Chapter 11 Enzyme Catalysts Flashcards
•What are single displacement and double displacement bimolecular reactions?
Bimolecular Reactions
■ Typically have 2 (or more) substrates
■ Bisubstrate reactions have unique rate equations
– Single-displacement / Sequential
■ Ordered:
-Leading substrate binds first
-Ordered release of products
■ Random:
-Substrates can bind in any order
-Products released in any order
■ Double Displacement (Ping-pong):
-Product of the first reaction is released prior to the binding of the
second substrate
-Forms a covalently modified enzyme intermediate, F
o What is the function of Oxidoreductases?
Catalyze Oxidation-reduction reactions
o What is the function of Transferases?
Catalyze Transfer of functional groups reactions
What is the function of Hydrolases?
Catalyze Hydrolysis reactions
What is the function of Lyases?
Catalyze Group elimination to form double bonds
What is the function of Isomerases?
Catalyze Isomerization
What is the function of Ligases?
Catalyze Bond formation coupled with ATP hydrolysis
•What is the catalytic site, binding site, exosites of an enzyme?
- The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) and residues that catalyze a reaction of that substrate (catalytic site)
- An exosite is a secondary binding site, remote from the active site, on an enzyme or other protein
•What are the general properties of enzymes and how do enzymatic reactions compare to chemical reactions?
General properties
-Rate Enhancement
■ Catalytic power (rate enhancement) - the ratio of the enzyme-catalyzed rate of a reaction to the uncatalyzed rate
-Specificity
■ Geometric and electronic
complementarity
– Lock and key
– Induced fit
Stereospecificity
■ May identify prochiral molecules
Some enzymes are less specific for substrate specificity and reaction type
– Exception to the rule (in picture)
Differ from chemical reactions
■ Higher reaction rates
■ Use milder reaction conditions
■ Reaction specificity
■ Tightly regulated
How do you calculate the catalytic power of an enzyme?
Look at video and practice everytime you get this card
• Define and recognize cofactors (metal ions, coenzymes, cosubstrates and prosthetic groups)
Cofactors basically act as an enzymes’ “chemical teeth”
-Metal ions- Cu2+, Fe3+, Zn2+ (if Cd2+ or Hg2+ replace Zn renders enzyme inactive)
Coenzymes
-cosubstrates(NAD+ and NADP+) and prosthetic groups (are permanently associated with their protein, often by covalent bonds,ex: cytochrome)
What is apoenzyme and holoenzyme?
A catalytically active enzyme-cofactor complex is called holoenzyme.
The enzymatically inactive protein resulting from the removal of a holoenzyme’s cofactor is referred to as an apoenzyme.
- Describe transition state theory, ΔG‡ and ΔG
■ ΔG - overall free energy change for a reaction, related to the equilibrium constant
■ ΔG‡ - free energy of activation for a reaction, related to the rate constant
- Given a free energy diagram of a reaction be able to determine the rate limiting step
The highest curve on graph
- Define ΔΔG‡cat and be able to calculate rate enhancement
Enzymes provide an alternative pathway for the reaction to occur that has a lower ΔGǂ
What is Acid-base catalysis? What is the mechanism?
■ Most enzymes are active in a narrow pH range
– Substrate binding
– Ionization state of amino acids
– Ionization state of substrate
– Variation in protein structure
May use acid and base catalysis simultaneously
■ RNAse A
■ His 12 – general base
■ His 119 – general acid
■ Second step
■ His 12 – general acid
■ His 119 – general base
What is Covalent catalysis? What is the mechanism?
Forms transient enzyme-substrate covalent bond
■ Primary amine attacks carbonyl forming Schiff base
Forms transient enzyme-substrate covalent bond
Nucleophiles
- Hydroxyl Group
- Sulfhydryl Group
- Amino Group
- Imidazole Group
What is Metal Ion (Metalloenzyme) catalysis? What is the mechanism?
Substrate orientation
■ Red-Ox reactions – can alter metal oxidation state
■ Electrostatically shielding or stabilizing negative charges
■ Carbonic Anhydrase
■ Zn2+ polarized a water molecule to form OH- which acts as a nucleophile to attack CO2
What is the Proximity and Orientation Effect? What is the mechanism?
■ Bring substrates together
By binding to their substrates enzymes:
– Facilitate proper orientation
– Electrostatic catalysis
– charged groups stabilize transition state – Limit relative movement of substrate
What is Transition state binding? What is the mechanism?
■ Enzyme may preferentially bind the transition state over the undistorted substrate
■ Drugs that resemble the transition state may inhibit the enzyme
