Enzymes Have properties shared by all catalysts

Enhance the rates of both forward and reverse reactions so equilibrium is achieved more rapidly

• Position of equilibrium is unchanged Reduce activation energy Not permanently altered during the reaction

• Can act over and over again = catalytically

Have unique properties Exhibit extreme substrate (reactant) specificity Exhibit reaction specificity, no side reactions Can couple reactions Can be regulated

Enzyme-catalyzed Reactions

ES = Enzyme-substrate complex

formed when substrates fit into the active site of the enzyme

E + S ES E + P

Michaelis-Menten Theory

• vo = initial velocity, ignore reverse reaction, measure rate before P accumulates

• k1 and k-1 represent rapid noncovalent association of substrate with enzyme’s active site

• k2 = rate constant for the chemical conversion of S to P, the rate-limiting step

vo = k2 [ES]

E + S ES E + P k1 k2

k-1

Michaelis-Menten Theory

Assumptions:• vo = initial velocity, ignore reverse reaction• [ES] is constant• Conversion of S to P is rate-limiting, vo = k2 [ES]

E + S ES E + P k1 k2

k-1

Vmax [S]Vo = -------------

Km + [S]

Fig. 5.4 Michaelis-Menten Plot

Vmax [S]Vo = -------------

Km + [S]

Km = Michaelis constant

Is a measure of the affinity of E for S

k-1 + k2

Km = k1

Inverse relationship: when Km is small affinity is great

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kcat = catalytic constant or turnover number

Moles of S converted to P per second per mole of enzyme ( or active site)

Inverse of kcat tells you how much time is required to convert one mole S to P

Fig. 5.6

Enzyme Regulation See both positive and negative regulation Small molecules interact with enzyme

Can bind to E to affect binding of S to form ES Can bind to ES to affect conversion of S to P

Consider inhibitors first reversible or irreversible inhibition

noncovalent vs. covalent interactions between E and I

Fig. 5.8

Fig. 5.9

Statins are competitive inhibitors of cholesterol synthesis

Fig. 5.9

Fig. 5.8

Fig. 5.11

Fig. 5.11

Fig. 5.8

Fig. 5.12

Fig. 5.12