Chapter8 Introduction to enzymology

The difference between enzatalyzed chemical reactions
Enzyme catalyzed reactions are much faster than uncatalyzed reactions. Enzyme catalyzed reactions display saturation kinetics with respect to substrate concentration. Enzyme catalyzed reactions are optimized for specific temperature and pH values.
1. Enzymes are not consumed or altered by the reaction they catalyze.
☺ Just as a construction worker can take a pile of lumber and build a home without being physically changed by the process, an enzyme takes substrates and converts them into products without being physically changed or consumed.
☺ At equilibrium, the ratio of substrates to products is the same regardless of whether an enzyme catalyst is present. Although the final equilibrium ratio of reactants to products is not altered by an enzyme, the rate at equilibrium is achieved is increased.
Enzymes lower ∆G‡ but don’t affect ∆G for the reaction
General Properties of Enzymes
2. Enzymes bind substrates to their active site and stabilize the transition state of the reaction.
Active site of chymotrypsin
What does the active site do?
The active site binds the substrates and positions them in the proper orientation for the reaction to occur. The active site contains chemical groups that stabilize the transition state of the reaction. The active site determines the specificity of the enzyme (i.e. it determines whether a particular substrate is bound and whether a particular product is made).
What is the transition state?
The transition state is the most unstable species on the reaction coordinate (i.e. the species with the highest energy)
The transition state is not an intermediate species
General Properties of Enzymes
1. Higher reaction rates: The rates of enzyme-mediated
processes are accelerated by a factor of 108-1010. Example1: carbonic anhydrase catalyzes: CO2 +H2OH2CO3 nonenzymatic rate constant = 1.3 x 10-1 s-1 enzymatic rate constant = 1 x 106 s-1 (x 7.7 x 106) Example2: Staphylococcal nuclease catalyzes nucleic acid hydrolysis nonenzymatic rate constant = 1.7 x 10-13 s-1 enzymatic rate constant = 95 s-1 (x 5.6 x 1014)
Transition State Theory
Reactants are required to reach a high-energy (unstable) state referred to as the transition state. Residence may last only 10-13 to 10-14s Only a limited number of molecules will possess sufficient energy to reach this transition state . Increasing temperature or other conditions may facilitate reactants reaching the transition state
What is a substrate? – A substrate is the compound that is converted into the product in an enzyme catalyzed reaction. – For the reaction catalyzed by aldolase, fructose 1,6phosphate is the substrate.
2. Enzymes catalyze both the forward and the reverse reaction.
☺ This is an important point. An enzyme does not determine which direction the reaction goes, it only increases the rate at which the reaction approaches equilibrium.
Enzyme catalyzed reactions are much faster than uncatalyzed reactions
Enzyme catalyzed reaction [Product]
Reaction Rate =
D[Product] D(time)
0
Uncatalyzed reaction 0 Time
The transition state cannot be trapped or isolated. Intermediates can be trapped or isolated.
The Transition State
Understand the difference between DG and DG‡ The overall free energy change for a reaction (∆G) is related to the equilibrium ratio of [S] and [P] The free energy of activation for a reaction (DG‡) is related to the reaction rate It is extremely important to appreciate this distinction!
Substrates, products, and enzymes
Enzymes catalyze the rate at which substrates are converted to product
Enzyme
Substrates
Product
Enzymes catalyze the conversion of substrates into products
What is the transition state of a reaction?
The transition state of a reaction is an unstable, high energy species that the substrates of the reaction must go through in order to form product. By lowering the energy of the transition state, enzymes increase the rate of the reaction.
Chapter 8 Introduction to Enzymology
Outline
What is enzyme? Chemical nature of enzymes Catalytic properties of enzymes -Features shared by enzymes and non-enzymatic catalysts -Unique features of enzymes Classification and nomenclature of enzymes
The active site of an enzyme is the place where all of the action occurs. It contains the functional groups (amino acid side chains) that bind the substrate(s) and catalyze it‟s conversion to product(s).
Enzymes catalyze both the reactions in both the forward and reverse direction
LDH
LDH
3. Enzymes do not alter the equilibrium (or equilibrium constant) between substrates and products.
Enzyme catalyzed reactions display saturation kinetics with respect to reactant concentration
Important things to remember about enzymes just like other catalysts
Chemical nature of enzymes
All enzymes are proteins except some RNAs and not all proteins are enzymes It was assumed that all enzymes are proteins until 1982 when Thomas Cech and Sydney Altman discovered catalytic RNAs (Nobel prize in Chemistry, 1989) Catalytic RNA, or ribozymes, satisfy several enzymatic criteria: substrate specificity, enhance reaction rate, and emerge from reaction unchanged
What is Enzymes?
Enzymes are biological catalysts that accelerate the rates of chemical reactions.
Snail without enzyme catalyst
Snail with enzyme catalyst