lecture – 4 the kinetics of enzyme-catalyzed reactions dr. saleha shamsudin
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Lecture – 4 The Kinetics of Enzyme-Catalyzed
ReactionsDr. Saleha Shamsudin
Effect of pH and temperature
-Enzyme are active only over small range of pH due to the active site functional group charges (ionic form) and the three dimensional shape of enzyme are pH-dependent-Certain enzyme have ionic group on their active sites, and these ionic group must be in a suitable form (acid or base) to function.-Variation in pH of medium result in changes of: -Ionic form of the active site -Activity of enzyme, hence the reaction rate -Affect the maximum reaction rate, Km
-stability of enzyme
-Scheme to describe pH dependence of the enzymatic reaction rate for ionizing enzymes.
-Some cases, the substrate may contain ionic groups, and the pH of medium affects the affinity of substrate to enzyme.-Refer to Eq. 3.44& Eq. 3.45
Variation of enzyme activity with pH for 2 different enzymes
-
Variation of reaction rate with temperature
ascending
descending
The rate varies according to Arrhenius equation
Thermal Denaturation occurred
Restriction of enzyme mobility in a fixed space = enzyme immobilization
Retain high MW compound
Allowing small MW compound access to enzyme
Prior to
Functional groups on support material are usually activated by using chemical reagent such as cyanogen bromide, carbodiimide and glutaraldehyde
Support materials with functional group
Chemical reagent
Cross-linking of enzyme molecule
The agent used are: ____________, ____________, ________________
Cross-linking can be achieved in several ways:1)2)3)
The disadvantages of cross-linking:1)2)
This is the schematic diagram of enzyme immobilization methods:Suggest the method of (a), (b), (c) and (d)?
Quiz
Diffusional Limitation in Immobilized Enzyme System
• Immobilized enzyme system normally includes- insoluble immobilized enzyme- soluble substrate, or product
• They are heterogeneous systems
CONCENTRATION D
IFFERENCE
FILM
TRANSFER
DRIVING FORCEDIFFUSION
HIGH
Immobilized Enzyme
Sb
Low S concentration
Substrate
external diffusion
CONCENTRATION D
IFFERENCE
FILM
TRANSFER
DRIVING FORCEDIFFUSION
HIGH
Immobilized Enzyme
Sb
REACTION
PRODUCT
external diffusion
FILM
TRANSFER
DRIVING FORCEDIFFUSION
HIGH
Immobilized Enzyme
Sb
INTRA-PARTICLE
TRANSFER
FILM
TRANSFER
PRODUCT
HIGH
Immobilized Enzyme
Sb
INTRA-PARTICLE
TRANSFER
REACTION
Diffusional Limitation in Immobilized Enzyme Systems
In immobilized enzyme systems,
the overall production rate is determined by
- liquid film mass transfer (external diffusion)
substrate, product
- intraparticle mass transfer (internal diffusion)
substrate, product in porous supports
- enzyme catalysis reaction
Substrate conc. (g/cm3)
Mass transfer coefficient (cm/s)
Diffusional Limitation in Immobilized Enzyme System
Ss: substrate concentration at the surface;Sb: substrate concentration in bulk solution.
Enzyme
SsSb
Liquid Film Thickness, L
EPk
ES 2E+S
Diffusion Effects in Surface-bound Enzymes on Nonporous Support Materials
Assume the enzyme catalyzed reaction rate follows Michaelis-Mententype kinetics.
Enzyme
SsSb
Liquid Film Thickness, L No intraparticle diffusion
Assume:
-Enzyme are evenly distributed on the surface of a nonporous support material.
-All enzyme molecules are equally active.
-Substrate diffuses through a thin liquid film surrounding the support surface to reach the reactive surface.
-The process of immobilization has not altered the enzyme structure and the intrinsic parameters (Vm, Km) are unaltered.
Diffusion Effects in Surface-bound Enzymes on Nonporous Support Materials
Enzyme are bound on surface
At steady state, the reaction rate=mass transfer rate
][
][''
sm
sm
SK
SVv
If the product formation rate is :
])[]([ sSbSLksJ The external diffusion rate (g/cm2-s):sJ
'mV the maximum reaction rate per unit surface area.(g/cm2-s)
Lk is the liquid mass transfer coefficient (cm/s).
Diffusion Effects in Surface-bound Enzymes on Nonporous Support Materials
Graphical solution for reaction rate per unit of surface area for enzyme immobilized on a non-porous support
][ bSLkv
When the system is strongly external diffusion (liquid film mass-transfer) limited, [Ss]≈0,the overall reaction rate is equal to the rate:
The system behaves as pseudo first order.
The rate is a linear function of bulk substrate concentration.
Da>>1
Diffusion Effects in Surface-bound Enzymes on Nonporous Support Materials
Diffusion Effects in Surface-bound Enzymes on Nonporous Support Materials
When the system is strongly reaction limited,[Sb] ≈ [Ss]
the overall reaction rate is equal to the rate:
][,
]['
bSappmKbSmVv
Da << 1
)]([1
'
,mbL
mmappm KSk
VKK
Km,app is increased. It is a function of mixing speed and Sb.
where
Diffusion Effects in Enzymes Immobilized in a Porous Matrix
- Substrate diffuses through the tortuous pathway within the porous support to reach the enzyme.
- Substrate reacts with enzyme on the pore surface.
Ex. Spherical support particles
Sr
Diffusion Effects in Enzymes Immobilized in a Porous Matrix
Assume:
- Enzyme is uniformly distributed in a spherical support particle.
- The reaction kinetics follows Michaelis-Menten kinetics.
- There is no external diffusion limitation,(no partitioning of the substrate between exterior and interior of support).
A steady state: Diffusion rate =reaction rate
.limitationdiffusion without ratereaction
limitationdiffusion cleintraparti with ratereaction
Relationship of effectiveness factor with the size of immobilized enzyme particle and enzyme loading