Dental Biochemistry

Front 1

1. What is enzyme inhibition?

Back 1

Reversible inhibition

Removal of inhibitor results in recovery of enzyme activity

Physiological enzyme inhibitors are generally reversible

Front 2

2. What is enzyme inactivation?

Back 2

Irreversible effects

Some drugs and many toxic substances (toxic metals)

Front 3

3. What are the two types of reversible inhibition?

Back 3

Competitive Inhibition

Non-Competitive Inhibition

Front 4

4. What is competitive inhibition?

Back 4

Compete for active site (inhibitor and substrate bind to same site)

Bind in mutually exclusive fashion with S

Inhibition can be overcome with high [S]

Front 5

5. What is non-competitive inhibition?

Back 5

Do not compete from same binding site

Can bind in presence or absence of substrate

Inhibition cannot be overcome with high [S]

Front 6

6. How does a linear v vs S plot change with competitive inhibition?

Back 6

Vmax remains the same so can still reach it if add enough S
*overcome effects if I

Km increases though
-need more [S] to achieve Vmax

Front 7

7. How does a linear v vs S plot change with non-competitive inhibition?

Back 7

Vmax decreases

Km remains the same
-adding more S does not make a different

Front 8

8. How does a double reciprocal plot change with competitive inhibition?

Back 8

Increase Km

Y-intercept remains the same

X-intercept moves closer to origin

Front 9

9. How does a double reciprocal plot change with non-competitive inhibition?

Back 9

Decrease Vmax

X-intercept remains the same

Y-intercept increases

Front 10

10. How do irreversible inactivators work?

Back 10

Form a covalent bond with enzyme to permanently inactivate it

Front 11

11. What is a "suicide substrate"?

Back 11

It binds to the active site and initiates catalysis

BUT if forms a covalently linked intermediate which the enzyme cannot release

*Enzyme catalyzes own inactivation

Front 12

12. How does penicillin act as a suicide substrate?

Back 12

Mimics structure of substrate involved in bacterial cell wall biosynthesis

It binds to enzyme and forms covalent intermediate that enzyme cannot hydrolyze

Front 13

13. How is aspirin a suicide substrate for cyclooxygenase?

Back 13

Permanently binds COCH3 group

Form a covalent bond

Front 14

14. What two problems does every cell have?

What additional problem do cells in the human body have?

Back 14

1. Balancing
-balance metabolic reactions

2. Responding
-respond to changes in environment

3. Responding
-respond to needs of other cells

Front 15

15. What are the three primary modes of enzyme regulation used by human cells?

Back 15

1. Regulation by reversible binding of metabolites

2. Regulation by covalent modification of enzymes

3. Regulation of the number of enzyme molecules

Front 16

16. What does regulation by reversible binding of metabolites entail?

Back 16

1. Involves substrates, products, and allosteric inhibitors

2. Non-covalent bonding and very fast

3. Main mechanism for balancing of metabolic reaction

Front 17

17. What does regulation by covalent modification of enzymes entail?

Back 17

1. Involves phosphorylation and dephosphorylation of enzymes

2. Requires other enzymes to catalyze covalent modification and is slower

3. Mainly used for responding to signals from other cells

*characteristic of multicelluar organisms

Front 18

18. What does regulation of the number of enzyme molecules (induction/repression) entail?

Back 18

1. Involves protein synthesis or degradation

2. Quite slow

3. Used for responding to signals from other cells or to environmental conditions

Front 19

19. What are the types of regulation by reversible binding of metabolites?

Back 19

1. Regulation by substrate availability

2. Regulation by product inhibition

3. Allosteric enzymes

Front 20

20. What is meant by regulation by substrate availability?

Back 20

If physiological range of [S] is around Km or less the Km, rate of product formation will depend on substrate concentration

If [S] is much greater than Km, rate of product formation will NOT depend on substrate availability

Front 21

21. How does an enzyme's Km relate to regulation by substrate availability?

Back 21

Enzymes with very low Km's (high affinity for S) have rate of product formation that is nearly constant

Others with higher Km's respond to changes in substrate level

Front 22

22. What is regulation by product inhibition?

Back 22

Product of enzymatic reaction is a competitive inhibitor of enzyme

Enzyme can be designed to be shut off by a certain level of product

Front 23

23. What are five important properities of allosteric enzymes?

Back 23

1. Sigmoid kinetics

2. Regulation by effectors

3. Effectors generally change S0.5 for substrate w/o changing Vmax

4. Multiple subunits with multiple active sites

5. Exist in two conformations that are in equilibrium with one another

Front 24

24. What is meant by sigmoid kinetics?

Back 24

Do not display hyperbolic velocity-substrate curve

Do not obey Michaelis-Menten equation

Are subject to substrate saturation

Front 25

25. What are effectors?

Back 25

Inhibitors and/or activators

They do not resemble the substrate(s) and bind to the enzyme at sites far from active site

Front 26

26. What is S0.5?

Back 26

Equivalent of Km

[S] at which velocity is 1/2 of Vma

Front 27

27. What is the mechanism by which allosteric enzymes work?

Back 27

One conformation can bind only substrates and activators

Other conformation can bind only inhibitors

Subunits all change conformation in a concerted fashion (all together)

Front 28

28. What is the threshold effect allosteric enzymes have?

Why do they have a threshold effect?

Back 28

Enzyme has little activity below threshold range of [S] and is activated steeply as [S] increases across threshhold

Because of cooperativity and sigmoid kinetics

Front 29

29. How do inhibitors and activators affect allosteric enzymes?

Back 29

Vmax is unchanged but S0.5 is changed

Inhibitors change threshold levels and increase S0.5

Activators lower threshold level and thus lower S0.5

Front 30

30. What is the concerted model of allosteric mechanism?

Back 30

1. Allosteric enzyme is mult-subunit that exists in relaxed (r) form and taut (t) form

2. T form is inactive form that cannot bind substrate

3. When enzyme undergoes conformational changes all subunits undergo the same conformational change

Front 31

31. What form is the enzyme in what it is all alone?

What does binding of substrate or activator do?

What does binding of inhibitor do?

Back 31

Equilibrium shifts to inactive T form (predominates)

Pulls equilibrium back towards R form

Pulls equilibrium further from R form to T form

Front 32

32. What happens with the first substrate molecule binds?

Back 32

Cooperative binding

First substrate molecule to bind locks all the subunits into active conformation

Front 33

33. What do activators do?

Back 33

Pull equilibrium in direction of active conformation

Facilitate substrate binding

Front 34

34. What are the general principles of pathway regulation?

Back 34

1. First committed step of pathway is usually the regulated step

2. Committed step is unique to that pathway and irreversible

3. Pathways are often regulated by feedback inhibition by the end product(s)

Front 35

35. What is enzyme activation by polypeptide cleavage?

Back 35

Accumulate a lot enzyme protein in inactive form

Activate it all at once by cleavage

*Generally used to activate extracelluar proteins

Front 36

36. In an uncatalyzed reaction how does the double-reciprocal plot look?

Back 36

Line passes through origin

Front 37

37. In a double reciprocal plot what is the x and y intercept?

Back 37

X-intercept = -1/Km

Y-intercept = 1/Vmax

Front 38

38. What is the Lineweaver-Burke equation?

Back 38

1/V = Km/Vmax x 1/S + 1/Vmax

Y = AX +B

A = slope