Plants pyrophosphatase

Dilute perchloric acid or trichloroacetic acid, or ethanol, is usually employed for extraction of the glycosyl esters of nucleoside pyrophosphates from biological materials.19 The high lability of these compounds in acidic media (see Section IV, p. 356) leads to unavoidable losses during extraction with acids. Extraction with ethanol can lead to difficulties, as ethanol may not completely inactivate pyrophosphatases present in the tissue the action of these enzymes may result in partial degradation of the nucleoside pyrophosphate derivatives. Such a situation has been encountered particularly with plant tissues.20... 

One remarkable difference between the cells of plants and animals is the absence in the plant cell cytosol of the enzyme inorganic pyrophosphatase, which catalyzes the reaction... 

For many biosynthetic reactions that liberate pyrophosphatase activity makes the process more favorable energetically, tending to make these reactions irreversible. In plants, this enzyme is present in plastids but absent from the cytosol. As a result, the cytosol of leaf cells contains a substantial concentration of PP,— enough ( 0.3 him) to make reactions such as that catalyzed by UDP-glucose pyrophosphorylase (Fig. 15-7) readily reversible. Recall from Chapter 14 (p. 527) that the cytosolic isozyme of phosphofructokinase in plants uses PPi, not ATP, as the phosphoryl donor. 

Alkaline pyrophosphatase dependent on Mg2+ was found in every sample examined from a broad spectrum of the plant kingdom (SI). Plants which fix C02 by the dicarboxylic acid pathway have characteristic high levels of alkaline pyrophosphatase in their chloroplasts presumably this performs the rather specific function of driving the synthesis of phosphoenolpyruvate, the immediate precursor of C02 fixation (32). Biosynthesis of the maize chloroplast enzyme is controlled by light acting through the phytochrome system (S3). Pyrophosphatase from spinach chloroplasts has been partially purified (34, 35). 

Answer [PPi] is high in the plant cell cytosol because the cytosol lacks inorganic pyrophosphatase, the enzyme that degrades PP, to 2 P,. Plants are unique in this regard. Animal cells have pyrophosphatase in their cytosol, and [PPJ is therefore kept too low for PP, to be a useful phosphoryl group donor. 

Den Hartog, M., Musgrave, A. and Munnik, T., 2001, Nod factor-induced phosphatidic acid and diacylglycerol pyrophosphatase formation a role for phospholipase C and D in root hair formation. Plant J. 25 55-66. 

Scolnick, PA. and Bartley, G.E. (1996) Two more members of an Arabidopsis geranylgeranyl pyrophosphatase synthase gene family. Plant Physiol, 110,1435. 

Additional evidence supporting the role of vacuolar transport in salt tolerance has been provided by A. thaliana plants overexpressing a vacuolar H -PPiase [40]. Transgenic plants overexpressing AVPl, coding for the vacuolar H -pyrophosphatase, displayed enhanced salt tolerance that was correlated with the increased ion content... 

Although pyrophosphatases are ubiquitous, there are organisms in which PPj is conserved by the cell and replaces ATP in several glycolytic reactions. These include Propionihacterium sulfate-reducing bacteria, the photosynthetic Rhodospirillum, and the parasitic Entamoeba histolytica.In the latter the internal concentration of PP is about 0.2 mM. Green plants also accumulate PPj at concentrations of up to... 

Cobalt, Co an essential bioelement present in traces in plants, animals and microorganisms. It is important as a constituent of vitamin B12. Traces of Co are required for microbial growth. It is a cofactor or prosthetic group of several enzymes, e.g. pyrophosphatases, peptidases, arginase, as well as certain enzymes involved in nitrogen fixation. 

Riboflavin needs to be present in the human typical diet, as animals, unlike many plants, fungi and bacteria, are unable to synthesize this molecule. Dietary intake of this vitamin includes free riboflavin and also its protein bound form, as FAD and FMN in flavoproteins (Figure 37.1 A). In the latter case, flavins need to be first released from carrier proteins during digestion and then hydrolysed to riboflavin by alkaline phosphatases and FMN/FAD pyrophosphatase in order to be absorbed at the small intestine. 

It appears that most nucleotide pyrophoqihatases from mammalian tissues cleave the reduced forms of the coenzymes faster than the oxidized nucleotides. The plant nucleotide pyrophosphatases, however, q>lit DPNH, TPNH, and the oxidized coenzyme forms at equal rates. It should be pointed out, however, that an enzyme such as the potato nucleotide pyrophosphatase also attacks ATP and adenosine diphosphate (ADP), whereas the purified pigeon liver enzyme does not. The snake venom enzyme seems to split the oxidized and reduced nucleotide at equal rates. 

Like the spinach enzyme, the pea ATPase is activated equally by Mg2+ and Mn2+ and hydrolyzes a broad range of nucleoside triphosphates, but not ADP, AMP, or monophosphorylated substrates. Although pea chloroplast envelope membranes have ADPase and pyrophosphatase activity, we conclude that the activities are distinct from the ATPase activity. The envelope ATPase differs from putative transport ATPases characterized in other plant membranes in that it is not inhibited by vanadate or DCCD, nor is it stimulated by potassium. However, a role for this activity in proton efflux and ion transport cannot be ruled out, because the envelope vesicles may be sufficiently leaky that protons and ions can diffuse freely across the membrane. This might limit any stimulatory effect of K+ and uncouplers. Evidence supporting a role for the ATPase in proton transport will depend on further characterization of the envelope vesicles, and/or purification and reconstitution of the ATPase into artificial lipid vesicles. 

In studies on higher plants the presence of APS kinase has not been demonstrated and the suggestion has been made that in plants the sulphate activating system consists only of ATP sulphurylase and pyrophosphatase. 

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