A complex iron-calcium cofactor catalyzing phosphotransfer chemistry

Abstract

Alkaline phosphatases play a crucial role in phosphate acquisition by microorganisms. To expand our understanding of catalysis by this class of enzymes we have determined the structure of the widely-occurring microbial alkaline phosphatase PhoX. The enzyme contains a complex active site cofactor comprising two antiferromagnetically-coupled Fe 3+ ions, three Ca 2+ ions, and a μ 3 -bridging oxo group. Notably, the main part of the cofactor resembles synthetic oxide-centered triangular metal complexes. Structures of PhoX-ligand complexes reveal how the active site metal ions bind substrate and implicate the cofactor oxo group in the catalytic mechanism. The presence of iron in PhoX raises the possibility that iron bioavailability limits microbial phosphate acquisition.Phosphate-containing macromolecules and metabolites are essential components of living cells. Under conditions of phosphate deficiency microorganisms obtain phosphate from biologically-derived organic compounds by producing extra-cytoplasmic alkaline phosphatases (1, 2). Prominent amongst these enzymes are phosphate monoesterases of the PhoA and PhoX families which are found in all three domains of life. The archetypal PhoA enzyme of Escherichia coli has been extensively studied (2) but PhoX alkaline phosphatases are minimally characterized and do not exhibit sequence similarity to other phosphotransfer enzymes. Genes encoding PhoX are abundant in ocean bacteria (3)(4)(5) and are also present in bloom-forming cyanobacteria (6), human pathogens (7,8), and eukaryotic green algae including the model organism Chlamydomonas reinhardtii (9).

Saharan dust nutrients promote Vibrio bloom formation in marine surface waters

“…Additionally, several Vibrio species are able to fix N, a process that has a very high Fe demand (38,39). Dust additions also likely added other limiting nutrients in addition to Fe, especially phosphate, which may help to explain the continued response even at very high levels (16). Finally, although the focus of this work was on Vibrio as a model for dust response, the relative abundance of other taxa (e.g., Pseudoalteromonas and Acinetobacter) also increased significantly with dust events and likely competed with Vibrio for uptake of additional Fe and nutrients.…”

“…This genetic repertoire suggests that Vibrio are effective competitors for Fe in many different environmental niches. The availability of Fe can also control the metabolism of other nutrients such as N and P, and can limit C utilization, effectively setting limits on growth for both open ocean and coastal heterotrophic bacteria (16,33,34). Potential colimitation is especially important in light of the fundamental contribution of heterotrophic bacteria to the cycling of C in marine ecosystems (33,37).…”

“…As an essential cofactor in many metabolic processes-including aerobic respiration, photosynthesis, and nitrogen fixation-its availability can be a determinant in the cycling of carbon (C) and biologically important macronutrients, like nitrogen (N) and phosphorus (P) (13,15,16). Dissolved Fe (dFe) is believed to be the most biologically available fraction of Fe, but is present in vanishingly low amounts in marine systems around the world, especially due to the low solubility of Fe(III) in seawater (17).…”

The acceleration of dissolved cobalt's ecological stoichiometry due to biological uptake, remineralization, and scavenging in the Atlantic Ocean

“…Indeed, there have even been reports that the activity of organic phosphorus acquisition may be constrained by zinc availability: recent field studies in the North Atlantic observed stimulation of alkaline phosphatase activity following the addition of zinc (Mahaffey et al, 2014). There are two isoforms of the alkaline phosphatase enzyme, the zinc PhoA and recently characterized iron-calcium PhoX (Duhamel et al, 2010;Shaked et al, 2006;Yong et al, 2014). PhoX was previously thought to be a calciumonly enzyme (Kathuria and Martiny, 2011).…”

Microbial Surface Colonization and Biofilm Development in Marine Environments

“…Furthermore, marine bacteria contain more iron per unit biomass than phytoplankton, and thus, bacterial iron assimilation may constitute another constraint on iron availability to phytoplankton (646). Iron is also a key resource limiting microbial N 2 fixation, phosphate acquisition, and, thus, productivity in the ocean (181,(647)(648)(649). It has been proposed that iron may control productivity in half of the world's oceans (650,651) and may have accounted for onequarter of the decrease in the atmospheric CO 2 concentration during the Earth's historical glacial maxima (652).…”