The two acireductone dioxygenase enzymes (ARD and ARD', previously known as E-2 and E-2') from Klebsiella pneumoniae share the same amino acid sequence Swiss:Q9ZFE7, but bind different metal ions: ARD binds Ni2+, ARD' binds Fe2+. ARD and ARD' can be experimentally interconverted by removal of the bound metal ion and reconstitution with the appropriate metal ion. The two enzymes share the same substrate, 1,2-dihydroxy-3-keto-5-(methylthio)pentene, but yield different products. ARD' yields the alpha-keto precursor of methionine (and formate), thus forming part of the ubiquitous methionine salvage pathway that converts 5'-methylthioadenosine (MTA) to methionine. This pathway is responsible for the tight control of the concentration of MTA, which is a powerful inhibitor of polyamine biosynthesis and transmethylation reactions [1,2]. ARD yields methylthiopropanoate, carbon monoxide and formate, and thus prevents the conversion of MTA to methionine. The role of the ARD catalysed reaction is unclear: methylthiopropanoate is cytotoxic, and carbon monoxide can activate guanylyl cyclase, leading to increased intracellular cGMP levels [1,2]. This family also contains other members, whose functions are not well characterised.

1: Mechanistic Studies of Two Dioxygenases in the Methionine Salvage Pathway of Klebsiella pneumoniae(,). Dai Y, Pochapsky TC, Abeles RH; Biochemistry 2001;40:6379-6387. PMID:11371200

2: One protein, two enzymes. Dai Y, Wensink PC, Abeles RH; J Biol Chem 1999;274:1193-1195. PMID:9880484