Question

The plasma membrane of E. coli is about 75% protein and 25% phospholipid by weight. How many molecules of membrane lipid are present for each molecule of membrane protein? Assume an average protein Mr of
50,000 and an average phospholipid Mr of 750. What more would you need to know to estimate the fraction of the membrane surface that is covered by lipids?

Answer 1

Mr of protein = 50,000 daltons

50,000 daltons = 1 molecule of protein

or

8.3x 10-20 g = 1 molecule of protein (since 1 dalton = 1.66 x 10-24 g)

therefore no. of protein molecules in 75 grams of protein by weight = 75g/8.3x 10-20 g

= 9.03 x 1020 molecules of protein

Mr of lipid = 750 daltons

750 daltons = 1 molecule of lipid

or

1.24 x 10-21g = 1 molecule of lipid (since 1 dalton = 1.66 x 10-24 g)

therefore no. of lipid molecules in 25 grams of lipid by weight = 25g/ 1.24x 10-21 g

= 2 x 1022 molecules of lipid

Number of molecules of lipid present for each molecule of membrane protein is given by

2 x 1022 molecules of lipid/ 9.03 x 1020 molecules of protein

therefore number of molecules of lipid present for each molecule of membrane protein is approximately

= 22.2

- to estimate fraction of membrane surface covered by lipids

We must know mass /nm2 of proteins and lipids