Question Description

1) Consider the enzyme mechanism represented by the following chemical equation and then answer the multiple choice questions that follow:

Schematic of steady state enzyme kinetics mechanism

  • Which of the following is kcat (aka, the “turnover number”)?
    A) k2
    B) k1
    C) k-1
    D) k-1/k1
    E) k1/k-1
  • Which is NOT an accurate description of the steady state approximation?
    A) d[ES]/dt = 0
    B) k1[E][S] = k-1[ES] + k2[ES]
    C) d[P]/dt
    D) the rate of ES complex formation and break down are equivalent
  • Which statement regarding KM is NOT true?
    A) It is defined as (k-1 + k2)/k1
    B) It is equal to [E][S]/ES] (KD), if k-1 >> k2
    C) It is the substrate concentration at 0.5 Vmax
    D) It has units of concentration/unit time (e.g., Molar/sec)
  • Which of the following is NOT equivalent to Vmax during steady state conditions?
    A) V0 as [S] approaches infinity
    B) KM/2
    C) k2[E]T
    D) d[P]/dt, [S] >> KM

2) The following kinetics data were obtained for an enzyme:

[S] (μM)

V0 (no inhibitor)

(μmol/min)

V0 (25 μM inhibitor X)

(μmol/min)

V0 (25 μM inhibitor Y)

(μmol/min)

3

10.4

4.1

2.1

5

14.5

6.4

2.9

10

22.5

11.3

4.5

30

33.8

22.6

6.8

90

40.5

33.8

8.1

  1. Plot V0 against [S] for these data (use graph paper or an Excel spreadsheet).
  2. Plot 1/V0 against 1/[S] for these data.
  3. What are the KM values of the enzyme without and with inhibitors?
  4. What are the Vmax values for the enzyme without and with inhibitors?
  5. What kind of inhibitor is X?
  6. What kind of inhibitor is Y?
  7. The enzyme prepartation used in the above experiment was only 15% pure. What is the Vmax of the pure enzyme?
  8. What is the KM of the pure enzyme?

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