Transketolase catalyzes the transfer of a ketol from a ketose (xylulose 5-phosphate, fructose 6-phosphate or sedoheptulose 7-phosphate) to an aldose (ribose 5-phosphate, erythrose 4-phosphate or glyceraldehyde 3-phosphate) and is a key enzyme in the pentose phosphate shunt (3, 4). Transketolase, together with transaldolase, creates a reversible link between two main metabolic pathways, the pentose phosphate pathway and glycolysis, which allows the cell to adapt its NADPH production, and ribose-5-phosphate production to meet its immediate needs (3). In Saccharomyces cerevisiae, TKL1 encodes the major isoform and TKL2 encodes a minor isoform (1). Double null mutants for tkl1 and tkl2 are viable, but auxotrophic for aromatic amino acids (1).

Transketolase from S. cerevisiae, is a homodimer, and is dependent on thiamine diphosphate as a cofactor and on divalent cations for activity (5, 3, 6, 7, 8, 9). Each subunit is folded into three consecutive alpha/beta-domains (3). The coenzyme-binding site is located in a deep cleft at the interface between the subunits, and residues of both subunits interact with the thiamine diphosphate cofactor (3, 10, 11, 12) to stabilize the holoenyme. Binding of Ca2+, or to a lesser extent of Mg2+, also stabilize the holoenzyme (12). In vivo, transketolase activity may be negatively regulated by RNA (13). Each is also capable of nonspecifically catalyzing the reactions in the non-oxidative branch of the pentose phosphate pathway (4).

Although transketolase activity is not detectable in tkl1 null mutants, Tkl2p is expressed and does play a physiological role (1). Expression of TKL2 is induced in carbon-limited (low glucose) cultures, in response to lithium chloride or dimethyl sulfoxide (DMSO), and at the diauxic transition in a Msn2/4p-dependent manner, and TKL2 expression is repressed in response to cAMP (14, 15, 16, 17). Tkl2p localizes to the nucleus and cytoplasm (18).

Tkl2p has similarity to S. cerevisiae Tkl1p, Escherichia coli transketolase, Rhodobacter sphaeroides transketolase, Streptococcus pneumoniae recP, Hansenula polymorpha dihydroxyacetone synthase, Kluyveromyces lactis TKL1 (which complements the phenotype of the tkl1 tkl2 double null mutant (19)), Pichia stipitis TKT, rabbit liver transketolase, rat TKT, mouse TKT, and human TKT (1, 5, 8, 19). Tkl2p is also related to E. coli pyruvate dehydrogenase E1 subunit, which is another vitamin B1-dependent enzyme (5).

Last updated: 2005-12-19