Enzye Kinetics

Question 1

Zero order

Answer 1

• rate of reaction is independent of (reactant or substrate) but proportional to (catalyst or enzyme)

Question 2

First order rate

Answer 2

• proportional to concentration of reactant to first power

Question 3

Second order reaction

Answer 3

• rate proportional to conc. Of 2 substrates or of 1 to 2nd power
• v=k[S1][S2]

Question 4

Pseudo first order

Answer 4

• rate appears to depend on concentration of only one substrate
• S1 + S2—-> P
• if [S2] is very high and [S1] is very low, the rate will be proportional to [S1] only

Question 5

How do enzymes lower activation energy

Answer 5

• by stabilizing transition state complexes

- no effect on Keq

Question 6

Michaelis-Menten

Answer 6

• v= Vmax[S]/ Km + [S]
• assumes formation of enzyme-substrate complex
• It assumes that the ES complex is in rapid equilibrium with free enzyme
• breakdown of ES to form products is slower than formation of ES and breakdown of ES to re-form E and S

Question 7

Saturation effect

Answer 7

• at low substrate concentration in enzyme catalyzed reactions the velocity slopes off from first order to zero order kinetics at high substrate concentrations
• in first order substrate-enzyme collision is rate limiting
• In zero-order portion the enzyme is saturated and the rate limiting step is occurring on the enzyme surface

Question 8

Lineweaver-Burk Equation

Answer 8

1/V0 = Km/Vmax X 1/[S] + 1/Vmax

Question 9

Km

Answer 9

• small = tight binding, high = weak binding
• different pH’s and temperatures result in different Km’s
• the value of Km approximate the physiological concentration of the true substrate in many cases
• allows for substrate control and elimination

Question 10

Turnover number

Answer 10

• kcat
• number of substrate molecules converted to product per enzyme molecule per unit of time
• measure of enzymes maximal catalytic activity

Question 11

Enzyme Efficiency comparison

Answer 11

• if the enzyme is saturated with S, the one with highest k2 is most efficient. But living cells don’t normally have saturated enzymes
• best enzyme is one with the highest k2/Km
• kcat/Km: specificity constant

Question 12

Effect of pH on enzyme activity

Answer 12

• enzymes have optimum pH’s
• depends on acid-base behavior of AA in active sites
• Extreme pH levels will produce denaturation

Question 13

Effect of Temperature on Enzyme Activity

Answer 13

• increases with temperature
• doubles with 10 degree rise
• most enzymes denatured at 70 degrees

Question 14

Sequential Reactions (single displacement)

Answer 14

• substrates must combine with the enzyme to form a ternary complex before catalysis can proceed

Question 15

Ping-pong mechanisms (double-displacement)

Answer 15

• one or more products are released from the enzyme before all the substrates have been added

Question 16

Competitive Inhibition

Answer 16

• inhibitor combines with free enzyme at active site
• effects can be overcome by raising the concentration of substrate
• changes Km, but Vmax stays the same
• this is because I binds the same site as S

Question 17

Mixed or no competitive

Answer 17

• inhibitor binds enzyme at a site other the active site
• decreases the maximum velocity by not Km
• inhibitor removes a certain fraction of the enzyme regardless of [S]

Question 18

Uncompetitive inhibition

Answer 18

• only binds to the ES complex

Question 19

Methotrexate

Answer 19

• strong competitive inhibitor of dihydrofolate reductase
• shuts down DNA synthesis by inhibiting dihydrofolate reductase
• used in cancer treatment

Question 20

Arsenite

Answer 20

• noncompetitive inhibitor
• blocks catalytic activity of lipoamide-containing enzymes such as the PDH and alpha-ketoglutarate dehydrogenase
• chronic poisoning can come from environmental sources such as arsenic-contaminated drinking water

Question 21

Statins

Answer 21

• lower cholesterol by inhibiting HMG-CoA reductase