Properties of Enzymes

Question 1

Catalysts

Answer 1

Increase the rate that equilibrium is reached by lowering the activation energy for a process

Question 2

Substrate

Answer 2

What an enzyme acts on

Question 3

Oxidoreductases

Answer 3

Class of enzymes
Catalyze oxidation-reduction reactions
Usually require a coenzyme like NAD+ or NADP+

Question 4

Coenzyme

Answer 4

Molecules that supply enzymes with additional functional groups needed for reactions

Question 5

Transferases

Answer 5

Class of enzymes
Catalyze functional group-transfer reactions
May require coenzymes

Question 6

Hydrolases

Answer 6

Class of enzymes

Catalyze hydrolysis of bonds

Question 7

Lyases

Answer 7

Class of enzymes

Catalyze lysis of a substrate (elimination reactions)

Question 8

Synthases

Answer 8

Lyases that catalyze addition reactions

Question 9

Isomerases

Answer 9

Class of enzymes

Catalyze interconversion between isomers

Question 10

Ligases

Answer 10

Class of enzymes
Catalyze joining of 2 substrates
Often require energy from ATP to drive reaction

Question 11

Initial velocity of enzymatic reaction

Answer 11

Vo= k2 [ES]

Question 12

Steady-state approximation

Answer 12

Period of time (usually at the beginning of a reaction) where the ES complex is formed at the same rate as it decomposes, so that there is no net change in the [ES]

Question 13

Michaelis-Menten equation

Answer 13

Vo=k2[ES]=(Vmax[S])/(Km+[S])

Question 14

kcat (catalytic constant)

Answer 14

kcat= (moles of substrate -> product)/[(second)(moles of enzyme)]
Vmax= kcat[E total]

Question 15

Km (Michaelis constant)

Answer 15

Measure of affinity of the enzyme for the substrate

Km=[S]=0.5Vmax

Question 16

Catalytic proficiency

Answer 16

Effectiveness of an enzyme

Question 17

Lineweaver-Burke plot

Answer 17

Plot 1/Vo (y-axis) vs. 1/[S] (x-axis)
Y-intercept is 1/Vmax
X-intercept is -1/Km

Question 18

Inhibitor

Answer 18

Compound that binds to an enzyme and interferes with its activity

Question 19

Reversible inhibitor

Answer 19

Binds enzyme via noncovalent forces

Question 20

Irreversible inhibitor

Answer 20

Binds enzyme covalently
“Kills” enzyme
Example: organophosphorus inhibitors (sarin: nerve gas)

Question 21

Competitive inhibitor

Answer 21

Most common mode of enzyme inhibition
Binds free enzyme molecule only
Competes with substrate for binding
Doesn't affect Vmax
Raises Km

Question 22

Uncompetitive inhibitor

Answer 22

Binds ES complex (not free enzyme)
Usually only occurs with multisubstrate reactions
Lowers both Vmax and Km

Question 23

Noncompetitive inhibitor

Answer 23

Can bind to enzyme or ES complex
Don’t bind at same site as substrate
Decreases Vmax
Doesn’t affect Km

Question 24

Affinity labels

Answer 24

Irreversible inhibitors with affinity for an enzyme’s active site
Useful in determining which residues are critical for enzymatic activity or are proximal to substrate binding

Question 25

Allosteric enzymes

Answer 25

Don’t follow Michaelis-Menten kinetics (sigmoidal plot instead of line)
Enzymatic activities are changed by metabolic activators and inhibitors (molecules that don’t resemble substrates or products)
Exhibit cooperativity

Question 26

Regulation by covalent modification

Answer 26

Method of controlling enzyme activity through phosphorylation
More permanent that allosteric modification, but can be reversible

Question 27

6 categories of enzymes

Answer 27

Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases