Category: biosynthesis
Cobalamin = vitamin B12. Absorption: dietary B12 in protein binds INTRINSIC FACTOR (IF, gastric parietal cells) in duodenum → IF-B12 complex absorbed at terminal ileum via cubilin/AMN receptor. Two coenzymes: methylcobalamin (methionine synthase cofactor, see methionine_sam_cycle) and adenosylcobalamin (methylmalonyl-CoA mutase cofactor, propionate catabolism). Deficiency causes: megaloblastic anemia (impaired DNA synthesis via THF trap), subacute combined degeneration of the spinal cord (impaired myelin maintenance from methylmalonate accumulation), hyperhomocysteinemia. Causes: pernicious anemia (autoimmune anti-IF antibodies), atrophic gastritis, terminal ileum disease/resection, strict vegan diet without supplementation. Treatment: IM cyanocobalamin (1000 µg monthly) or high-dose oral; underlying cause-specific.