Transketolase and Related Enzymes

Other thiamine diphosphate-dependent enzymes have recently been scrutinized for their preparative value [166]. Although pyruvate decarboxylase (PDC) (EC 4.1.1.1) 

Transketolase (EC 2.2.1.1) is involved in the oxidative pentose phosphate pathtvay in tvhich it catalyzes the reversible transfer of a hydroxyacetyl nucleophile bettveen a variety of sugar phosphates. The enzyme, tvhich requires thiamine diphosphate and divalent Mg as cofactors [248], is commercially available from baker s yeast and can be readily isolated from many natural or recombinant sources [249, 250]. The yeast enzyme has been structurally tvell characterized [251], including protein tvith a carbanion intermediate covalently bound to the cofactor [252]. Large-scale enzyme production has been investigated for the transketolase from Escherichia coli [253-255]. Immobilization vas sho vn to significantly increase stability against inactivation by aldehyde substrates [256]. The enzyme is quite tolerant to organic cosolvent, and preparative reactions have been performed continuously in a membrane reactor [255], vith potential in-situ product removal via borate complexation [257]. 

Enzymes from yeast, spinach, and Escherichia coli have been sho vn to tolerate a broad substrate spectrum, vith the ne vly formed chiral center al- vays having an absolute (S) configuration as a result of re-face attack [258]. 

Kinetic resolution by transketolase, and non-equilibrium C-C bond formation by decomposition of hydroxypyruvate. 

Although generic aldehydes are converted tvith full stereocontrol, and even a,j5-unsaturated aldehydes are acceptable to some extent, hydroxylated acceptors are usually converted at higher rates (Table 5.6) [259]. 

Multi-enzymatic scheme for stereoselective synthesis of t /o equivalents of xylulose 5-phosphate from fructose 1,6-bisphosphate. 

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