Aquacobalamin reductase or cob(II)alamin:NAD+ oxidoreductase participates in tetrapyrrole and chlorophyll metabolism.
Aquacobalamin reductase (EC ) is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are cob(II)alamin and nicotinamide adenine dinucleotide ion, whereas its 3 products are aquacob(III)alamin, nicotinamide adenine dinucleotide, and hydrogen ion. Nicotinamide adenine dinucleotide (NAD) is a cofactor central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. Tetrapyrroles are a class of chemical compounds that contain four pyrrole or pyrrole-like rings. The pyrrole/pyrrole derivatives are linked by (=(CH)- or -CH 2- units), in either a linear or a cyclic fashion. Pyrroles are a five-atom ring with four carbon atoms and one nitrogen atom. Some tetrapyrroles form the active core of compounds with crucial biochemical roles in living systems, such as hemoglobin and chlorophyll. Linear tetrapyrroles (called bilanes) include: Porphin, the simplest tetrapyrrole Porphyrins (/prfrn/ POR-fr-in) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their  carbon atoms via methine bridges (=CH). The parent of porphyrin is porphine, a rare chemical compound of exclusively theoretical interest. The name "porphyrin" derives from the Greek word  (porphyra), meaning purple. One of the best-known families of porphyrin complexes is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. Four PBGs are then combined through deamination into hydroxymethyl bilane (HMB), which is hydrolysed to form the circular tetrapyrrole uroporphyrinogen III. Porphyrins, including heme, the core of hemoglobin Porphine or porphin is an organic chemical compound with formula C 20H 14N 4. Porphine does not occur in nature, and is almost only of theoretical interest; however, substituted derivatives include many biochemically significant compounds called porphyrins, with the dominant example being protoporphyrin IX. 
