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Problem Set 7
1) Consider the enzyme mechanism represented by the following chemical equation and then answer the multiple choice questions that follow:
- 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) the substrate is in equilibrium with the product
D) d[P]/dt is constant
E) 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 |
- Plot V0 against [S] for these data (use graph paper or an Excel spreadsheet).
- Plot 1/V0 against 1/[S] for these data.
- What are the KM values of the enzyme without and with inhibitors?
- What are the Vmax values for the enzyme without and with inhibitors?
- What kind of inhibitor is X?
- What kind of inhibitor is Y?
- The enzyme preparation used in the above experiment was only 15% pure. What is the Vmax of the pure enzyme?
- What is the KM of the pure enzyme?
Additional suggested problems from Voet, Voet & Pratt: Chapter 11 Problems 1, 5-7, 10-12, 18 (pages 358-359). Chapter 12 Problems 2, 7, 11, 22-24 and 30 (pages 398-400).