A manganese-dependent 3,4-dihydroxyphenylactate 2,3-dioxygenase from Arthrobacter globiformis strain CM-2 (MndD) cloned in Escherichia coli has been purified to homogeneity. Sedimentation equilibrium analysis indicates an alpha 4 homotetrameric holoenzyme structure (4 x 38,861 Da). Steady-state kinetic analysis of MndD with a variety of substrates and inhibitors yields very similar relative rates to the known Fe(II)- and Mn(II)-dependent 3,4-dihydroxyphenylacetate 2,3-dioxygenases from Pseudomonas ovalis and Bacillus brevis, respectively. Yet, unlike the Fe(II)-dependent enzyme, MndD retains almost all activity in the presence of H2O2 and CN- and is inactivated by Fe(II). ICP emission analysis confirms the presence of 3.0 +/- 0.2 g-atoms Mn (and only 0.7 +/- 0.2 g-atoms Fe) per tetrameric holoenzyme molecule. Comparison of MndD samples with varying metal content, including an apo and partial-apo enzyme preparation, shows a strong positive correlation between specific activity and Mn content. EPR spectra of MndD as isolated exhibit a nearly isotropic g = 2.0 signal having 6-fold hyperfine splitting (A = 95 G) typical of octahedrally coordinated Mn(II) in a protein. Quantitation of the EPR spin yields 3.4 +/- 0.3 g-atoms of Mn(II) per holoenzyme. When exposed anaerobically to its natural substrate, 3,4-dihydroxyphenylacetate (3,4-DHPA), the EPR spectrum undergoes a dramatic change characterized by the attenuation of the g = 2 signal and the appearance of new signals at g = 1.2, 2.9, 4.3, and 16. The g = 4.3 signal displays 6-fold hyperfine splitting (A = 95 G) that unambiguously assigns it to the Mn(II) center. The appearance of these new signals indicates a large increase in zero-field splitting suggestive of a change in ligand coordination to the Mn(II) center. Similarly perturbed signals are seen in the EPR spectra of MndD complexed with the comparably active substrate analog, D,L-3,4-dihydroxymandelate, or the tight-binding inhibitor, p-nitrocatechol, but not in the complexes with weaker binding substrates and inhibitors. The fact that only strong-binding substrates and inhibitors significantly perturb the Mn(II) EPR signal strongly suggests that the substrate coordinates to the Mn(II) center in the catalytic pathway.

Almost all bacterial ring cleavage dioxygenases contain iron as the catalytic metal center. We report here the first available sequence for a manganese-dependent 3,4-dihydroxyphenylacetate (3,4-DHPA) 2,3-dioxygenase and its further characterization. This manganese-dependent extradiol dioxygenase from Arthrobacter globiformis CM-2, unlike iron-dependent extradiol dioxygenases, is not inactivated by hydrogen peroxide. Also, ferrous ions, which activate iron extradiol dioxygenases, inhibit 3,4-DHPA 2,3-dioxygenase. The gene encoding 3,4-DHPA 2,3-dioxygenase, mndD, was identified from an A. globiformis CM-2 cosmid library. mndD was subcloned as a 2.0-kb SmaI fragment in pUC18, from which manganese-dependent extradiol dioxygenase activity was expressed at high levels in Escherichia coli. The mndD open reading frame was identified by comparison with the known N-terminal amino acid sequence of purified manganese-dependent 3,4-DHPA 2,3-dioxygenase. Fourteen of 18 amino acids conserved in members of the iron-dependent extradiol dioxygenase family are also conserved in the manganese-dependent 3,4-DHPA 2,3-dioxygenase (MndD). Thus, MndD belongs to the extradiol family of dioxygenases and may share a common ancestry with the iron-dependent extradiol dioxygenases. We propose the revised consensus primary sequence (G,T,N,R)X(H,A)XXXXXXX(L,I,V,M,F)YXX(D,E,T,N,A)PX(G,P)X{2,3}E for this family. (Numbers in brackets indicate a gap of two or three residues at this point in the sequence.) The suggested common ancestry is also supported by sequence obtained from genes flanking mndD, which share significant sequence identity with xylJ and xylG from Pseudomonas putida.

Whereas all other members of the extradiol-cleaving catechol dioxygenase family are iron-dependent, the 3,4-dihydroxyphenylacetate 2,3-dioxygenase (MndD) from Arthrobacter globiformis CM-2 is dependent on manganese for catalytic activity. Recently, the endogenous iron ligands of one family member, the 2,3-dihydroxybiphenyl 1,2-dioxygenase (BphC), were identified crystallographically as two histidines and a glutamic acid [Sugiyama, K., et al. (1995) Proc. Jpn. Acad., Ser. B 71, 32-35; Han, et al. (1995) Science 270, 976-980; Senda, T., et al. (1996) J. Mol. Biol. 255, 735-752]. Though BphC and MndD have low overall sequence identity (23%), the three BphC metal ligands are all conserved in MndD (H155, H214, and E266). In order to determine whether these residues also act as ligands to manganese in MndD, site-directed mutants of each were constructed, purified, and analyzed for activity and metal content. Mutations H155A, H214A, and E266Q yielded purified enzymes with specific activities of <0.1% of that of the wild-type dioxygenase and bound 0.4, 1.8, and 33% of the wild-type level of manganese, respectively. The relatively high level of manganese [with a Mn(II) EPR signal distinctly different from that of the wild-type enzyme] observed for E266Q suggests that the glutamine may act as a weak ligand to the metal. Mutant E266D, which retains the potential metal binding capability of a carboxylate group, exhibited 12% of the wild-type activity in crude extracts, suggesting that Mn remains bound; however, this mutant protein was too unstable to be purified and analyzed for metal content. On the basis of the low activity and metal content of mutant proteins, we propose that the conserved residues H155, H214, and E266 ligate manganese in MndD. As is the case with the superoxide dismutases, the extradiol-cleaving catechol dioxygenases appear to utilize identical coordinating residues for their iron- and manganese-dependent enzymes.