Enzymes Lecture 3 Flashcards Preview

Biochemistry > Enzymes Lecture 3 > Flashcards

Flashcards in Enzymes Lecture 3 Deck (37):
1

Note: For competitive inhibitions Km stays the same because α = α' (equation for apparent Km is = α*Km/α')

!

2

How can we learn more about the way an enzyme
binds a substrate molecule in a reaction?

We can study the inhibitory effect of small molecules (other than the substrate) on the reaction.

3

______ are important pharmaceutical agents

For example, _____ inhibits the enzyme that catalyzes the first step in the synthesis of prostaglandins (involved in some processes that produce pain)

Enzyme Inhibitors

Aspirin

4

There are two main classes of inhibitors.

This one is noncovalent binding of the inhibitor; can be reversed by removing the inhibitor

What is one common type?

Reversible Inhibition

Competitive

5

There are two main classes of inhibitors.

These bind covalently with or destroy an essential functional group on an
enzyme, or form a very stable noncovalent
association

Irreversible Inhibition

6

This type of inhibitor molecules competes with the substrate molecule for the SAME active site of an enzyme

The inhibitor is very similar to the

Competitive Inhibitor

Substrate

7

By binding to the enzyme’s active site, the
inhibitor prevents the substrate from binding.

It forms an _____ complex and increases/decreases the rate of formation of ES complex.

Can the inhibitor undergo catalysis?

EI complex

decreases the rate of formation of ES complex

No.

8

Competitive inhibitors can be analyzed by _____ kinetics

The M-M equation becomes...

At high [S], the ES complex is UNFAVORED/FAVORED so
that the maximum rate can be obtained?

Steady State Kinetics

Vα = Vmax[S]/(αKm + [S])

Favored

9

What is the equation for α?

What is αKm?

α = 1 + [I]/KI and KI =[E][I]/[EI]

The apparent Km

10

Figure 11.2 describes the effects of competitive inhibition on enzyme kinetics

____ is the same at high [S]

Lineweaver-Burke plots are _____ as expected for steady state kinetics

Vmax

Linear

11

Is Vmax changed by the presence of a competitive inhibitor?

No, it stays the same (the y-intercept is the same)

12

An example of a competitive inhibitor is
shown in Fig 11.3. The molecule UpA is a very good substrate for the enzyme ribonuclease

If the oxygen atom at the cleavage site in the
substrate UpA is replaced by a ____ group to form a phosphonate analog UpcA, a strong competitive inhibitor is formed.

Ribonuclease binds the analog, but it cannot
cleave the _____ bond.

CH2

phosphonate bond

13

Another example of a competitive inhibitor, this is used in medicine as a treatment for patients who have ingested methanol

It competes effectively with methanol as an alternative substrate for ________

Ethanol

alcohol dehydrogenase

14

This is another type of reversible inhibitor, it is seen only with enzymes having two or more substrates

It binds at a site different from the substrate active site

Uncompetitive Inhibitor

15

Unlike a competitive inhibitor, an uncompetitive inhibitor binds only to the ____ complex forming an

Uncompetitive

ES complex

ESI complex

16

In the M-M equation, in the presence of an uncompetitive inhibitor, the equation is altered to....

Where α’ =

Vα= Vmax[S]/(Km + α’[S])

α’ = 1 + [I]/KI’ and KI’=[ES][I]/[ESI]

17

Does an uncompetitive inhibitor lower or increase Vmax?

Does the apparent Km increase or decrease? Why?

Lowers Vmax

Decreases because the [S] needed to reach 0.5 Vmax increases by the factor α’

18

This type of inhibitor binds at a site different from that of the substrate active site, but it binds to either E or ES

Mixed Inhibitor

19

What is the rate equation describing mixed inhibition?

Vo = Vmax[S]/αKm + α’[S]

20

How does mixed inhibition affect Km and Vmax?

A decrease in the apparent affinity of the enzyme for the substrate (a decrease in apparent affinity means the Km value appears to increase) or or in an increase in the apparent affinity (an increase in apparent affinity means the Km value appears to decrease) when the inhibitor binds favorably to the enzyme-substrate complex. In either case the inhibition decreases the apparent maximum enzyme reaction rate

21

When α=α’, it is known as ______ inhibition.
Note this is a special case of mixed inhibition

Noncompetitive

22

What is the only thing affected during noncompetitive inhibition?

Vmax

23

What equation can be used to summarize the effects of all reversible inhibitors?

Vo = Vmax[S]/αKm + α’[S]

24

This category of inhibitors binds covalently with enzymes or destroy a functional group on an enzyme that is essential for the enzyme's activity, or form a very stable noncovalent association

Irreversible Inhibitors

25

In one example of irreversible inhibition, ___ reacts with diisopropylfluorophosphate (DIFP) and irreversibly inhibits the enzyme

This has led to the finding that ______ is the key acctive site Ser residue in the above molecule

Chymotrypsin

Ser195

26

These are a special type of irreversible inhibitor that are unreactive until they bind to the active site of a specific enzyme

Suicide Inactivator

27

The suicide inactivator undergoes the first
few steps of the reaction, but then it’s
converted into a reactive compound that
combines irreversibly with the enzyme.

They are important in drug design

!

28

Enzymes have an optimal ____ for enzyme activity

Above or below the optimal value, activity decreases

pH

29

An ionic interaction essential for stabilization of the enzyme’s active conformation might be eliminated.

For example, consider removing a proton from a his residue. The pH range over which an enzyme undergoes changes in activity can provide information as to the type
of amino acid involved

!

30

____ is a digestive enzyme located in the small intestine

Chymotrypsin

31

Chymotrypsin catalyzes the hydrolytic cleavage of ____ bonds

Peptide

32

Chymotrypsin is specific for peptide bonds adjacent to _____ residues

Aromatic AA resiudes (Trp, Phe, Tyr)

33

Chymotrypsin has a widely studied mechanism which includes....

1) General acid-base catalysis
2) Covalent catalysis
3) Transition-state stabilization

34

Chymotrypsin does not catalyze a direct attack
of water on the peptide bond; a transient
covalent ______ intermediate is formed

acyl enzyme

35

What are the two phases of the chymotrypsin mechanism?

Acylation, deacylation.

36

This phase of the chymostrypsin mechanism cleaves peptide bond and form an ester linkage between peptide carbonyl carbon and the enzyme


Acylation

37

This phase of the chymostrypsin mechanism hydrolyzes the ester linkage and regenerate the nonacylated enzyme

Deacylation