The vitamin coenzymes are unstable in light. Intracellular vitamin B is maintained in two active coenzymes, methylcobalamin and 5-deoxyadenosylcobalamin. Cobalt is the active center of a group of coenzymes called cobalamins. Different methanogenic reactions are catalyzed by unique sets of enzymes and coenzymes. Organic cofactors are sometimes further divided into "coenzymes" and "prosthetic groups". Different sources give slightly different definitions of coenzymes, cofactors, and prosthetic groups. There are three well-known and -studied classes of SOD metallic coenzymes that exist in plants. Accordingly, many p97/CDC48 coenzymes and adaptors have domains that can recognize ubiquitin. The B complex vitamins function as enzyme cofactors (coenzymes) or the precursors for them. This continuous regeneration means that small amounts of coenzymes can be used very intensively. When the enzyme is inhibited by methotrexate, the cellular levels of folate coenzymes diminish. The electron transport chain oxidizes coenzymes NADH and FADH2. Nucleotides also act as coenzymes in metabolic-group-transfer reactions. It covalently binds FAD, and does require coenzymes & molecular oxygen for the oxidocyclization reaction. These group-transfer intermediates are the loosely bound organic cofactors, often called "coenzymes".