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
