生物反应工程：Chap2_Enzyme_2

v = k2[ES]
we can get,
V ,app[S] m v= K′ [S] m+
Vm Vm,app = [I ] . Where 1 + KI
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Enzyme
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Plots on Non-competitive Inhibitions
The net effect of non-competitive inhibitions is a reduction in with V ,app m same Michaelis-Menten constant. High substrate concentration would not overcome noncompetitive inhibitions. Other reagents need to be added to block binding of it to the enzyme.
The inhibition effect is dominant. The substrate concentration at which the maximum reaction rate achieves can be obtained by setting
vmax =
dv = 0 , and we can get, d[S]
Chapter 2 Enzymes
Lysozyme
Reduced DsbA from E. coli
§2.3.3 Models for More Complex Enzyme Kinetics
Inhibition Enzyme Kinetics
Inhibitors: compounds may bind to enzymes and reduce their activity. Irreversible: Heavy metals (lead, cadmium, mercury) Only reversible by chelating agents, such as EDTA and citrate. Reversible: Dissociate more easily from the enzyme.
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With similar equations and derivations as before,
[E][S] K′ m = [ES]
[ES][S] KSI = [ESS]
[E0 ] = [E] +[ES] +[ESS]
v = k2[ES]
we can get,
v=
V [S] m [S]2 K′ [S] + m+ KSI V m K′ [S] m +1 + [S] KSI
II. On the other hand, when the substrate contains ionic groups and the pH of the medium affects the affinity of the substrate to the enzyme.
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Enzyme
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Enzyme
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Plots on Allosteric Enzymes
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Enzyme
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§2.3.4 Effects of pH and Temperature
Many factors can influence the catalytic activity of enzymes, presumably by affecting the enzyme’s structural or chemical state. pH Temperature Fluid factors (hydrodynamic forces, hydrostatic pressures, interfacial tensions) Chemical agents (alcohol, urea and hydrogen peroxide) Irradiation (light, sound, ionizing radiation)
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Enzyme
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Uncompetitive Inhibitions反竞争 反竞争
This kind of inhibitors can only bind to the ES complex and have no affinity for the enzyme, scheming as follow,
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Enzyme
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Allosteric Enzyme
Some enzymes have more than one substrate binding sites. The binding of one substrate to the enzyme facilitates binding other substrate molecules. The rate expression:
′ Km,app
and
V ,app , m
and the net result is
a reduction in reaction rate.
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Enzyme
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Non-competitive Inhibitions非竞争 非竞争
This kind of inhibitors can bind on sites other than the active site and reduce enzyme affinity to the substrate, scheming as follow,
′ Km,app = ′ Km [I ] . 1 + KI
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Vm Vm,app = [I ] and where 1 + KI
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Enzyme
Plots on Uncompetitive Inhibitions
Obviously, the net effect of uncompetitive inhibitions is a reduction in both
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Enzyme
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With similar equations and derivations as before,
[E][S] K′ m = [ES]
[ES][I] KI = [ESI]
[E0 ] = [E] +[ES] +[ESI ]
v = k2[ES]
we can get,
Vm,app[S] v= ′ Km,app +[S]
Enzyme 13
=
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Discussions about Substrate Inhibitions
Low substrate concentration
V V [S] m m 1 1 K′ 1 [S] v= = = + m Namely << 1 , then K′ [S] + K′ or m v V V [S] KSI 1+ m m m [S]
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Enzyme
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pH Effects
I. Many enzymes have ionic groups on the active sites, then
Variation of pH Enzyme activity
Changes
Modifies
Ionic form
Influences
Reaction rate
d[S] V [S]n m v=− = dt K′′ +[S]n m
where n is the cooperativity coefficient, and n>1 indicates positive cooperativity. By rearranging, we have, v ln = n⋅ ln[S] − ln K′′ m V −v m
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Enzyme
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Substrate Inhibitions底物 底物
High substrate concentrations may cause inhibitions in some enzymatic reactions, scheming as follow,
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Enzyme
when [S]max = K′ SI , mK
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V m . K′ m 2 +1 KSI
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Enzyme
Plots on Substrate Inhibitions
At low substrate concentration, the rate expression approximates to the Michaelis-Menten equation, then the curve in doublereciprocal plot is parallel with the linear line denoted the instance without substrate inhibition. Similarly, at high substrate concentration, the curve in doublereciprocal plot is in hyperbolic mode.
′ Km,app
with reduced reaction rate, but same
maximum reaction rate. Furthermore, the competitive inhibition can be overcome by high substrate concentration.
v= ′ Km,app +[S]
m
[I ] where Vm = k2[E0 ] and Km,app = Km 1+ ′ ′ . KI
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Enzyme
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Plots on Competitive Inhibitions
The net effect of competitive inhibitions is an increased value of
The original Michaelis-Menten equation obtained, thus no inhibition observed. High substrate concentration Namely
Km' << 1 , then [S]
v=
Vm [S] 1+ KSI
or
1 1 [S] = + v V V KSI m m
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Enzyme
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With similar equations and derivations as before,
[E][S] [EI][S] K′ = m = [ES] [ESI]
[E][I] [ES][I] KI = = [EI] [ESI]
[E0 ] = [E] +[ES] +[EI] +[ESI]
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Enzyme
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Competitive Inhibitions竞争性 竞争性
The inhibitors are the substrate analogues, scheming as follow,
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Enzyme
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Using rapid equilibrium assumption:
′ Km = [E][S] [ES] KI = [E][I ] [EI]
and the conservation equation of the enzyme,
[E0 ] = [E] +[ES] +[EI]
and the rate expression,
v = k2[ES]
Combining the above equations to eliminate the [ES] gives, V [S]
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For CASE I (Ionic Enzyme): The reaction scheme is as follow:
E- + H+ K2 EH + S + H+ K1 EH2+
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K m'
EHS
k2
EH + P
Where the meanings of the denotations are: EH: active form E-, EH2+: inactive form E-: deprotonation form EH2+: protonation form