This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
At isoelectric pH protein 
Select and explain. 
A. Have Net charge 'O', B. Are positively charged, C. Are negatively charge, D. Don't migrate
A: Have Net charge 'O'
Answer: A: Have Net charge 'O' 
Explanation: Proteins are least soluble at their isoelectric pH . Significance of isoelectric proteind : The amonoacid composition will determine the isoelectric pH of protein . The alpha amino acids and carboxyl group are utilized for peptide bond formation, and hence are not ionizable.All other ionizable groups present in the protein will influence pl of the protein. At the isoelectric point,the number of anioins andcations present on the protein molecule will be equal and the net charge is zero. At the pl value,the proteins will not migrate in an electrical field.At the pl, solubility,buffering capacity and viscosity will be minimum; and precipatation will be maximum. On the acidic side of pl, the proteins are cations and on alkaline side, they are anions in nature. REFERENCE : DM VASUDEVAN TEXTBOOK SEVENTH EDITION ; PAGE NO :41