Enzyme Inhibitors

Question 1

enzyme inhibition

Answer 1

1) information about enzyme regulation
2) Identifies ligand binding determinants
3) structure of the transition state; thus, helps define the catalytic mechanism
4) Helps determine catalytic strategies, i.e. acid-base,hydrogen bonds, etc.
5) Provides leads for drug discovery

Question 2

Reversible enzyme inhibitors: competitive

Answer 2

Characteristics:
* Inhibitor (I) binds reversibly to active site
* Inhibitor prevents substrate (S) from binding

Both the inhibitor and substrate are competing for the active site i.e. same chemical structure

Question 3

Kinetic determination of competitive inhibition

Answer 3

If an enzyme follows M-M
kinetics….
competitive inhibitors
RAISE/INCREASE
the apparent Km;…… different X intercept
Vmax is NOT CHANGED….. same Y intercept

Question 4

Competitive inhibition example

Answer 4

viral RNA-dependent RNA polymerase
coronavirus
Remdesivir triphosphate

What does Remdesivir do?- nitrile Grp at C1
 Prevents new RNA synthesis – no
new RNA, means no new viral proteins!
 Essentially once Remdesivir is
incorporated, the new RNA chain
synthesis is terminated

Question 5

Reversible enzyme inhibitors: uncompetitive

Answer 5

Characteristics:
* Inhibitor (I) and substrate (S) can bind to the
enzyme simultaneously
* Binds at a site distinct from the active site
* ONLY binds to ES complex

Question 6

Kinetic determination of uncompetitive inhibition

Answer 6

uncompetitive inhibitors
LOWER : Vmax ….
DECREASE : Km…. further left on X

removes the some of the enzyme molecules from the reaction

Since it binds toES complex, not free enzyme (E), Km decreases (less substrate needed to reach Vmax).

Question 7

Reversible enzyme inhibitors: mixed inhibitors

Answer 7

Characteristics:
* Inhibitor (I) and substrate (S) can bind to the enzyme
simultaneously
* Binds at a site distinct from the active site- allosteric
* BUT binds to both free enzyme (E) AND ES complex

Question 8

Kinetic determination of mixed inhibition

Answer 8

Mixed inhibitors usually affect both Vmax and Km

Km can increase or decrease, depending upon if
the inhibitor binds to free enzyme-increase OR ES complex-decrease

Question 9

Reversible enzyme inhibitors: mixed inhibitor - noncompetitive

Answer 9

Noncompetitve inhibition is rarely encountered in
experiments, BUT this inhibitor affects Vmax, NOT Km
* This is a special type of mixed inhibition
* In noncompetitive inhibition, the factor α = α’

Question 10

IRREVERSIBLE enzyme inhibitors

Answer 10

Characteristics:
* Free enzyme is NOT regenerated
after interaction with these inhibitors
* Form covalent interactions
OR highly stable non-covalent interactions
* Useful for studying reaction
mechanisms; identify amino acids in enzymes with key catalytic functions

Question 11

Active site-directed irreversible inhibitors

Answer 11

also called “affinity labels”
* are chemically reactive compounds that are designed to resemble substrate
* bind specifically to the
active site AND form covalent bonds with the enzyme’s amino acid residues

Question 12

Suicide/mechanism-based inhibitors

Answer 12

are chemically UNREACTIVE compounds, until they bind to the active site of a specific enzyme

undergo the first steps of the enzyme’s catalytic mechanism BEFORE being converted into a reactive compound that interacts IRREVERSIBLY with enzyme

Example : Allopurinol is a suicide inhibitor for xanthine oxidase

Question 13

Treatment of Gout

Answer 13

combination of nutritional and drug
therapies
 Food rich in nucleotides withheld (liver, glandular products)
 Drugs include colchicine (antimitotic), uricosuric drugs (to enhance excretion of uric acid), and allopurinol

allopurinol–> Oxypurinol

Question 14

Penicillin

Answer 14

Penicillin and its derivatives are
IRREVERSIBLE inhibitors of
bacterial transpeptidase

Question 15

Beta Lactamase suicide inhibitor

Answer 15

b lactamase - cleaves b-lactam ring and makes antibiotics inactive= resistance

CLAVULANIC ACID

Question 16

Transition state analogs: IRREVERSIBLE
enzyme inhibitors

Answer 16

non-covalent binding is so tight that the
inhibitor dissociates only rarely

Transition state analogs are stable molecules made to “look” more like TS species and bind more tightly to the enzyme than substrate

Question 17

Transition state analogs- example

Answer 17

HIV-protease inhibitors

HIV protease inhibitors:
- OH mimics the oxygen in the
tetrahedral intermediate
- benzyl ring helps to position
the drug into the active site

Question 18

Allosteric Enzymes