Abstract
The molybdenum cofactor (Moco) is the prosthetic group of all molybdenum-dependent enzymes except for nitrogenase. The multistep biosynthesis pathway of Moco and its function in molybdenum-dependent enzymes are already well understood. The mechanisms of Moco transfer, storage and insertion, on the other hand, are not. In the cell, Moco is usually not found in its free form and remains bound to proteins because of its sensitivity to oxidation. The green alga Chlamydomonas reinhardtii harbors a Moco carrier protein (MCP) that binds and protects Moco but is devoid of enzymatic function. It has been speculated that this MCP acts as a means of Moco storage and transport. Here, the search for potential MCPs has been extended to the prokaryotes, and many MCPs were found in cyanobacteria. A putative MCP from Rippkaea orientalis (RoMCP) was selected for recombinant production, crystallization and structure determination. RoMCP has a Rossmann-fold topology that is characteristic of nucleotide-binding proteins and a homotetrameric quaternary structure similar to that of the MCP from C. reinhardtii. In each protomer, a positively charged crevice was identified that accommodates up to three chloride ions, hinting at a potential Moco-binding site. Computational docking experiments supported this notion and gave an impression of the RoMCP-Moco complex.
Highlights
Molybdenum is an essential trace element in most species (Mendel, 2013)
This notion is in line with the fact that the members of the former two phyla are oxygen-evolvers and as such are in need of an Moco carrier protein (MCP) to protect the highly labile molybdenum cofactor (Moco) (Witte et al, 1998), while the members of the latter two phyla live in microaerophilic or anaerobic environments and are able to dispense with Moco protection through an MCP
We identified and characterized the first confirmed Moco carrier protein originating from a prokaryotic organism, the cyanobacterium R. orientalis
Summary
The abundant chemical form of molybdenum in the environment is the biochemically inactive oxyanion molybdate, which gains activity when complexed by the dithiolene motif of a specific tricyclic pterin called molybdopterin (MPT). The compound formed, named the molybdenum cofactor (Moco; Fig. 1), is able to operate as a prosthetic group of Moco-dependent enzymes (Moenzymes). The corresponding chemical structure initially remained elusive, as Moco is not amenable to the procedures used in the isolation of natural products, and was solved as part of the aldehyde oxidoreductase holoenzyme complex from Desulfovibrio gigas (Romao et al, 1995). Researchers have identified and structurally elucidated a large number of Mo-enzymes (Hille et al, 2014) and have made considerable progress towards understanding the Moco-biosynthesis pathway
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