<p>The two acireductone dioxygenase enzymes (ARD and ARD', previously known as E-2 and E-2') from <taxon tax_id="573">Klebsiella pneumoniae</taxon> share the same amino acid sequence <db_xref db="SWISSPROT" dbkey="Q9ZFE7"/>, but bind different metal ions: ARD binds Ni2+, ARD' binds Fe2+ [<cite idref="PUB00007489"/>]. 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 [<cite idref="PUB00007490"/>]. 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 [<cite idref="PUB00007490"/>, <cite idref="PUB00007489"/>]. </p><p>This family also contains other proteins, whose functions are not well characterised.</p>