MCAT Biochemistry Question 31: Answer and Explanation
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Question: 31
1. At what pH can protein A best be obtained through electrophoresis? (Note: MM = molar mass)
| Protein | pI | MM |
| Protein A | 4.5 | 25,000 |
| Protein B | 6.0 | 10,000 |
| Protein C | 9.5 | 12,000 |
- A. 2.5
- B. 3.5
- C. 4.5
- D. 5.5
Correct Answer: D
Explanation:
In most electrophoresis experiments, we attempt to separate out one component from the others. Because we are attempting to isolate protein A only, a pH that causes protein A to be negative while proteins B and C are neutral or positive will be best. pH 5.5 accomplishes this goal; proteins B and C will be positively charged. A pH of 4.5, choice (C), would make protein A neutral, and it would thus not migrate across the gel. Any neutral impurities would also remain in the well with protein A, making this pH not the best choice.