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(20 pts) Sphingosine-1-phosphate (SPP) has recently been discovered to be important for cell survival. The synthesis of SPP form 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 (5 mM) and absence of threo-sphingosine, a stereoisomer of sphingosine. The results are shown below. The sphingosine kinase used in all the reactions was 10 mg. Sphingosine (mM) v0 (mg. min-1) v0 (mg. min-1) (no inhibitor) (with threo-sphingosine) 2.5 32.3 7.6 3.5 40 10.5 5 50.8 14.6 10 72 25.4 20 87.7 43.9 115.4 70.8 What is the KI value and what is the Kcat (turn over number)?
Added by Alfredo J.
Step 1
First, we need to determine the type of inhibition. To do this, we can plot the data on a double reciprocal (Lineweaver-Burk) plot. Plot 1/v0 (y-axis) against 1/[Sphingosine] (x-axis) for both sets of data (with and without the inhibitor). Show more…
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Sphingosine-1-phosphate (S1P) is important for cell survival. The synthesis of S1P 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. $$\begin{array}{ccc} \hline \begin{array}{c} \text { Sphingosine] } \\ \text { ( } \mathbf{\mu} \mathbf{M} \text { ) } \end{array} & \begin{array}{c} \boldsymbol{v}_{\mathbf{o}}\left(\mathbf{m g} \cdot \mathbf{m i n}^{-\mathbf{1}}\right) \\ \text { (no inhibitor) } \end{array} & \begin{array}{c} \boldsymbol{v}_{\mathbf{o}}\left(\mathbf{m g} \cdot \mathbf{m i n}^{-1} \mathbf{)}\right. \\ \text { (with } \text {threo} \text { -sphingosine) } \end{array} \\ \hline 2.5 & 32.3 & 8.5 \\ \hline 3.5 & 40 & 11.5 \\ \hline 5 & 50.8 & 14.6 \\ \hline 10 & 72 & 25.4 \\ \hline 20 & 87.7 & 43.9 \\ \hline 50 & 115.4 & 70.8 \\ \hline \end{array}$$ Construct a Lineweaver-Burk plot to answer the following questions: (a) What are the apparent $K_{M}$ and $V_{\max }$ values in the presence and absence of the inhibitor? (b) What kind of an inhibitor is threo -sphingosine? Explain.
Consider the ezyIe Phosphatidylinositol-4,5-bisphosphate kinase (PI3K), which as its name suggests, is a lipid kinase that is responsible for phosphorylating the inositol phospholipid (PI4,5P2) at its position. The enzyme that reverses this reaction by catalyzing the removal of the phosphate is Phosphatase and tensin homolog (PTEN). Thus, PTEN is a phosphatidylinositol-3,4,5-trisphosphate 3-phosphatase. Write the reaction catalyzed by PI3K: PI4,5P2 + ATP -> PI3,4,5P3 + ADP Write the reaction catalyzed by PTEN: PI3,4,5P3 + H2O -> PI4,5P2 + Pi How is it that the value for ΔG is negative for both kinase and phosphatase reactions? The negative value for ΔG indicates that both reactions are energetically favorable and spontaneous. What would the net reaction be if both enzymes (PI3K and PTEN) are active for 10 cycles? Explain. If both enzymes are active for 10 cycles, the net reaction would be the conversion of PI4,5P2 to PI4,5P2, as the phosphorylation and dephosphorylation reactions would cancel each other out. Lipid rafts "organize and concentrate proteins." Use this cartoon to briefly explain how lipid rafts are able to "select" for certain proteins and exclude others.
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