This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of amino acids, Subject name: Biochemistry
What is Isoelectric point? 
Select and explain. 
A. When pH = pI, B. When zwitterion exists, C. Protein precipitation occurs, D. All
D: All
Answer: D: All 
Explanation: At Isoelectric pH (pI), protein's net charge is 0, known as Zwitterion. This structure is insoluble, so precipitates. At Acidic pH, protein has positive charge. At Alkaline pH, protein has negative charge.