Plants, aldolases

Plant aldolases are similar to mammalian aldolases, but have smaller molecular weights.347 The aldolases from Lactobacillus casei and Micrococcus aerogenes348 are similar to the mammalian enzymes with respect to Schiff-base formation349 and to involvement of specific amino acid residues.350 However, the amino acid composition of the enzyme from M. aerogenes is different from those of rabbit aldolases.351... 

As a consequence of the widespread occurrence of the enzyme aldolase in plants, Tewfik and Stumpf 64) concluded that this enzyme played a major role in the sugar transformations of plants. Aldolase from mammalian tissue had been shown 66) to catalyze the following reaction ... 

Aldolases cataly2e the asymmetric condensation of intermediates common in sugar metaboHsm, such as phosphoenolpymvic acid, with suitable aldehyde acceptors. Numerous aldolases derived from plants or animals (Class I aldolases) or from bacteria (Class II) have been examined for appHcations (81). Efforts to extend the appHcations of these en2ymes to the synthesis of unusual sugars have been described (2,81). 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

Aldol reactions occur in many biological pathways, but are particularly important in carbohydrate metabolism, where enzymes called aldolases catalyze the addition of a ketone enolate ion to an aldehvde. Aldolases occur in all organisms and are of two types. Type 1 aldolases occur primarily in animals and higher plants type II aldolases occur primarily in fungi and bacteria. Both types catalyze the same kind of reaction, but type 1 aldolases operate place through an enamine, while type II aldolases require a metal ion (usually 7n2+) as Lewis acid and operate through an enolate ion. 

Figure 10.34 Aldolase-based creation of two independent chiral centers in the total synthesis of the complex microbial plant defence elicitor (—)-syringolide.

Goyer, A. et al.. Folate biosynthesis in higher plants. cDNA cloning heterologous expression and characterization of dihydroneopterin aldolases, Plant. Physiol, 135, 103, 2004. 

A syringolide 45, an elicitor of the bacterial plant pathogen Pseudomonas Siringae pv. tomato, has been synthesized in five steps via a fructose 1,6-diphosphate aldolase reaction (Scheme 95) <2000JOC4529>. 

Figure 2.20 The two mechanisms of aldolases. Group 1 enzymes from animals and higher plants use an amino group in the enzyme to form a Schiff s base intermediate to activate the aldol donors. Group II enzymes from lower organisms, use a metal ion, usually Zn " in the active site to form an enolate intermediate. The two mechanisms are examplified by fiuctose-1,6-diphosphate aldolase, a very important aldolase in synthesis and breakdown of sugars.

Folate biosynthesis has also been studied in plants and the dihydroneopterin aldolase from Arabidopsis thaliana has been crystallized and its structure determined the construction of the active site has similarities with those of other... 

From the many enzymes that are known to make and break C-C bonds, we first chose the two transferases, transketolase (TKT) and transaldolase (TAL), both from the Gram-negative bacterium Escherichia coli. While project B21 evolved, we learned that this microorganism holds other and so far unknown enzymes which are of interest for asymmetric syntheses. One transketolase-like enzyme, 1-deoxy-D-xylulose 5-phosphate synthase (DXS), turned out to be the first enzyme of a novel biosynthetic pathway leading to isoprenoids in bacteria, algae, and plants. The other, fructose 6-phosphate aldolase (ESA) - while similar to transaldolase - allows the direct use of the inexpensive dihydroxyacetone in aldol condensations. 

There are two classes of aldolases. Class I aldolases, found in animals and plants, use the mechanism shown in Figure 14-5. Class II enzymes, in fungi and bacteria, do not form the Schiff base intermediate. Instead, a zinc ion at the active site is coordinated with the carbonyl oxygen at C-2 the Zn2+ polarizes the carbonyl group... 

These enzymes have been found in all plant and animal tissues examined and are absent only from a few specialized bacteria. Three closely related isoenzymes are found in vertebrates.185 186 The much studied rabbit muscle aldolase A is a 158-kDa protein tetramer of identical peptide chains.186 187 Aldolase B, which is lacking in hereditary fructose intolerance, predominates in liver and isoenzyme C in brain.185... 

Loewus, M.W., and Loewus, F.A., 1973a, D-Glucose 6-phosphate cycloaldolase Inhibition studies and aldolase function. Plant Physiol. 51 263-266. 

The enzymatic aldol reaction represents a useful method for the synthesis of various sugars and sugar-like structures. More than 20 different aldolases have been isolated (see Table 13.1 for examples) and several of these have been cloned and overexpressed. They catalyze the stereospecific aldol condensation of an aldehyde with a ketone donor. Two types of aldolases are known. Type I aldolases, found primarily in animals and higher plants, do not require any cofactor. The x-ray structure of rabbit muscle aldolase (RAMA) indicates that Lys-229 is responsible for Schiff-base formation with dihydroxyacetone phosphate (DHAP) (Scheme 13.7a). Type II aldolases, found primarily in micro-organisms, use Zn as a cofactor, which acts as a Lewis acid enhancing the electrophilicity of the ketone (Scheme 13.7b). In both cases, the aldolases accept a variety of natural (Table 13.1) and non-natural acceptor substrates (Scheme 13.8). 

---