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Flashcards in Enzymes Deck (42):
1

Enzymes (4)

-proteins that act as catalysts
-increase the rate of chemical reactions
-bind reactants (substrates), convert them to products, and release the products
-they return to their original form at the end.

2

Catalytic power of enzymes

-Enzymes do not invent new reactions.
-Enzymes make reactions occur faster.
-The catalytic power of an enzyme is 106 to 1014.

3

Turnover number

the number of molecules of substrate converted to product per enzymes molecule per second.

4

Specificity

the ability of an enzyme to select just one substrate and distinguish this substrate from a group of very similar compounds.

5

Classification of enzymes
-numbers and names

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

6

Oxidoreductases
-number
-catalyze...

-1
-catalyze oxidation and reduction reactions.

7

Transferases
-number
-catalyze...

-2
-Catalyze transfer of C-, N-, or P-containing groups.

8

Hydrolases:
-number
-catalyze....

-3
-Catalyze hydrolytic cleavage of C-C, C-O, C-N and other covalent bonds.

9

Lyases
-number
-catalyze...

-4
-Catalyze cleavage of C—C, C—O, C—N and other covalent bonds by atom elimination, generating double bonds.

10

Isomerases
-number
-catalyzes...

-5
-Catalyze geometric or structural changes within a molecule.

11

Ligases
-number
-catalyzes...

-6
-Catalyze the joining together (ligation) of two molecules in reactions coupled to the hydrolysis of ATP.

12

Mechanism of enzyme action (2)

-substrate must gain activation energy to reach a point the transition state of the reaction, at which the energy level is maximum.
-transition state of the enzyme-catalyzed reaction has a lower energy than that of the uncatalyzed reaction, the reaction can proceed faster.

13

An enzyme cannot alter the ......, IT CAN ONLY.......

-alter the equilibrium
-can only accelerate the reaction rate.

14

3 basis steps of enzyme- catalyzed reaction

(1)binding of substrate: E +S → ES.

(2)conversion of bound substrate to bound product: ES → EP.

(3)release of product: EP → E + P.

15

The active site (3)

-Within the active site, cofactors and functional groups participate in transforming the bound substrate molecules into products.
-Three-dimensional changes of binding sites allows the reacting portions of the substrates to approach each other from the appropriate angles
-contains functional groups that directly participate in the reaction.

16

Active sites
-functional groups are donated by...

-by the polypeptide chain
-by bound cofactors (coenzymes)

17

Transition state complex
-formed by...
-characteristics (3)

-the activated substrates + the enzyme

-an unstable high-energy complex.
-decomposes to products, which dissociate from the enzyme.
-The enzyme returns to its original form. The free enzyme then binds another set of substrates and repeats the process.

18

Models of the binding interaction between the enzyme and substrate:

-“lock-and-key”.
-“induced-fit”.

19

Lock and Key model

Substrate and enzyme active sites have complementary shapes and binds it:
-Multiple hydrophobic interactions,
-Electrostatic interactions,
-Hydrogen bonds.

20

“INDUCED FIT” MODEL

Enzyme active site forms a complementary shape to the substrate after binding and increases the number of binding interactions

21

Factors affecting reaction velocity

-substrate concentration
-temperature
-pH

22

The rate or velocity of a reaction

the number of substrate molecules converted to product per unit time;

23

Factors affecting reaction velocity
-Substrate concentration

-The rate of reaction increases with substrate concentration until a maximal velocity (V-max ) is reached.
-The leveling off of the reaction rate at high substrate concentrations reflects the saturation with substrate of all available binding sites.

24

Factors affecting reaction velocity
-temperature

-The reaction velocity increases with temperature until a peak velocity is reached
-Further elevation of the temperature results in a decrease in reaction velocity
(optimum temperature)

25

Factors affecting reaction velocity
-pH

-an effect on the ionization of the active site.

26

Holoenzymes vs. Apoenzyme

Holoenzyme --> the active enzyme with its nonprotein component.

Apoenzyme --> is an enzyme without its nonprotein moiety and is inactive.

27

Holoenzymes
-3 "types"

-Prosthetic groups
-Cofactors
-Coenzymes

28

Prosthetic groups (4)

-tightly and stably incorporated into a protein's structure by covalent or noncovalent forces.
-required for the biological function of some proteins.
-organic or inorganic.
-not composed of amino acids.

29

Prothetic groups
-most common ones

Metalloenzymes --> metals (Co, Cu, Mg, Mn, and Zn)

Vitamins (FMN, FAD, Biotin)

30

Cofactors (3)

-non- substrates that bind enzymes and help catalyze reactions
-must be present in the surrounding the enzyme for catalysis to occur.
-most common: metal ions

31

Coenzymes
-characteristics
-function

Serve as recyclable shuttles or group transfer agents (organic carrier molecules). Function of these shuttles is twofold:
1) they stabilize species
2) serve as an adaptor or handle that facilitates the recognition and binding of small chemical groups.

32

Chemical moieties transported by coenzymes:

-Acetate (coenzyme A).
-Methyl groups (folates).
-Oligosaccharides (dolichol).

33

Isoenzymes

-distinct enzyme forms that catalyze the same reaction
-arise through gene duplication
-exhibit differences in properties

34

Michaelis- Menten equation

-Reflects the affinity of the enzyme for that substrate.
-Km = to the substrate concentration at which the reaction velocity is equal to 1⁄ 2 V max.

35

Significance of Km
1. Small Km
2. Large Km

1. high affinity of the enzyme for substrate (a low concentration of substrate is needed to half-saturate the enzyme).

2.low affinity of the enzyme for substrate (a high concentration of substrate is needed to half-saturate the enzyme).

36

Relationship of velocity to enzyme concentration

The rate of the reaction is directly proportional to the enzyme concentration at all substrate concentrations.

37

Irreversible inhibitors

They act irreversibly by chemically modifying the enzyme. Making or breaking covalent bonds with AA residues essential for:
-Substrate binding,
-Catalysis,
-Maintenance of the enzyme's functional conformation.

38

"Suicide" inhibitors (mechanism- based inhibitors)

-bind to the active site and initiate the catalysis process as if it was the normal substrate.
HOWEVER
-a reactive intermediate is formed that modifies the residue on the active site, blocks the function of a catalytically essential residue

39

Reversible inhibition
-most common types

-Competitive.
-Non- competitive.
-Uncompetitive.

40

Competitive inhibition
-what happens
-vmax and km

-competes with the substrate for that site and more substrate is needed

-Vmax: does not change
-Km increases

41

Non- competitive inhibition
-what happens
-vmax and km

-Inhibitor and substrate bind at different sites on the enzyme.

-Vmax: decrease
-Km: same

42

Uncompetitive inhibition
-what happens
-vmax and km

-An inhibitor that is uncompetitive with respect to a substrate will bind only to enzyme containing that substrate.

-Vmax: decrease
-Km: decrease