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Flashcards in Enzye Kinetics Deck (21):

Zero order

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


First order rate

- proportional to concentration of reactant to first power


Second order reaction

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


Pseudo first order

- 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


How do enzymes lower activation energy

- by stabilizing transition state complexes
- no effect on Keq



- 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


Saturation effect

- 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


Lineweaver-Burk Equation

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



- 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


Turnover number

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


Enzyme Efficiency comparison

- 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


Effect of pH on enzyme activity

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


Effect of Temperature on Enzyme Activity

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


Sequential Reactions (single displacement)

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


Ping-pong mechanisms (double-displacement)

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


Competitive Inhibition

- 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


Mixed or no competitive

- 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]


Uncompetitive inhibition

- only binds to the ES complex



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



- 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



- lower cholesterol by inhibiting HMG-CoA reductase