Abstract
All eukaryotic molybdenum (Mo) enzymes contain in their active site a Mo Cofactor (Moco), which is formed by a tricyclic pyranopterin with a dithiolene chelating the Mo atom. Here, the eukaryotic Moco biosynthetic pathway and the eukaryotic Moco enzymes are overviewed, including nitrate reductase (NR), sulfite oxidase, xanthine oxidoreductase, aldehyde oxidase, and the last one discovered, the moonlighting enzyme mitochondrial Amidoxime Reducing Component (mARC). The mARC enzymes catalyze the reduction of hydroxylated compounds, mostly N-hydroxylated (NHC), but as well of nitrite to nitric oxide, a second messenger. mARC shows a broad spectrum of NHC as substrates, some are prodrugs containing an amidoxime structure, some are mutagens, such as 6-hydroxylaminepurine and some others, which most probably will be discovered soon. Interestingly, all known mARC need the reducing power supplied by different partners. For the NHC reduction, mARC uses cytochrome b5 and cytochrome b5 reductase, however for the nitrite reduction, plant mARC uses NR. Despite the functional importance of mARC enzymatic reactions, the structural mechanism of its Moco-mediated catalysis is starting to be revealed. We propose and compare the mARC catalytic mechanism of nitrite versus NHC reduction. By using the recently resolved structure of a prokaryotic MOSC enzyme, from the mARC protein family, we have modeled an in silico three-dimensional structure of a eukaryotic homologue.
Highlights
All eukaryotic molybdenum (Mo) enzymes contain in their active site a Mo Cofactor (Moco), which is formed by a tricyclic pyranopterin with a dithiolene chelating the Mo atom
Arabidopsis CNX1 is able to bind to the cytoskeleton, where it interacts with other proteins that are involved in Moco biosynthesis, pointing to CNX1 as anchor protein for this metabolic pathway [35,36]
Once eukaryotic Moco is synthesized, it can bind directly to sulphite oxidase (SO), nitrate reductase (NR), or mitochondrial amidoxime reducing component (mARC) apoenzymes, or it can undertake a final sulfuration in order to bind to the apoenzymes aldehyde oxidase (AO) or XOR/XD (Figure 2)
Summary
Among the transition metals of the fifth row of the periodic table, Mo is the only one that has been reported as an essential element. Mo-related lead the pleiotropic loss of the activity of all Mococomponent enzymes It can be caused either by deficiencies an insufficient. This phenotype is mainlyleads caused the lossplant of NR severely thefor overall plant growth as [5].aThis is mainly caused by anddevelopment, AO activities, and is impacting responsible nitrate reduction first phenotype step of nitrate assimilation and the loss of NR and. Moco-deficiency causes been reported as an important player in plant development and stress adaptation [8]. Moco-deficiency causes a severe neurological disorder that leads to death shortly after birth, mainly deficiency, resulting in sulphite accumulation and subsequent brain damage [9]. Problems due to the SO activity deficiency, resulting in sulphite accumulation and subsequent brain damage that are related to Mo-derived toxicity are very rare and have been described mainly in ruminants. Resulting in copper-deficiency in these animals [10]
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