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Chapter 12: Q2. (page 373)
If you know a reaction鈥檚 half-life, can you determine its rate constant? What other information do you need?
Yes, half-life of a reaction can be determined through its rate constant without any other information.
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Sphingosine-1-phosphate (SIP) is important for cell survival. The synthesis of SIP from sphingosine and ATP is catalyzed by the enzyme sphingosine kinase. An understanding of the kinetics of the sphingosine kinase reaction may be important in the development of drugs to treat cancer. The velocity of the sphingosine kinase reaction was measured in the presence and absence of threo-sphingosine, a stereoisomer of sphingosine that inhibits the enzyme. The results are shown below.
[Sphingosine] (饾泹惭) | v鈧� (mg min鈦宦�) (no inhibitor) | v鈧� (mg min鈦宦�) (with threo-sphingosine) |
2.5 | 32.3 | 8.5 |
3.5 | 40 | 11.5 |
5 | 50.8 | 14.6 |
10 | 72 | 25.4 |
20 | 87.7 | 43.9 |
50 | 115.4 | 70.8 |
Construct a Lineweaver-Burk plot to answer the following questions:
(a) What are the apparent KM and Vmax values in the presence and absence of the inhibitor?
(b) What kind of an inhibitor is threo-sphingosine? Explain.
From the reaction data below, determine whether the reaction is first order or second order and calculate the rate constant.
Time | Reactant |
0 | 6.2 |
1 | 3.1 |
2 | 2.1 |
3 | 1.1 |
4 | 1.3 |
| 5 | 1.1 |
For an enzyme-catalyzed reaction, the presence of 5 nM of a reversible inhibitor yields a Vmax value that is 80% of the value in the absenceof the inhibitor. The KM value is unchanged. (a) What type of inhibition is likely occurring? (b) What proportion of the enzyme molecules have bound inhibitor? (c) Calculate the inhibition constant.
You are trying to determine the KM for an enzyme. Due to a labmishap, you have only two usable data points:
Substrate concentration (渭M) | Reaction velocity (渭M s鈦宦�) |
1 | 5 |
100 | 50 |
Use these data to calculate an approximate value for KM. Is this valuelikely to be an overestimate or an underestimate of the true value? Explain.
Based on some preliminary measurements, you suspect that a sample of enzyme contains an irreversible enzyme inhibitor. You decide to dilute the sample 100-fold and remeasure the enzyme's activity. What would your results show if an irreversible inhibitor is present?
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