A protein made from an expression vector can be so plentiful as to account for about 10% of total cell protein. You would like to produce a protein of interest that can be expressed at such a level in Escherichia coli. You plan to induce the expression and collect the cells when cell density in the culture medium reaches about 10? cells/mL. Proteins make up about 15% of total cell weight, and each E. coli cell typically has a mass of about 1 picogram (i.e. 10-¹² g). Assuming that almost 50% of the protein is lost during purification, how many liters of culture should you prepare in order to obtain 15 milligrams of pure protein? Write down your answer in liters, with two decimal places and without units, e.g. 15.00.

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Quizplus · Accepted Answer

The amount of purified protein obtained per milliliter of culture is calculated as: 10⁹ cell/mL × 10-¹² g/cell × 15% × 10% × 50%= 7.5 × 10^-6 g/mL To obtain 15 mg of the protein, therefore, one would need to start with about 2000 milliliters (= 2 L) of culture.

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