Plants enzymes bound

C-methyl-D-erythritol 2,4-cyclodiphosphate synthase catalyses the conversion of 4-diphospho-cytidyl-2-C-methyl-D-erythritol 2-phosphate to 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MECDP) (Equation (7)). This reaction is part of the isoprenoid biosynthesis pathway in many plants and bacteria. The structure of the E. coli enzyme bound to Mn, cytosine monophosphate, and 2-C-methyl-D-erythritol 2,4-cyclodiphosphate has been determined. The enzyme in the crystal and probably in solution is trimeric, three monomers are packed in a circular assembly with three-fold symmetry. The active site is at the interface of two adjacent monomers all the ligands bound to the Mn + come from one monomer and a MECDP molecule. The structure of this active site is shown in Figure 29 ... 

The preparation of extracts of plant tissue containing active DPOs can be fraught with problems. In the intact plant tissue, both enzyme and substrate are present but are thought to be compartmentalized, with the enzyme bound to membranes and the native substrate ) present in the vacuole. As soon as the tissue is disrupted, these can react together with the very real risk of a suicidal inactivation of the enzyme by its own reaction products. As a result, most isolation procedures for DPOs include additions of ascorbate and/or cysteine to prevent the formation of the reactive quinones. Assuming that the enzyme is... 

Fig. 2.13 Biosynthesis of saturated fatty acids in plants and animals. Palmitate is formed by successive additions of malonyl coenzyme A to the enzyme-bound chain, with C02 being lost at each addition.This results in chain elongation by a (CH2)2 unit at each step. Details of the formation of butyryl (C4) from acetyl (C2) are shown, while the subsequent six further additions, terminating in palmitate, proceed similarly.

Plants use NADH as the electron donor, whereas fimgi and bacteria use NADPH. The electrons are transferred to enzyme-bound FAD, then to cytochrome 557, then to molybdenum, and finally to the substrate. 

In general, reported molecular weights for plant cysteine synthases have been in the range of 62,000-70,000 daltons (Table II). The only report of a value outside this range is that of 41,000 daltons for a partially purified preparation from blue lupin (Hendrickson and Conn, 1%9). The plant enzyme can be dissociated into two identical subunits, each with a molecular weight approximately one-half that of the intact enzyme. Pyridoxal phosphate is tightly bound to the enzyme, and its resolution from the enzyme and... 

Cystathionine-/3-lyase has been purified 430-fold from spinach leaves (Giovanelli and Mudd, 1971). The enzyme contains a firmly bound pyridoxal phosphate prosthetic group, and was isolated as the holoenzyme. The substrate specificity of the plant enzyme resembles that of the bacterial enzyme, being most active with cystathionine and djenkolate, and far less active with cystine, 5-methylcysteine, and serine, but differs from that of the fungal enzyme, the activity of which with cysteine, serine, and lanthionine is about the same as that with cystathionine. Conversely, with respect to sensitivity to inhibition by 5, 5 -dithiobis-2-nitrobenzoic acid and A-ethylmaleimide, the plant enzyme resembles the fungal enzyme. 

Equation (18), catalyzed by glutathione synthetase (E.C. 6.3.2.3), has been demonstrated in acetone powders of bean seedlings (Webster, 1953). Activity required Mg " ", and was stimulated by K+. The stoichiometry and the mechanism of this reaction have not been studied in plants. Studies with purified GSH synthetase from other systems indicate that the reaction proceeds through enzyme-bound y-glutamylcysteinyl phosphate (Meister, 1974). 

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