Lecture 12 - Enzymatic catalysis Flashcards
Enzymes are good catalysts due to their
specificity of substrate binding and optimal arrangement of catalytic groups
Enzymatic catalytic mechanisms are divided into what six classes?
- Acid-base catalysis
- Covalent catalysis
- Metal ion catalysis
- Electrostatic catalysis
- Proximity and orientation effects
- Preferential binding of the transition state complex
General acid catalysis involves
partial proton transfer from a Brønsted acid
General base catalysis involves
partial proton abstraction from a Brønsted base
RNase A is a digestive enzyme that functions to
hydrolyze RNA to individual nucleotides
RNase A contains what two important residues?
His 12 and His 119
In RNase A, what acts as the general base?
His 12
In RNase A, what acts as the general acid?
His 119
In covalent catalysis, reaction rates are accelerated through
the transient formation of a catalyst-substrate covalent bond
In covalent catalysis, the covalent bond is usually formed by the reaction of a nucleophilic group on the ______ with an electrophilic group on the ______
catalyst; substrate
A good covalent catalyst must combine the properties of
high nucleophilicity and the ability to form a good leaving group
Groups with ________ make good covalent catalysts
highly mobile electrons
Which amino acids make good covalent catalysts?
- Histidine
- Cysteine
- Aspartate
- Serine
Serine proteases are a class of
proteolytic enzymes in which a serine hydroxyl plays an essential role in catalysis
In chymotrypsin, the oxygen of the serine side chain
attacks the carbonyl group of the peptide
Two classes of enzymes that contain metal ions
- Metalloenzymes
- Metal-activated enzymes
Metalloenzymes include
tightly bound ions and transition metal ions
Metal-activated enzymes include
loosely bound ions from the solution and alkali and alkaline earth metal ions
Metal ions often act to
neutralize negative charges
Metal ions act much in the same way as a ______ to neutralize negative charges.
proton
A metal ion’s charge makes a bound water molecule more
acidic
The Zn+2 ion of carbonic anhydrase lies at the
bottom of an active site cleft
The most important function of carbonic anhydrase in animals is to
maintain acid-base balance in blood and other tissues
Carbonic anhydrase helps to transport
carbon dioxide out of tissues
Zn+2 polarizes a water molecule to form
OH-
Another important role of metal ions is
charge shielding
DNA polymerases require ______ for activity
metal ions
In electrostatic catalysis, an active site often excludes
water
Electrostatic interactions are much stronger than in
aqueous solutions
In electrostatic catalysis, charge distribution guides polar substrates to the
active site
In proximity, the reaction between bound molecules doesn’t require
an improbably collision of 2 molecules
In orientation effects, reactants are not only close to each other, they’re
oriented in optimal position to react
When imidazole is attached to the reactant, the reaction is _____ times faster
24
In catalysis by preferential transition state binding, the rate is _____ times faster when R is CH3 rather than H
315
Transition state analogs are often
enzyme inhibitors
