Donor substrate

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Figure 10.2 Nucleophilic donor substrates of preparatively useful aldolases.

Extensive studies have indicated that only pyruvate is acceptable as the NeuA donor substrate, with the exception of fluoropyruvate [49], but that the enzyme displays a fairly broad tolerance for stereochemically related aldehyde substrates as acceptor alternatives, such as a number of sugars and their derivatives larger or equal to pentoses [36,48,50,51]. Permissible variations include replacement of the natural D-manno configured substrate (4) with derivatives containing modifications such as epimerization, substitution, or deletion at positions C-2, -4, or -6 [16,27]. Epimeriza-tion at C-2, however, is restricted to small polar substituents owing to strongly... 

Transketolase (TKase) [EC 2.2.1.1] essentially catalyzes the transfer of C-2 unit from D-xylulose-5-phosphate to ribose-5-phosphate to give D-sedoheptulose-7-phosphate, via a thiazolium intermediate as shown in Fig. 16. An important discovery was that hydroxypyruvate works as the donor substrate and the reaction proceeds irreversibly via a loss of carbon dioxide (Fig. 17). In this chapter, we put emphasis on the synthesis with hydroxypyruvate, as it is the typical TPP-mediated decarboxylation reaction of a-keto acid. ... 

The reactions were carried out under the same conditions as solubilization (see sec. 1), except that a constant 2 1 donor substrate (molar) ratio was used and for comparative purposes, all runs with Tetralin were repeated with 1,2,3,4-tetrahydro-... 

Figure 6.10 Broad donor substrate tolerance of sialic acid aldolase in synthesis of fluorinated sialic acid analogues...

It is interesting to note that the photosubstitution intermediate 11 appears to be significantly more selective toward reaction with various two electron donor substrates than is the photofragmentation intermediate 1. One speculative rationalization of this is that the Ru3(C0) intermediate has the opportunity to "delocalize" its unsaturation by having one CO bridge an edge of the metal triangle with concomitant formation of a multiple metal-metal bond. 

G. Meng and K. Futterer, Donor substrate recognition in the raffinose-bound E342A mutant of fiructosyltransferase Bacillus subtilis levansucrase, BMC Struct. Biol., 8 (2008) 16. 

Attempts to rationalize the role of ligands in these reactions and in the even more intriguing case of cooligomerizations have been successful only in part. Steric and electronic effects of the substrate should not be considered without taking in account both the other ligands present and the oxidation state of the metal. Donor substrates are generally best stabilized by acceptor ligands and vice versa (10c). in accordance with the electroneutrality principle. 

A number of excellent reviews have recently been published [1] consequently, this chapter will consider mainly the practical aspects of asymmetric transfer hydrogenation by reviewing each of the components of the reaction, namely catalyst, hydrogen donor, substrate, product and other elements such as solvent, reaction conditions and scale-up. 

R is an electron-donor substrate such as purine or xanthine and A is an electron acceptor such as 02 or NAD+. It is thought that the in vivo mammalian form of xanthine oxidase uses NAD+ as acceptor and is therefore, more appropriately named xanthine dehydrogenase. No evidence exists for a dehydrogenase form of aldehyde oxidase. The specificities of xanthine oxidase and aldehyde oxidase have been extensively catalogued (96), and the mechanism and properties of these enzymes have been reviewed (97, 98). 

A practical, inexpensive one-step procedure was developed for the RhaD-catalyzed gram-scale synthesis of L-fructose. The requirement for DHAP as the donor substrate was circumvented by use of borate buffer, presumably by in situ formation of borate esters as a phosphate ester mimic. Racemic glyceraldehyde was also used, as the enzyme preferentially accepted the L-enantiomer as a substrate. The method can also be apphed to other products, including L-rhamnulose, and towards a two-step synthesis of L-iminocychtols. ... 

This enzyme [EC 2.7.1.74] catalyzes the reaction of a nucleoside triphosphate with deoxycytidine to produce a nucleoside diphosphate and dCMP. This enzyme can use any nucleoside triphosphate (except dCTP) as the phosphate-donor substrate and it can phosphorylate cytosine arabinoside as well. 

The model has numerous short-comings . Chiefly, it fails to account for the fact that U- and R-enzymes frequently have other phosphoryl-acceptor and -donor substrates, and the model does not explain how fluctuations in these cosubstrate concentrations may alter the so-called responsiveness of enzymes. Moreover, the concentration of orthophosphate is wholly omitted from the model, despite the fact that key regulatory steps in ATP utilization and regeneration are exquisitely sensitive to changes in orthophosphate concentration. The energy-charge model also fails to consider magnesium ion com-... 

Glutamate synthase (NADPH) [EC 1.4.1.13], an iron-sulfur flavoprotein, catalyzes the reaction of L-glutamine with a-ketoglutarate (or, 2-oxoglutarate) and NADPH to produce NADP+ and two glutamate molecules. Ammonia can act as the nitrogen donor substrate instead of L-glutamine, albeit weaker. 

This enzyme [EC 1.11.1.7] catalyzes the reaction of a donor substrate with hydrogen peroxide to produce an oxidized donor and two water. 

Aldolases are part of a large group of enzymes called lyases and are present in all organisms. They usually catalyze the reversible stereo-specific aldol addition of a donor ketone to an acceptor aldehyde. Mechanistically, two classes of aldolases can be recognized [4] (i) type I aldolases form a Schiff-base intermediate between the donor substrate and a highly conserved lysine residue in the active site of the enzyme, and (ii) type II aldolases are dependent of a metal cation as cofactor, mainly Zn, which acts as a Lewis acid in the activation of the donor substrate (Scheme 4.1). 

The stereochemistry of the aldol reaction is highly predictable since it is generally controlled by the enzyme and does not depend on the structure or stereochemistry of the substrates. Aldolases generally show a very strict specificity for the donor substrate (the ketone), but tolerate a broad range of acceptor substrates (the aldehyde). Thus, they can be functionally classified on the base of the donor substrate accepted by the enzyme. 

The main drawback of the DHAP-dependent aldolases is their strict specificity for the donor substrate. Apart from the scope limitation that this fact represents, DHAP is expensive to be used stoichiometrically in high-scale synthesis, and labile at neutral and basic pH, and therefore its effective concentration decreases over time in enzymatic reaction media, hindering the overall yield of the aldol reaction. In addition, due to the presence of a phosphate group in both DHAP and the... 

N. meningitides and human ST6Gal-I, EC2.4.99.1) were exploited to add sialic acid in a2-3 and a2-6 linkages to the terminal galactose residue of mono-, di- and tri-LacNAc. The fusion of the bacterial ST3 with CMP-Neu5Ac synthetase [83] generated the donor substrate in situ from added NeuSAc and CTP. 

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