enzymes
DESCRIPTION
Enzymes. Fall 2007 Lecture 2. Enzymes. High molecular weight proteins 15,000 < MWTRANSCRIPT
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Enzymes
Fall 2007
Lecture 2
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Enzymes• High molecular weight proteins
15,000 < MW <4,000,000
• Catalysts• Nomenclature - end with “ase”• Holoenzyme - enzyme containing a non-protein group like a metal
– (apoenzyme + cofactor)– (protein + metal)
• Isoenzymes - catalyze same rxn• Classified based on type of rxn catalyzed (Table 3.1)
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Examples(enzyme - use - source)
• Trypsin - anti-inflammatory, meat tenderizers - animal pancreas
• amylase - syrup, glucose production - Bacillus subtilis• protease - detergents, silver recovery - B. subilis• invertase - confectionaries- Sacharomyces cerevisiae• cellulase - breaks down cellulose - bacteria/yeast/mold• penicillinase - remove penicillin from allergic individuals
- bacteria
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Enzyme Function• Lower activation energy of a reaction by binding to the
substrate and forming a substrate-enzyme complex• Interaction is due to van der Waals and H-bonding at the
active site• Interaction is complex - studied via Raman Spectroscopy
and X-ray• Enzyme does not affect equilibrium constant or free energy
change
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Lowers the activation energy of a reaction - highly specificDownloaded from
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Lock and Key Mechanism
Specific binding site
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Enzyme Kinetics
S P r = v = dP/dt = k1(S)E
S + E P + E r = v = dP/dt = k1(S)(E)
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Substrate
v
First order
Zero order
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Enzyme Kinetics
Michaelis Menten Approach
(Henri)
S + E <------------> ES -----------> E + P
Based on experimental data
SK
Sv
dt
dPvrate
M max
k1
k2
k3
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Rapid Equilibrium MM
k1 (E)(S) = k2(ES)
or (1)
Rate Equation (2)
substituting for (ES) from Eq 1 into Eq 2
))(()(2
1 SEk
kES
)(3 ESkdt
dPv
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Using the total enzyme balance E0 = E + ES
or
substituting into the rate equation
))((2
13 SEk
kk
dt
dP
))((2
10 SE
k
kEE
Skk
EE
2
1
0
1)(
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we obtain
KM = k2/k1 here is a dissociation constant, it characterizes the interaction of an enzyme with a given substrate S= KM when v = ½ vmax
vmax = k3E0 - maximum reaction rate, proportional to the initial enzyme concentration
SS
kk
EkS
Skk
Ekk
k
dt
dPv
1
2
03
2
1
02
13
1
SK
Svv
M max
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Vmax and KM
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7
Substrate
v
Vmax
KM
Figure 3.3 in book
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In many cases the assumption of rapid equilibrium is not valid although the enzyme-substrate reaction still shows saturation type kinetics.
•In Class Exercise
S + E <----------> ES <-----------> E + P
Assume rapid equilibrium and determine the rate expression for product formationk2
k1
k4
k3
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Quasi Steady State Approach
• Briggs and Haldane – another mathematical approach to the observed experimental MM eqn
S + E <--------> ES -----------> E + P
Assume that the change in (ES) with time is very small compared with to changes in S or P
Rate Equation of (ES)
k2
k1k3
))(())(()()())(()(
321321 ESkkSEkESkESkSEkdt
ESd
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Assuming quasi steady state d(ES)/dt = 0Solve equation for (ES)
Rate Equation for P is substituting for (ES)
Using the total enzyme balance E0 = E + ES
))(()(32
1 SEkk
kES
))(()(32
133 SE
kk
kkESk
dt
dPv
)(132
10 S
kk
kEE
Hint: Solve SS eqn for whatever quantity set equal to zero
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Or
Substituting into the rate equation for E
Rearranging results in the following:
)(132
1
0
Skk
k
EE
)()(
1)(
)())((
32
132
031
32
31
Skk
kkk
SEkkSE
kk
kk
dt
dPv
)(
)(
)(
1
32
03
Sk
kk
SEk
dt
dPv
Hint: Do not multiply through k values
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The equation has the same form as the MM eqn
Where vmax = k3E0 and
Or Figure 3.4
• In Class Exercise
S + E <----------> ES <-----------> E + P
Assume rapid quasi steady state and determine the rate expression for product formation
)(
)(
)(
1
32
03
Sk
kk
SEk
dt
dPv
1
32
k
kkKM
SK
Svv
M max
k1
k2k4
k3
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