Metallation and mismetallation of iron and manganese proteins in vitro and in vivo: the class I ribonucleotide reductases as a case study

Abstract

How cells ensure correct metallation of a given protein and whether a degree of promiscuity in metal binding has evolved are largely unanswered questions. In a classic case, iron- and manganese-dependent superoxide dismutases (SODs) catalyze the disproportionation of superoxide using highly similar protein scaffolds and nearly identical active sites. However, most of these enzymes are active with only one metal, although both metals can bind in vitro and in vivo. Iron(II) and manganese(II) bind weakly to most proteins and possess similar coordination preferences. Their distinct redox properties suggest that they are unlikely to be interchangeable in biological systems except when they function in Lewis acid catalytic roles, yet recent work suggests this is not always the case. This review summarizes the diversity of ways in which iron and manganese are substituted in similar or identical protein frameworks. As models, we discuss (1) enzymes, such as epimerases, thought to use FeII as a Lewis acid under normal growth conditions but which switch to MnII under oxidative stress; (2) extradiol dioxygenases, which have been found to use both FeII and MnII, the redox role of which in catalysis remains to be elucidated; (3) SODs, which use redox chemistry and are generally metal-specific; and (4) the class I ribonucleotide reductases (RNRs), which have evolved unique biosynthetic pathways to control metallation. The primary focus is the class Ib RNRs, which can catalyze formation of a stable radical on a tyrosine residue in their β2 subunits using either a di-iron or a recently characterized dimanganese cofactor. The physiological roles of enzymes that can switch between iron and manganese cofactors are discussed, as are insights obtained from the studies of many groups regarding iron and manganese homeostasis and the divergent and convergent strategies organisms use for control of protein metallation. We propose that, in many of the systems discussed, “discrimination” between metals is not performed by the protein itself, but it is instead determined by the environment in which the protein is expressed.

Rapid X-ray Photoreduction of Dimetal-Oxygen Cofactors in Ribonucleotide Reductase

“…Similar MnFe sites meanwhile have been found in a number of related enzymes (7,8,(33)(34)(35)(36) and in addition, MnMn cofactors were described in the tyrosine-radical forming class-Ib RNRs (37)(38)(39). An important common feature of all three types of cofactors is the participation of high-valent intermediates, for example, M(III)M(III) and M(IV)M(III), in the catalytic reactions (25,28,35,36).…”

Genetic Characterization and Role in Virulence of the Ribonucleotide Reductases of Streptococcus sanguinis

“…It has been shown in E. coli, B. subtilis, and B. anthracis that a flavodoxin-like protein named NrdI is required along with O 2 for formation of the NrdF-Mn III 2 -Y ⅐ cofactor in vitro (4,7,45,46). Virtually all bacterial species analyzed to date that possess class Ib RNRs also contain at least one nrdI gene, and in several species, the gene is in the same operon as nrdE and nrdF (14).…”

Direct observation of structurally encoded metal discrimination and ether bond formation in a heterodinuclear metalloprotein

“…Their primary ligand preferences are very similar, and their binding sites in enzymes often appear virtually identical (15). However, their redox potentials differ greatly, and correct discrimination between them is therefore paramount for redox-active enzymes (16). Metal specificity is commonly discussed in terms of the intracellular availability of metal ions and the Irving-Williams series of metal complex stabilities (Mn II < Fe II < Ni II < Co II < Cu II > Zn II ) (17).…”

“…The biochemical selection of manganese over iron thus presents an intricate problem. Metallochaperones are required for correct metallation of some metalloproteins (19)(20)(21)(22), but to date no chaperones have been identified for manganese, and it is unclear whether they are required for diiron clusters (16). Protein-folding location or general metal status can also control metallation (16,23).…”