Enzymes Flashcards
Most enzymes are…
Globular proteins
Examples of enzyme cofactors
Inorganic (Mg+, Zn+), organic (called coenzymes), prosthetic groups, apoenzymes.
Types of coenzymes
Coenzyme A and NAD+.
Coenzyme bound to enzyme protein =
Prosthetic group.
Apoenzyme
Without coenzyme
Holoenzyme
With coenzyme.
How does an apoenzyme become active
By binding of coenzyme or cofactor to enzyme. Holoenzyme is formed when it binds to the active site.
Oxidoreductases
Catalyze the transfer of electrions.
Transferases
Catalyze group transfer reactions.
Hydrolases
Catalyze hydrolysis reactions.
Lyases
Catalyse cleavage of C-C, C-O, and C-N or other bonds by elimination. Leave behind double bonds or rings.
Isomerases
Catalyze the transfer of groups within molecules to yield isomeric forms.
Ligases
Catalyze the formation of C-C, C-S, C-O and C-N bonds by condensation reaction coupled to cleavage of ATP or similar cofactor.
Enzymes do not…
Affect equilibrium, cannot change free energy.
Enzymes do affect…
Rates of reaction, decrease activation energy.
How do enzymes lower activation energy?
They organize reactive groups into close proximity and proper orientation (into a fairly rigid ES complex) by using enzyme substrate binding energy.
Enzyme active sites are complimentary too…
The transition state of the reaction.
Enzyme substrate binding results in…
Reduction of entropy, hindering the reaction.
Acid-base catalysis
Give and take protons
Covalent catlysis
Change reaction paths. Requires a nucleophile on the enzyme
Metal ion catalysis
Use redox cofactors, pKa shifters. Stabilizes negative charges.
In steady state conditions…
ES formed = ES consumed
ES association and dissociation depend on…
Constants (K) which reflect E and E properties.
Reaction rate is proportional to…
ES complex.
Kcat
Turnover rate, number of S molecules converted to P per time. Unit is 1/s .
Kcat and Km reflect the…
Cellular environment.
Enzyme efficieny
The specificity constant, Kcat/Km.
Enzyme efficient is limited by…
Specificity.
Kinetic mechanism
The order of binding of substrates and release products
Competitive inhibition
Competes with substrate for binding, binds to active site, does not affect catalysis. No change in Vmax, apparent increase in Km.
Uncompetitive inhibition
Only binds to ES complex, does not affect substrate binding, inhibits catalytic function. Decrease in Vmax, apparent decrease in Km.
Mixed inhibition
Binds to enzyme with or without subsrate, binds to regulatory site, inhibits both substrate binding and catalysis. Decrease in Vmax, apparent change in Km.
Noncompetitive inhibition
Type of mixed inhibitors where this is no change in Km.
Irreversible inhibitors react with…
The enzyme. One molecule can permanently shut off one enzyme molecule.
Noncovalent modification: allosteric regulators
Can be positive (improve catalysis) or negative (reduce catalysis).
