Question

The data below were collected during a passive membrane transport experiment where two solutes (A and B) were evaluated relative to their separate transport proteins.

Solute A [S] mM	Transport rate (mmol/min)	Solute B [S] mM	Transport rate (mmol/min)
2.0	0.3	2.0	0.5
4.0	0.7	4.0	1.0
6.0	1.0	6.0	1.4
8.0	1.3	8.0	1.6
10.0	1.5	10.0	1.7
12.0	1.7	12.0	1.8
14.0	1.9	14.0	1.9
16.0	2.0	16.0	2.0
18.0	2.0	18.0	2.0

1. Identify the Km for Solute B. (your answer must include the number and the units)    _______________________
2. Which transport protein is present in the highest concentration?   ___________________________
3. Which ligand has the greatest affinity for its protein? ________________________

Answer 1

Km is the substrate concentration at half maximal velocity. For solute B, the maximum transport rate observed is 2 mmol/min. Thus, half maximal velocity is 1 mmol/min and the corresponding solute B concentration is 4 mM.

Transport protein in higher concentration, will have increased rate at lowest substrate concentration. With this logic, at transport rate of 0.5 mmol/min, transport protein for solute B is at a higher concentration.

Ligand with lowest Km has higher affinity for the protein. With maximum transport rate observed of 2 mmol/min for solute A, the half maximal velocity is 1 mmol/min. Thus Km for solute A is 6 mM. Therefore, solute B with lower Km has higher affinity for its protein.