42
Mo
Molybdenum
95.94
Required: most Bacteria, Archaea, and Eukarya
Molybdenum
Major functions in cells: (1)
-Moco-dependent enzymes in Bacteria, Archaea, and Eukaryotes
-FeMoco for Mo-nitrogenases
Environmental and health impacts:
-Mo influences nitrogen fixation
-Moco deficiency is an inherited human disease
Learn More!
(1) Molybdenum: A Redox Cofactor
Mo is a required element for many organisms throughout all three domains of life. Mo often functions as part of a metal-binding pterin (MPT)-based cofactor known as molybdopterin (Moco), which is a required redox cofactor in as many as 50 different enzymes, mostly in bacteria, from four different families (Zhang and Gladyshev, 2008). In some enzymes, MPT functions instead with tungsten (W) to generate Wco (or Tuco), which is analogous to Moco.
​
Mo is very broadly required for life. The major Moco-dependent enzyme families are sulfite oxidase, xanthine oxidase, dimethylsulfoxide reductase, and aldehyde:ferredoxin oxidoreductase (Schwarz et al., 2009). Genomic surveys reveal that 72% of bacteria, 95% of Archaea, and 63% of Eukarya encode Moco enzymes.
​
Mo is also required in nitrogenase, the key enzyme of nitrogen fixation (Hernandez et al., 2009).
(2) Substituting Moco with Wco-dependent Enzyme
The chemical properties of W and Mo are, in many ways, quite similar, and they can catalyze many of the same reactions. There are several examples of enzyme substitutions of these metals as part of a metal-sparing response.
​
The sulfate-reducing bacterium Desulfovibrio alaskensis encodes two formate dehydrogenases. One isozyme (W-FDH) requires W, whereas the other can function with either W or Mo (Mo/W-FDH). Growth in the presence of Mo leads to the upregulation of the Mo/W isozyme whereas addition of 10mM W to the medium led to the exclusive synthesis of the W-FDH isozyme. These results suggest that this organism modulates synthesis of these two enzymes in response to metal availability for cofactor synthesis (Mota et al., 2011).
​
Other organisms may also have functionally redundant pathways that can utilize either Mo- or W-cofactored enzymes. This has been suggested, for example, in the Archaean Methanosarcina acetivorans, where genome analysis suggests the presence of paralogous gene clusters encoding Mo- and W-dependent forms of formylmethanofuran dehydrogenases (Rohlin and Gunsalus, 2010).
(3) Substituting MoFe with VFe/FeFe in Nitrogenases
Mo is required in nitrogenases, the key enzyme of nitrogen fixation. The soil bacterium Azotobacter vinelandii secretes a variety of catechols (metallophores) originally identified as siderophores but which are also thought to help mobilize Mo and other metals to support nitrogenase synthesis (Kraepiel et al., 2009). When Mo is available, and there is no fixed nitrogen, A. vinelandii induces the synthesis of a Mo-nitrogenase (has MoFe subunit) and, conversely, if Mo is unavailable, alternative nitrogenases are synthesized instead (VFe or FeFe subunits).
The Mo-nitrogenase is the most active of the three nitrogenase forms and cells hyperaccumulate Mo in excess of their immediate needs to maintain synthesis of this preferred enzyme. Cytosolic Mo activates expression of the Mo-nitrogenase while repressing the expression of activator proteins needed for expression of both the V-nitrogenase and Fe-nitrogenase (Masepohl and Hallenbeck, 2010).
​