enzyme protein which lowers activation energy for accelerating chemical reaction.
Distribution among A, B and (AB)* in equilibrium is determined by change of Gibbs
free energy and enzyme does not affect the distribution.
5.4.2 Kinetics of Reactions Mediated by Enzyme
Michaelis and Menten proposed kinetics to understand enzyme’s reaction from
experimental result of hydrolysis of sucrose by invertase [1]. Following conditions
are described in the kinetics. Reaction velocity is proportional to enzyme concentration at constant substrate concentration. At constant enzyme concentration, reaction velocity is proportional to substrate concentration during low substrate
concentration and the velocity is gradually saturated in higher substrate concentration, and to constant velocity. They introduced reaction mechanism by assuming that
intermediate complex (ES) is in equilibrium state with enzyme (E) + product (P) and
there is no reverse reaction from ES at initial state where rate constant is k.
E þ S
k À1
!
k þ1
ES !
k þ2
E þ P
From equilibrium condition, Michaelis constant (equilibrium constant) is shown
as following equation.
E
½ S
½
ES
½
¼
k À1
k þ1
¼ K m
When condition of [S] ) [E] 0 , substrate concentration can be replaced with
equation, [S] ¼ [S] 0 À [ES] ¼ [S] 0 , and Michaelis constant becomes following
equation.
E
½ 0 À ES
½
S
½
ES
½ ¼ K m and this equation is converted to ES
½ ¼
E
½ 0 S
½
K m þ S
½ , then reaction
velocity to generate product is shown as following equation.
v ¼ k þ2 ES
½ ¼
k þ2 E
½ 0 S
½
K m þ S
½ And v ¼
V max S
½
K m þ S
½ . V max is the maximum velocity of the
chemical reaction mediated by enzyme showing catalytic efficiency of enzyme.
Michaelis constant K m shows affinity of enzyme to substrate. Smaller value of K m
means higher affinity.
On the other hand, Briggs and Haldane assume different condition in reaction
mechanism mediated by enzyme. Concentration of intermediate complex is in
equilibrium state and there is no reverse reaction from E + P to ES at initial state.
And reaction mechanism is shown as following equation [2].
E þ S
k À1
!
k þ1
ES !
k þ3
E þ P
74
5 Structure and Function of Protein
Distribution among A, B and (AB)* in equilibrium is determined by change of Gibbs
free energy and enzyme does not affect the distribution.
5.4.2 Kinetics of Reactions Mediated by Enzyme
Michaelis and Menten proposed kinetics to understand enzyme’s reaction from
experimental result of hydrolysis of sucrose by invertase [1]. Following conditions
are described in the kinetics. Reaction velocity is proportional to enzyme concentration at constant substrate concentration. At constant enzyme concentration, reaction velocity is proportional to substrate concentration during low substrate
concentration and the velocity is gradually saturated in higher substrate concentration, and to constant velocity. They introduced reaction mechanism by assuming that
intermediate complex (ES) is in equilibrium state with enzyme (E) + product (P) and
there is no reverse reaction from ES at initial state where rate constant is k.
E þ S
k À1
!
k þ1
ES !
k þ2
E þ P
From equilibrium condition, Michaelis constant (equilibrium constant) is shown
as following equation.
E
½ S
½
ES
½
¼
k À1
k þ1
¼ K m
When condition of [S] ) [E] 0 , substrate concentration can be replaced with
equation, [S] ¼ [S] 0 À [ES] ¼ [S] 0 , and Michaelis constant becomes following
equation.
E
½ 0 À ES
½
S
½
ES
½ ¼ K m and this equation is converted to ES
½ ¼
E
½ 0 S
½
K m þ S
½ , then reaction
velocity to generate product is shown as following equation.
v ¼ k þ2 ES
½ ¼
k þ2 E
½ 0 S
½
K m þ S
½ And v ¼
V max S
½
K m þ S
½ . V max is the maximum velocity of the
chemical reaction mediated by enzyme showing catalytic efficiency of enzyme.
Michaelis constant K m shows affinity of enzyme to substrate. Smaller value of K m
means higher affinity.
On the other hand, Briggs and Haldane assume different condition in reaction
mechanism mediated by enzyme. Concentration of intermediate complex is in
equilibrium state and there is no reverse reaction from E + P to ES at initial state.
And reaction mechanism is shown as following equation [2].
E þ S
k À1
!
k þ1
ES !
k þ3
E þ P
74
5 Structure and Function of Protein