Chlamydomonas reinhardtii-A Reference Microorganism for Eukaryotic Molybdenum Metabolism

Abstract

Molybdenum (Mo) is vital for the activity of a small but essential group of enzymes called molybdoenzymes. So far, specifically five molybdoenzymes have been discovered in eukaryotes: nitrate reductase, sulfite oxidase, xanthine dehydrogenase, aldehyde oxidase, and mARC. In order to become biologically active, Mo must be chelated to a pterin, forming the so-called Mo cofactor (Moco). Deficiency or mutation in any of the genes involved in Moco biosynthesis results in the simultaneous loss of activity of all molybdoenzymes, fully or partially preventing the normal development of the affected organism. To prevent this, the different mechanisms involved in Mo homeostasis must be finely regulated. Chlamydomonas reinhardtii is a unicellular, photosynthetic, eukaryotic microalga that has produced fundamental advances in key steps of Mo homeostasis over the last 30 years, which have been extrapolated to higher organisms, both plants and animals. These advances include the identification of the first two molybdate transporters in eukaryotes (MOT1 and MOT2), the characterization of key genes in Moco biosynthesis, the identification of the first enzyme that protects and transfers Moco (MCP1), the first characterization of mARC in plants, and the discovery of the crucial role of the nitrate reductase-mARC complex in plant nitric oxide production. This review aims to provide a comprehensive summary of the progress achieved in using C. reinhardtii as a model organism in Mo homeostasis and to propose how this microalga can continue improving with the advancements in this field in the future.

Keywords: Chlamydomonas; Moco; homeostasis; microalga; molybdenum.

Figure 1

Molybdenum homeostasis in Chlamydomonas. The biosynthetic machinery, maturation, and distribution of Moco in a Chlamydomonas cell are shown. The basic steps of Moco biosynthesis, from GTP to Moco, including enzymes that contain Moco, are depicted. Proteins catalyzing the individual steps are shown in different colors. The names of proteins that have been notably studied in Chlamydomonas are shown in green, and the others are shown in gray. All known intermediates of the pathway are presented sequentially in the three steps in which Moco is synthesized. The MOT1 and MOT2 proteins transport the molybdate-anion to the cytosol. The CNX2 and CNX3 proteins catalyze the conversion of GTP to cPMP. The mitochondrial ABC transporter ATM3 is involved in transporting cPMP from the mitochondria to the cytosol. MPT-synthase, consisting of CNX6 and CNX7, converts cPMP to MPT, which is then sulfurated by CNX5. CNX1G activates MPT by converting it to MPT-AMP, which is then transferred to CNX1E. CNX1E deadenylates MPT-AMP and incorporates Mo into MPT to produce Moco. ABA3 catalyzes the addition of a sulfur atom coordinated to Mo to the XDH and AO families of molybdoenzymes. Moco can bind to the Moco carrier protein MCP1, where it is accumulated or transferred to form the different Mo enzymes nitrate reductase (NR), sulfite oxidase (SO), xanthine dehydrogenase (XDH), aldehyde oxidase (AO), and mARC. AMP (adenosine monophosphate); for a more detailed explanation, refer to the text.

Figure 2

Schematic representation of the enzymatic activity catalyzed by the five molybdoenzymes of Chlamydomonas. Not all substrates for each of the enzymes are shown. A characteristic example has been chosen as the substrate for each of the molybdoenzymes. The electron donors (NADH and O₂) or acceptors (NAD⁺ and H₂O) involved in each of the reactions are indicated for each of the enzymes. To simplify the figure, it has not been shown that NR can also act as an electron donor for mARC. For more details, refer to the text.