Question

1. A transporter protein has two conformations, A and B. Binding site for Slug is open inside the cell for conformation A, while it is open outside the cell for conformation B.
i) Which conformation should have the lowest Kd for Slug, A or B?

ii) Explain why the Kd for Slug cannot be the same in conformations A and B in order for efficient transport to occur.

2. Imagine another protein that couples transport of Slug to the energetically favorable transport of a second molecule, Speedy, down its own concentration gradient. The concentration of Speedy is higher inside the cell than out. Is this new protein a uniporter, antiporter, or symporter? Explain answer

Answer 1

Answer:

1):
i):

• Kd is a measure of affinity. Kd is inversely proportional to affinity.
• Greater the Kd value, lower the affinity.
• The carrier protein must contain different Kd values for the slug for it to bind outside and release it inside. If the Kd value is same in both conformations, the slug would not be released into the cell.

ii):

• Since the protein is released into the cell, The conformation A must have higher Kd value.

2):

• Since speedy is moving from inside to outside and slug is moving outside to inside, it is an antiport system.

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