Plants molybdenum enzymes

Long recognized as an essential element for the growth of plants, molybdenum has never been directly demonstrated as a necessary animal nutrient. Nevertheless, it is found in several enzymes of the human body, as well as in 30 or more additional enzymes of bacteria and plants.632 Aldehyde oxidases,633 xanthine oxidase of liver and the related xanthine dehydrogenase, catalyze the reactions of Eqs. 16-58 and 16-59 and contain molybdenum that is essential for catalytic activity. Xanthine oxidase also contains two Fe2S2 clusters and bound FAD. The enzymes can also... 

Non-metallic trace nutrients include boron (for plants only), iodine and fluorine (for vertebrates), selenium, and silicon. Selenium forms an essential part of the important mammalian enzyme glutathione peroxidase which, in mammals, protects membranes from oxidation and is one of the microbiocidal enzymes in phagocytes (Stadtman, 1980). A selenium, iron, and molybdenum enzyme was described above under molybdenum . Silicon is thought to contribute to the architecture and resiliance of connective tissues of vertebrates. In rats, silicon is essential for growth and development. It seems to be present as a silanolate, with Si-O-R bridges to such polysaccharides as heparin and hyaluronic acid (Schwarz, 1973). 

Molybdenum is an essential element for several enzymes important in plant and animal metabolism. It has a unique role in nitrogenase, an enzyme that converts molecular nitrogen into ammonia in nitrogen fixation. Molybdenum enzymes provide the key steps in the fixation of nitrogen by microorganisms and its utilization by higher plants (Table 1). 

Because of its position in the Periodic Table, molybdenum has sometimes been linked to chromium (see Chromiumand chromium alloys) or to other heavy metals. However, unlike those elements, molybdenum and its compounds have relatively low toxicity, as shown in Table 3. On the other hand, molybdenum has been identified as a micronutrient essential to plant life (11,12) (see Fertilizers), and plays a principal biochemical role in animal health as a constituent of several important enzyme systems (see Mineral nutrients). 

Molybdenum, recognized as an essential trace element for plants, animals, and most bacteria, is present in a variety of metaHo enzymes (44—46). Indeed, the absence of Mo, and in particular its co-factor, in humans leads to severe debility or early death (47,48). Molybdenum in the diet has been impHcated as having a role in lowering the incidence of dental caries and in the prevention of certain cancers (49,50). To aid the growth of plants. Mo has been used as a fertilizer and as a coating for legume seeds (51,52) (see FERTILIZERS Mineral NUTRIENTS). 

Molybdenum (Mo) is present in all plant, human, and animal tissues, and is considered an essential micronutrient for most life forms (Schroeder et al. 1970 Underwood 1971 Chappell and Peterson 1976 Chappell et al. 1979 Goyer 1986). The first indication of an essential role for molybdenum in animal nutrition came in 1953 when it was discovered that a flavoprotein enzyme, xanthine oxidase, was dependent on molybdenum for its activity (Underwood 1971). It was later determined that molybdenum is essential in the diet of lambs, chicks, and turkey poults (Underwood 1971). Molybdenum compounds are now routinely added to soils, plants, and waters to achieve various enrichment or balance effects (Friberg et al. 1975 Friberg and Lener 1986). 

Figure 16-31 (A) Structure of molybdopterin cytosine dinucleotide complexed with an atom of molybdenum. (B) Stereoscopic ribbon drawing of the structure of one subunit of the xanthine oxidase-related aldehyde oxidoreductase from Desulfo-vibrio gigas. Each 907-residue subunit of the homodimeric protein contains two Fe2S2 clusters visible at the top and the molybdenum-molybdopterin coenzyme buried in the center. (C) Alpha-carbon plot of portions of the protein surrounding the molybdenum-molybdopterin cytosine dinucleotide and (at the top) the two plant-ferredoxin-like Fe2S2 clusters. Each of these is held by a separate structural domain of the protein. Two additional domains bind the molybdopterin coenzyme and there is also an intermediate connecting domain. In xanthine oxidase the latter presumably has the FAD binding site which is lacking in the D. gigas enzyme. From Romao et al.633 Courtesy of R. Huber.

The opposite sequence, reduction of nitrate and nitrite ions, provides a major route of acquisition of ammonia for incorporation into cells by bacteria, fungi, and green plants (Fig. 24-1). Assimilatory (biosynthetic) nitrate reductases catalyze the two-electron reduction of nitrate to nitrite (Eq. 16-61). This is thought to occur at the molybdenum atom of the large 900-residue highly regulated793 molybdopterin-dependent enzyme. In green plants the reductant is... 

Nitrogen fixation in the root nodules of peas and other leguminous plants is carried out by the molybdenum-containing enzyme nitrogenase. What is the molecular mass of nitrogenase if the enzyme contains two molybdenum atoms and is 0.0872% Mo by mass ... 

Molybdenum 5 10 0.2 Many redox enzymes nitrogenase (plants)... 

There were 37 distinct enzymes that contain molybdenum or tungsten known by the end of 1997. The enzymes are diverse in function, broadly distributed, and include oxidases, reductases, dehydrogenases, a transhydroxylase, and a hydratase. The Mo enzymes are found in eubacteria, archae, protista, fungi, plants, and animals (including humans) and are essential for respiration and carbon and nitrogen assimilation. Several of the enzymatic substrates and products are key components in the nitrogen, sulfur, selenium, carbon, and arsenic cycles and have major biological and environmental impact. 

The potentials found for nitrate reductases [96] vary with the role of the particular enzyme. Assimilatory nitrate reductase, found in plants, algae, and fungi, is involved in the first step in nitrogen assimilation and has a molybdenum center that operates at around 0 mV. Respiratory (dissimilatory) nitrate reductase, utilized by bacteria in energy yielding processes, has a molybdenum center that operates at around +200 mV [97,98],... 

All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). 

Molybdenum is very important in the biochemistry of animals, plants, and microorganisms. It is the only element in the second transition series known to have natural biological functions. It occurs in more than 30 enzymes, in some of which it may be replaced by tungsten or vanadium. Tungsten is the only element in the third transition series known to have natural biological functions. Not only does it sometimes occur in enzymes that usually contain molybdenum, but there are some enzymes that are known only with tungsten. 

Molybdenum is required in the diet. It is required by three enzymes in mammals sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. Molybdenum occurs in these enzymes as part of the molybdenum cofactor (Figure 10.52). This cofactOT is biosynthes z.ed in the body with GTP as the starling material. All known Mo mclalloenzymes, with the exception of nitrogenase (a plant enzyme), use Mo in the form of the molybdenum cofactor. 

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