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Transcript of Catalysis
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Catalysis
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Facts and Figures about Catalysts
Life cycle on the earth– Catalysts (enzyme) participates most part of life cycle
e.g. forming, growing, decaying– Catalysis contributes great part in the processes of converting sun energy to
various other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2
– Catalysis plays a key role in maintaining our environment
Chemical Industry– ca. $2 bn annual sale of catalysts– ca. $200 bn annual sale of the chemicals that are related products– 90% of chemical industry has catalysis-related processes– Catalysts contributes ca. 2% of total investment in a chemical process
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Catalysis
• Catalysis– Catalysis is an action by catalyst which takes part in a chemical reaction process and
can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions (This is one of many definitions)
Three key aspects of catalyst action taking part in the reaction
• it will change itself during the process by interacting with other reactant/product molecules altering the rates of reactions
• in most cases the rates of reactions are increased by the action of catalysts; however, in some situations the rates of undesired reactions are selectively suppressed
Returning to its original form• After reaction cycles a catalyst with exactly the same nature is ‘reborn’• In practice a catalyst has its lifespan - it deactivates gradually during use
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Catalysis action - Reaction kinetics and mechanism
Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction)
– Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that
• The reaction activation energy is altered
• The intermediates formed are different from those formed in non-catalytic reaction
• The rates of reactions are altered (bothdesired and undesired ones)
– Reactions proceed under less demanding conditions • Allow reactions occur under a milder conditions, e.g. at lower temperatures for those
heat sensitive materials
reactant
reaction process
uncatalytic
product
ener
gy
catalytic
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Types of catalysts– Classification based on the its physical state, a catalyst can be
• gas • liquid• solid
– Classification based on the substances from which a catalyst is made• Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.)• Organic (organic acids, enzymes etc.)
– Classification based on the ways catalysts work• Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq)• Heterogeneous - reaction system involves multi-phase (catalysts + reactants/products)
– Classification based on the catalysts’ action• Acid-base catalysts• Enzymatic• Photocatalysis• Electrocatalysis, etc.
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Catalysis: General Mechanistic Outlook
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Alternate Case
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Van’t Hoff Intermediate
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Comparative Activation Energy Analysis
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Acid Catalyzed Reactions
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Mechanism for Acid-base Catalysis
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Acid Catalyzed Reaction
At low pH At high pH At an optimized condition
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Acid-Base Catalysis Graphical Representation
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Acid Catalyzed Fast Pre-equilibria
fSH = fraction of protonated substrate
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Base Catalyzed reaction under fast pre-equilibrium
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Alternate A-B Catalyzed Reaction (Van’t Hoff Intermediate)
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Graphical Representation
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Hydrolysis of Aspirin and its ester Derivative
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Enzyme Kinetics• measurement of velocity = reaction rate
• compare enzymes under different conditions, or from different tissues or organisms– understand how differences relate to physiology/function of organism– e.g., physiological reason for different Km values
• compare activity of same enzyme with different substrates (understand specificity)
• measure amount or concentration of one enzyme in a mixture by its activity
• measure enzyme purity (specific activity = amount of activity/amount of protein)• study/distinguish different types of inhibitors
– info about enzyme active sites and reaction mechanism– development of specific drugs (enzyme inhibitors)
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Enzyme Catalysis
• Six major classes of reaction catalyzed enzymes are Classified as– Oxidoreductase– Transferase– Hydrolase– Lyases– Isomerases– Ligases
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Enzyme Catalysis
Lock and Key Theory• Enzymes are Exact Size fit for
substrates -- conformational changes do not occur when substrate binds during the reaction.
Induced Fit Theory• Enzymes flexible -- conformational
changes can occur when substrate binds during the reaction, to get maximal complementarity to the transition state.
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Initial velocity
The initial velocity increases with [S] at low [S].
Enzyme Kinetics
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Michealis-Menten Model
1. First step: The enzyme (E) and the substrate (S) reversibly and quickly form a non-covalent ES complex.
2. Second step: The ES complex undergoes a chemical transformation and dissociates to give product (P) and enzyme (E).
3. v=k2[ES]4. Many enzymatic reactions follow Michaelis–Menten
kinetics, even though enzyme mechanisms are always more complicated than the Michaelis–Menten model.
5. For real enzymatic reactions use kcat instead of k2.
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The Enzyme-Substrate Complex (ES)
• The enzyme binds non-covalently to the substrate to form a non-covalent ES complex– The ES complex is known as the Michaelis complex.– A Michaelis complex is stabilized by molecular
interactions (non-covalent interactions).– Michaelis complexes form quickly and dissociate
quickly.
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Assumptions1. k1,k-1>>k2 (i.e., the first step is fast and is always at equilibrium).
2. d[ES]/dt ≈ 0 (i.e., the system is at steady state.)
3. There is a single reaction/dissociation step (i.e., k2=kcat).
4. STot = [S] + [ES] ≈ [S]
5. There is no back reaction of P to ES (i.e. [P] ≈ 0). This assumption allows us to ignore k-2. We measure initial velocities, when [P] ≈ 0.
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Michealis-Menten Equation
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M-M Eqn• The final form of M-M equation in the case of a single
substrate is
• kcat (turnover number): how many substrate molecules can one enzyme molecule convert per second
• Km (Michaelis constant): an approximate measure of substrate’s affinity for enzyme
][]][[
SKSEkv
m
totcat
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Linear representation of Enzyme Kinetics
Lineweaver Burk Plot Eadie Plot
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Concentration of Various Species
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Zoomed outlook
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Two Substrate Enzyme Kinetics
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Enzyme Inhibitors
Reversible versus Irreversible • Reversible inhibitors interact with an enzyme
via non-covalent associations • Irreversible inhibitors interact with an enzyme
via covalent associations
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Classes of InhibitionTwo real, one hypothetical
• Competitive inhibition - inhibitor (I) binds only to E, not to ES
• Noncompetitive inhibition - inhibitor (I) binds either to E and/or to ES
• Uncompetitive inhibition - inhibitor (I) binds only to ES, not to E. This is a hypothetical case that has never been documented for a real enzyme, but which makes a useful contrast to competitive inhibition
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Competitive Inhibition
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Uncompetitive Inhibition
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Non-Competitive InhibitionE ES E + P
E I ESI
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Example of Irreversible Inhibition
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Effect of pH on Enzyme Kinetics
Low pH High pH
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Photochemical Processes and Kinetics
• Photochemical process – The initiation of a chemical reaction through the absorption of a photon by an atom or molecule.
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Jablonski Diagram
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Franck Condon Principle
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Photochemical ProcessesAfter population of S1, the fate of the excited species
via photophysical processes:(1) Thermal equilibration of the vibrational Energy =
vibrational relaxation (VR, ~100 fs)(2) Fluorescence: A radiative transition to lose the
excess electronic energy through emission of a photon. (10-9 to 10-6 s)
(3) Intersystem crossing (ISC): A change of spin state to T1, which is forbidden by quantum mechanics.
• Rate of S1 VR > fluorescence ~ ISC.• T1 VR(4) Phosphorescence from T1 to S0, a spin
forbidden transition occurs in 10-6 to seconds timescales.
(5) Internal conversion: A non-radiative process, the S1 state decays to high vibrational levels of S0 followed rapid VR.
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Fluorescence and Fluorescence Quenching
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Photochemical Process
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