Ketopantoic acid

The conversion of a-ketoisovalerate (32) to ketopantoate (21) is cataly2ed by ketopantonate hydroxymethyltransferase and a cofactor tetrahydrofolate (65). Further reduction of ketopantoate (21) to (R)-pantoate (22) is cataly2ed by ketopantoic acid reductase (66). 

Precursors in the biosynthesis of pantothenic acid include a-ketoiso valeric add (pantoic acid), uracil (/J-alanine), and aspartic acid. Intermediates in the synthesis include ketopantoic acid, pantoic acid, and -alanine. 

Fig. 8. Possible routes for the synthesis of D-pantothenic acid through the enzymatic transformation. PL, pantolactone KPL, ketopantolactone KPA, ketopantoic acid d-PA, D-pantoic acid KPaOEt, ethyl 2 -ketopantothenate KPaCN, 2 -ketopantothenonitrile D-PaOEt, ethyl D-pan-tothenate D-PaCN, D-pantothenonitrile...

Fig. 10. Diversity of microbial reduction of ketopantolactone (a), ketopantoic acid (b), and ethyl 2 -ketopantothenate (c). Symbols A, yeasts O, molds , bacteria , actinomycetes , basidiomycetes...

The enzyme that catalyzes the stereospecific reduction of ketopantoic acid to D-pantoic acid was isolated in a crystalline form from Pseudomonas maltophilia [109] (see Tables 4 and 5). It is an NADPH-dependent enzyme and is strictly specific to ketopantoic acid. The observation that mutants lacking this enzyme require either D-pantoic acid or pantothenic acid for growth and that the revertants regain this activity indicates that the enzyme is ketopantoic acid reductase (EC 1.1.1.169) and is involved in the pantothenate biosynthesis. 

Reduction of ketopantoic acid to D-pantoic acid (0, (4) in Fig. 8). Agrobacterium sp. S-246 is a good source of ketopantoic acid reductase. The yield of D-pantoic acid reached 119 g/1 (molar yield, 90% optical purity, 98% e.e.) on incubation with washed cells of the bacterium [102]. From a practical point of view, ketopantoic acid reduction with Agrobacterium cells has several advantages over ketopantoyl lactone reduction with Candida (or Rhodotorula) cells. The former results in a higher product yield, molar conversion and optical purity of the product than the latter. It is necessary to maintain the substrate level at lower than 3% in the case of the ketopantolactone reduction, but not for the ketopantoic acid reduction. 

Significant diversity exists among the CoA biosynthetic pathways of various organisms. For example, the discussions of the CoA biosynthesis enzymes above highlighted the existence in some cases of two or more distinct nonhomologous proteins that all exhibit the same activity. The transformation of ketopantoic acid 4 to pantoic acid 5 in the biosynthesis of pantothenic acid can also be catalyzed by one of the two proteins a strict KPR enzyme (the gene product EC 1.1.1.169) or the ketol-acid reductoisomerase (tfoCgene product EC... 

Fig. 198. Formation of pantoic and echimidinic acids 1 Ketopantoate hydroxymethyltransferase 2 ketopantoate oxidoreductase...

---