This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'A decrease in the level of heme leads to a reduction in globin synthesis in reticulocytes. Which of the following best explains this phenomenon? \nSelect and explain. \nA. RNA polymerase activity is decreased in reticulocytes by low heme, B. The initiation factor eIF-2 becomes phosphorylated,reducing its level of activity, C. Heme normally activates peptidyl transferase in reticulocytes, D. A tRNA degrading enzyme is active in the absence of heme', 'role': 'user'}, {'content': "Answer: B \nExplanation: One mechanism by which initiation of translation in eukaryotes is effected is by phosphorylation of a small set (S) residue in the alpha subunit of eIF-2. The factor eIF-2 requires activation by interaction with GTP. The energy of GTP hydrolysis is used during translational initiation, thereby allowing eIF-2 to have GDP bound instead of GTP. In order to reactivate eIF-2, the GDP must be exchanged for DTP. This requires an additional protein of the guanine-nucleotide exchange factor (GEF) family known as eIF-2B. The phosphorylated form of eIF-2, in the absence of the eIF-2B, is just as active an initiator of translation as the non-phosphorylated form. However, when eIF-2 is phosphorylated, the GDP-bound complex is stabilized and exchanged for GTP is inhibited. When eIF-2 is phosphorylated, it binds eIF-2B more tightly, thus slowing the rate of exchange. It is this inhibited exchange that affects the rate of initiation. Within reticulocytes, the phosphorylation of eIF-2 is the result of the activity called heme-controlled inhibitor (HCI). Ref: Kennelly P.J., Rodwell V.W. (2011). Chapter 6. Proteins: Myoglobin & Hemoglobin. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.", 'role': 'assistant'}]