Plant enzymes, function

ALBRECHT M, KLEIN A, HUGUENEY p, SANDMANN G and KUNTZ M (1995) Molecular cloning and functional expression in E. coli of a novel plant enzyme mediating -carotene desaturation , FEES Lett, 372, 199-202. 

Horseradish peroxidase (HRP) is an extracellular plant enzyme that acts in regulation of cell growth and differentiation, polymerization of cell wall components, and the oxidation of secondary metabolites essential for important pathogenic defense reactions. Because of these essential functions, and also because of its stability and ready availability, HRP has attracted considerable attention.13 It has been involved in a number of applications, such as diagnostic assays,14 biosensors,15 bioremediation,16 polymer synthesis,17 and other biotechnological processes.18 More applications in which HRP catalysis is translated into an electrochemical signal are likely to be developed in the near future. 

This enzyme [EC 3.5.3.12], also known as agmatine imi-nohydrolase, catalyzes the hydrolysis of agmatine to produce A/ -carbamoylputrescine and ammonia. The plant enzyme also catalyzes the reactions of EC 2.1.3.3 (ornithine carbamoyltransferase), EC 2.1.3.6 (putrescine car-bamoyltransferase) and EC 2.122 (carbamate kinase), thereby functioning as a putrescine synthase, converting agmatine and ornithine into putrescine and citrulline, respectively. 

The oxime, 34, is glycosylated by a UDPG-thiohydroximate glucosyltransferase, and subsequently sulfonated by sulfotransferase to yield a glucosinolate of the type Thus, aldoximes generated by initial oxygenation of amino acids are found to serve as substrates of a wide variety of plant enzymes that act directly at the hydroximino function. 

McCue, K. F., Shepherd, L. V. T., Allen, P. V, Maccree, M. M., Rockhold, D. R., Corsini, D. L., Davies, H. V, Belknap, W. R. (2005). Metabolic compensation of steroidal glycoalkaloid biosynthesis in transgenic potato tubers using reverse genetics to confirm the in vivo enzyme function of a steroidal alkaloid galactosyltransferase. Plant science, 168, 267-273. 

Other herbicides are selectively inactivated by the target crop whilst the weeds that they control either do not metabolise them or they do it so slowly that the weed is killed before it can inactivate the herbicide. There are a number of key plant enzymes that are used in the inactivation of herbicides. Microsomal mixed function oxidases are able to hydroxylate a wide range of herbicides such as bentazone and diclofop-methyl (Figure 2.30). It is often the case that these hydroxylated metabolites are subsequently glucosylated by sugars in the tissue and these conju-gants can be stored in the cell vacuole where they can have no phytotoxic effects. 

The fatty acid synthases of yeast and of vertebrates are also multienzyme complexes, and their integration is even more complete than in E. coli and plants. In yeast, the seven distinct active sites reside in two large, multifunctional polypeptides, with three activities on the a subunit and four on the /3 subunit. In vertebrates, a single large polypeptide (Afr 240,000) contains all seven enzymatic activities as well as a hydrolytic activity that cleaves the finished fatty acid from the ACP-like part of the enzyme complex. The vertebrate enzyme functions as a dimer (Afr 480,000) in which the two identical subunits lie head-to-tail. The subunits appear to function independently. When all the active sites in one... 

The fructose bisphosphatase of green plants has an amino acid sequence which is very similar to those of the corresponding enzymes isolated from other sources such as yeast or mammals, except that the plant enzyme has an additional sequence of 20 or so amino acids that has no counterpart in the enzymes found in the other species. What function might this additional sequence have in the plant enzyme ... 

It is surprising that so little basic work concerning the relationship of gibberellic acid to enzyme function and synthesis has been reported. The field of study is very fertile and should be extremely rewarding for an individual who wishes to investigate some of the basic plant growth-regulating processes. 

In conclusion, it should be pointed out that in marked contrast to the very extensive studies on rabbit muscle phosphorylase, little attention has been paid to enzymes from other sources. However, primary structures of plant phosphorylases have now been determined and bacterial expression systems for the plant enzymes have also been made available as reviewed in this article. We hope that future studies on the structure and function of plant phosphorylases without allosteric regulation and comparison with those of the highly regulated animal enzyme will provide valuable information on this interesting group of enzymes, phosphorylases. 

Sucrose sucrose 1-fructosyl transferase appears to be a glycoprotein with a pH optimum of 5.0 and a molecular weight of 65 to 70 kDa (Scott, 1968). It has a lower temperature optimum than many plant enzymes and a relatively low Q10, allowing it to function effectively at lower temperatures. For example, the enzyme s activity decreases slowly (i.e., by only a factor of 2) between 28 and 8°C (Wagner and Wiemken, 1986). 

It is worth noting that the understanding of the starch synthases lags behind that of the ADPGlc PPase and the branching enzymes. To cover that ground, it will be necessary to achieve expression of the plant enzymes in E. coli so that studies of structure-function relationships can be facilitated. 

Lipoxygenases (LOs) are nonheme, mononuclear iron enzymes that catalyze the regio- and stereoselective conversion of polyunsaturated fatty acids with a di.di-1,4-diene functionality into products having a l-hydroperoxy-tra 5, cM-2,4-diene functionality. The mammalian LOs typically act on arachidonic acid and produce alkyl hydroperoxides that are converted into leukotrienes and lipoxins, which are involved as messengers in the inflammatory response. Plant enzymes act on linoleic acid, but the role of the product alkyl hydroperoxide is less well understood. 

The finding that Lys and Arg residues are important in allosteric effector binding and are situated at the C-terminus in ADP-Glc PPases of plants and cyanobacteria is different with what is observed for the bacterial ADP-Glc PPases. Lys39 (E. coli) and Arg residues in the N-terminal of the A. tumefaciens enzyme were shown to be important for the interaction of the activators and inhibitors (97, 98, 107, 112). These results suggest that the regulatory domains may be at different sites in the bacterial and plant enzymes. Other studies, however, with chimeric ADP-Glc PPases constructed from E. coli and A. tumefaciens have shown that the C-terminus in the bacterial ADP-Glc PPases are also functional in determining effector specificity and affinity (113). Regulation of ADP-Glc PPases most likely is determined by interactions that occur between the N- and C-termini in the enzyme. 

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