a. phosphotriesterase. Attack by water [derived from Escherichia coli] The degradation reaction of organophosphates works via .

(yellow in solution)