As we saw in our previous unit, a simple enzyme reaction can be represented by:

Question

As we saw in our previous unit, a simple enzyme reaction can be represented by: [E] + [S] [ES] rightarrow [E] + [P] Where E is the enzyme, S is the substrate, P is the product, and ES is the enzyme substrate complex. Write an analogous equation describing the passive transport of a solute (S) across the plasma membrane and into the cytosol by way of a transporter protein (T) Your friend's lab is studying a new pharmaceutical agent they hope will cure a hereditary kidney disease. You know nothing about this drug, except your friend gave you a peek at their data (figure on the right) regarding the kinetics of drug uptake into kidney cells. She thinks the data support her hypothesis that a transporter protein is required for uptake into cells, her labmate, however, thinks the data show that this drug simply uses simple diffusion to enter the cell. Briefly explain which idea you think is correct and why. Predict and draw what type of data you would expect for the other mechanism of drug import. Given what you have learned from part A and B, what other parameters could you suggest that your friend extract from her current data?

Explanation / Answer

In some passive transport mechanisms for example glucose transport, the process can be described by analogy with an enzymatic catalysis in which the substrate is glucose outside the cell (Sout), the product is glucose inside (Sin), and the enzyme is the transporter, T. The specificity and kinetics of transporters is closely analogous to that of enzymes. When the rate of substrate (glucose) uptake is measured as a function of external substrate (glucose) concentration, it results in the formation of a hyperbolic plot; if the external substrate (glucose) concentrations is high then the rate of uptake approaches Vmax.

k1   k2 k3

Sout + T ---------------) Sout . T --------------) Sin . T --------------) Sin + T

(--------------- (--------------- (--------------

k-1 k-2 k-3

k1, k-1 ...etc are the forward and reverse rate constants for each step.

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