Abstract
Tungsten is the heaviest element used in biological systems. It occurs in the active sites of several bacterial or archaeal enzymes and is ligated to an organic cofactor (metallopterin or metal binding pterin; MPT) which is referred to as tungsten cofactor (Wco). Wco-containing enzymes are found in the dimethyl sulfoxide reductase (DMSOR) and the aldehyde:ferredoxin oxidoreductase (AOR) families of MPT-containing enzymes. Some depend on Wco, such as aldehyde oxidoreductases (AORs), class II benzoyl-CoA reductases (BCRs) and acetylene hydratases (AHs), whereas others may incorporate either Wco or molybdenum cofactor (Moco), such as formate dehydrogenases, formylmethanofuran dehydrogenases or nitrate reductases. The obligately tungsten-dependent enzymes catalyze rather unusual reactions such as ones with extremely low-potential electron transfers (AOR, BCR) or an unusual hydration reaction (AH). In recent years, insights into the structure and function of many tungstoenzymes have been obtained. Though specific and unspecific ABC transporter uptake systems have been described for tungstate and molybdate, only little is known about further discriminative steps in Moco and Wco biosynthesis. In bacteria producing Moco- and Wco-containing enzymes simultaneously, paralogous isoforms of the metal insertase MoeA may be specifically involved in the molybdenum- and tungsten-insertion into MPT, and in targeting Moco or Wco to their respective apo-enzymes. Wco-containing enzymes are of emerging biotechnological interest for a number of applications such as the biocatalytic reduction of CO2, carboxylic acids and aromatic compounds, or the conversion of acetylene to acetaldehyde.
Highlights
Tungsten and molybdenum are transition metals of the sixth group and occur in nature predominantly in form of their oxyanions—tungstate (WO4 2− ) and molybdate (MoO4 2− ). While their average abundance in the earth’s crust is highly similar, their bioavailability may differ in various aqueous environments. Both metals are present in biological systems in ligation to the so-called metallopterin, which occurs as a three-ring pyranopterin compound in most known molybdo- or tungstoenzymes
Orthologous genes putatively encoding aldehyde oxidoreductase (AOR)-like enzymes are found in many hyperthermophilic archaea and anaerobic bacteria, and some of them have been biochemically characterized, such as AORs and formaldehyde oxidoreductase (FOR) from Thermococcus litoralis, AORs from T. paralvinellae, Clostridium formicoaceticum, Eubacterium acidaminophilum, Desulfovibrio gigas, Methanobacterium thermoautotrophicum, and glyceraldehyde-3-phosphate oxidoreductases (GAPOR) from Pyrobaculum aerophilum [7,29,30,31]
A recent report shows that a thiosulfate reductase (TSR) of the dimethyl sulfoxide reductase (DMSOR) family is involved in thiosulfate respiration in the archaeon P. aerophilum
Summary
Tungsten and molybdenum are transition metals of the sixth group and occur in nature predominantly in form of their oxyanions—tungstate (WO4 2− ) and molybdate (MoO4 2− ). II benzoyl-CoA catalyzing hydride or hydrogen atom transfers (e.g., formate dehydrogenase or class II benzoyl-CoA reductase) sulfuratom atomtransfer transferreactions reactions (polysulfide (polysulfide reductase) It is evident hydratase, all Wco- and Moco-containing enzymes catalyze redox reactions (Figure 2). Discuss thatprocesses may be involved discriminating between molybdenum and tungsten during uptake, Mo/W-MPT synthesis and the between molybdenum and tungsten during uptake, Mo/W-MPT synthesis and the incorporation into their respective apo-enzymes This aspect is of particular importance for the emerging number of organisms that simultaneously produce Wco- and Moco-dependent enzymes. This aspect is of particular importance for the of organisms that simultaneously produce Wco- and Moco-dependent enzymes
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