Enzymes

Question 1

Key Points About Enzymes

Answer 1

1) Lower the activation energy
2) Increase the rate of reaction
3) Do not alter the equilibrium constant
4) Are not changed or consumed in the reaction
5) Are pH and temperature-sensitive, with optimal activity at specific pH ranges and temperatures
6) Do not alter the overall change in G of the reaciton
7) Are specific for a particular reaction or class of reactions

Question 2

Major Enzyme Classifications

Answer 2

LI'L HOT"
Ligase
Isomerase
Lyase
Hydrolase
Oxidoreductase
Transferase

Question 3

Oxidoreductases

Answer 3

Catalyze oxidation-reduction reactions: often have a cofactor that acts as an electron carrier, such as NAD+ or NADP+.

Question 4

Reductant

Answer 4

Electron donor in a reaction catalyzed by oxidoreductases

Question 5

Oxidant

Answer 5

Electron acceptor in a reaction catalyzed by oxidoreductases

Question 6

Transferases

Answer 6

Catalyze the movement of a functional group from one molecule to another.
Includes kinases

Question 7

Kinases

Answer 7

A transferase: catalyzes the transfer of a phosphate group, generally from ATP, to another molecule

Question 8

Hydrolases

Answer 8

Catalyze the breaking of a compound into 2 molecules using the addition of water

Question 9

Lyases

Answer 9

Catalyze the cleavage of a single molecule into 2 products: do not require water as a substrate and do not act as oxidoreductases.
The reverse reaction is catalyzed by a synthase

Question 10

Isomerases

Answer 10

Catalyze the rearrangement of bonds within a molecule: catalyze reactions between stereoisomers as well as constitutional isomers

Question 11

Ligases

Answer 11

Catalyze addition or synthesis reactions, generally between large similar molecules, and often require ATP.

Question 12

Endergonic reaction

Answer 12

One that requires energy (positive change in G)

Question 13

Exergonic reaction

Answer 13

One in which energy is given off

Question 14

Apoenzymes

Answer 14

Enzymes without their cofactors

Question 15

Holoenzymes

Answer 15

Enzymes containing their cofactors

Question 16

Cofactors

Answer 16

Generally inorganic molecules or metal ions, and are often ingested as dietary minerals

Question 17

Coenzymes

Answer 17

Small organic groups, the vast majority of which are vitamins or derivatives such as NAD+, FAD, and coenzyme A.

Question 18

Km/Michaelis constant

Answer 18

The substrate concentration at which half of the enzyme’s active sites are full.

Question 19

kcat

Answer 19

Has units of s-1. Measures the number of substrate molecules “turned over”, or converted to product, per enzyme molecule per second.

Question 20

Kcat/Km (catalytic efficiency)

Answer 20

Referred to as the catalytic efficiency of the enzyme. A large kcat (high turnover) or a small Km (high substrate affinity) will result in a higher catalytic efficiency, which indicates a more efficient enzyme

Question 21

Hill’s coefficient

Answer 21

Numerical value that quantifies cooperativity.
If Hill’s coefficient > 1, positively cooperative binding is occurring
If Hill’s coefficient < 1, negatively cooperative binding is occurring
If Hill’s coefficient = 1, enzyme does not exhibit cooperative binding

Question 22

Competitive inhibition

Answer 22

Simply involves occupancy of the active site: substrates cannot access enzymatic binding sites if there is an inhibitor in the way.
Km increases, and Vmax is unchanged

Question 23

Noncompetitive inhibitors

Answer 23

Bind to an allosteric site instead of the active site, which induces a change in enzyme conformation. Binds equally well to the enzyme and the enzyme-substrate complex.
Km remains unchanged
Vmax decreases

Question 24

Mixed inhibition

Answer 24

Results when an inhibitor can bind to either the enzyme or the enzyme-substrate complex, but has different affinity for each.
Binds to an allosteric site
Vmax is always decreased
If inhibitor prefentially binds to enzyme, it increases Km value (lowers affinity)
If inhibitor binds to enzyme-substrate complex, it lowers Km value (increases affinity)

Question 25

Uncompetitive inhibition

Answer 25

Binds only to the enzyme-substrate complex: essentially locks the substrate in the enzyme, preventing its release: can be interpreted as increasing affinity between the enzyme and substrate..
Binds to allosteric site
Decreases both Km and Vmax

Question 26

Zymogens

Answer 26

Inactive enzymes. Contain a catalytic (active) domain and regulatory domain. Regulatory domain must be either removed or altered to expose the active site.