Fructose acceptor products

Even though the reaction scheme is complex, attempts have been made to model the reaction and develop kinetic equations. Mooser et al.lu developed kinetic equations for a limited case based on the primer mechanism. Reh et a/.145 developed kinetic equations for the synthesis of dextran and for the maltose acceptor reaction based on the two-site insertion mechanism. Likewise, Boker et al,146 developed a kinetic equation for the formation of the fructose acceptor product, leucrose, based on the two-site insertion mechanism that included an acceptor site. An equation was derived for the overall reaction rate that was a function of the consumption of sucrose by dextran formation and acceptor-product formation. 

In the presence of such weak acceptors as fructose, initial overall reaction rates decrease significantly with increasing concentration of acceptor, whereas they increase with the substrate concentration. Initial rates of acceptor-product formation increase with both substrate and acceptor formation, the formation of dextran being... 

Summarizing the results of many investigations, monosaccharides and such derivatives as D-mannitol and D-glucitol are rather weak acceptors. Disaccharides, including such acceptor products as isomaltose, are much better acceptors, except for certain molecules, for instance leucrose, which is not an acceptor.29,46,47 The decrease of enzyme activity with time has been described in terms of a first-order reaction. The inactivation parameters have been calculated for the immobilized enzyme. The inactivation constants kd were 0.0135 (1/d) when maltose was the acceptor (stabilizing), and 0.029 (1/d) when fructose was the acceptor.38... 

In the presence of weak acceptors, such as glucose or fructose, initial overall reaction rates decrease significantly with increasing acceptor concentration, whereas they increase with the substrate concentration. Initial rates of acceptor product formation increase with both substrate and acceptor concentration, the dextran formation being suppressed to a major extent only at very high acceptor concentration (at about 3 M glucose or fructose concentration) [20, 21], The acceptor product of fructose, leucrose, does not act as an acceptor, so that high yields (up to about 70%) can be obtained, which is of interest for industrial application. Leucrose has been developed up to the pilot scale (500 kg leucrose produced) for application as an alternative sweetener [24, 25]. 

Homopolysaccharides are synthesised by relatively few spedfic enzymes and are not constructed from subunits. The commerdally important homo polymer dextran is synthesised extracellularly by the enzyme dextransucrase. In Leuconostoc mesenteroides the enzyme is induced by the substrate sucrose. This is deaved to release free fructose and link the glucose to the redudng end of the acceptor dextran chain, which is bound to the enzyme. The product from this bacterium is composed almost exdusively of... 

Fig. 3. Product spectra of the wild-type (WT) GTFR and mutant S628D enzymes (200 U/L) incubated (7 days at 30 °C) with sucrose (146 mM) and different acceptor substrates (292 mM, Glc glucose Fru fructose). Yields are given in percentage (mol/mol Glc). Yields of higher oligosaccharides (DP >5) and...

Fructose 1,6-biphosphate aldolase from rabbit muscle in nature reversibly catalyzes the addition of dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The tolerance of this DHAP-dependent enzyme towards various aldehyde acceptors made it a versatile tool in the synthesis of monosaccharides and sugar analogs [188], but also of alkaloids [189] and other natural products. For example, the enzyme-mediated aldol reaction of DHAP and an aldehyde is a key step in the total synthesis of the microbial elicitor (—)-syringolide 2 (Fig. 35a) [190]. 

The reaction is essentially irreversible. The main products are a high-molecular-weight (1 X 107— 1 X 10 Da) glucan, together with fructose, and the minor products are glucose and leucrose (where, n m or w). Glucose arises from an acceptor reaction with water, and leucrose [5-O-a-D-glucopyranosyl-D-fructopyr-anose] arises from an acceptor reaction with the primary product, fructose. (The acceptor reactions are discussed in Section IV.)... 

The aldehyde substrates may be used as racemic mixtures in many cases, as the aldolase catalyzed reactions can concomitantly accomplish kinetic resolution. For example, when DHAP was combined with d- and L-glyceraldehyde in the presence of FDP aldolase, the reaction proceeded 20 times faster with the D-enantiomer. Fuc 1-P aldolase and Rha 1-P aldolase show kinetic preferences (greater than 19/1) for the L-enantiomer of 2-hydroxy-aldehydes. Alternatively, these reactions may be allowed to equilibrate to the more thermodynamically favored products. This thermodynamic approach is particularly useful when the aldol products can cyclize to the pyranose form. Since the reaction is reversible under thermodynamic conditions, the product with the fewest 1,3-diaxial interactions will predominate. This was demonstrated in the formation of 5-deoxy-5-methyl-fructose-l-phosphate as a minor product (Scheme 5.5).20a 25 The major product, which is thermodynamically more stable, arises from the kinetically less reaction acceptor. 

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