A deficiency of substance B12 can result in an anemia. Choose the type of anemia that would occur if the substance were deficient.

Functions of vitamin B12 a. Vitamin B12 contains cobalt in a corrin ring that resembles a porphyrin.(1). Vitamin B12 is the cofactor for methylmalonyl-CoA mutase, which catalyzes the rearrangement of methylmalonyl-CoA to succinyl-CoA (see Fig below).(a). This reaction is involved in the production of succinyl-CoA from valine, isoleucine, threonine, methionine, and the propionyl-CoA formed by the oxidation of fatty acids with an odd number of carbons.(2). Vitamin B12 is involved in the transfer of methyl groups from FH4 to homocysteine to form methionine (see Figure below).A vitamin B12 deficiency results in a megaloblastic anemia plus demyelination of nerves, because of reduced levels of SAM in the nervous system. These cells are large because the vitamin deficiency interferes with DNA synthesis, and the cells double in size without being able to replicate their DNA. Once the anemia begins, the large blast cells are released by the marrow in an attempt to control the anemia. A hypochromic, microcytic anemia can result from the lack of iron, or lack of pyridoxal phosphate. Both conditions lead to a reduction in the synthesis of heme, so the red cells cannot carry as much oxygen (which gives them the pale color). The cells are small in order to maximize the concentration of hemoglobin present in the cells. Hemolytic anemia occurs when the red cell membrane fragments, which can occur with pyruvate kinase deficiencies or a lack of glucose-6-phosphate dehydrogenase activity (which results in reduced NADPH levels). Sickle cell anemia is caused by a point mutation in the b-globin gene, substituting a valine for a glutamic acid.The transfer of a 1-carbon unit from serine to deoxyuridine monophosphate (dUMP) to form deoxythymidine monophosphate (dTMP). FH4 is oxidized to FH2 in this reaction. FH2 is reduced to FH4 by dihydrofolate reductase. The rectangles indicate the steps at which the antimetabolites methotrexate and 5-fluorouracil act. FH2 , dihydrofolate; FH4 , tetrahydrofolate; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, reduced NADP.