Amino acid biosynthesis and metabolic flux profiling of Pichia pastoris

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Abstract
Amino acid biosynthesis and central carbon metabolism of Pichia pastoris were studied using biosynthetically directed fractional 13C labeling. Cells were grown aerobically in a chemostat culture fed at two dilution rates (0.05 h−1, 0.16 h−1) with glycerol as the sole carbon source. For investigation of amino acid biosynthesis and comparison with glycerol cultivations, cells were also grown at 0.16 h−1 on glucose. Our results show that, firstly, amino acids are synthesized as in Saccharomyces cerevisiae. Secondly, biosynthesis of mitochondrial pyruvate via the malic enzyme is not registered for any of the three cultivations. Thirdly, transfer of oxaloacetate across the mitochondrial membrane appears bidirectional, with a smaller fraction of cytosolic oxaloacetate stemming from the mitochondrial pool at the higher dilution rate of 0.16 h−1 (for glucose or glycerol cultivation) when compared to the glycerol cultivation at 0.05 h−1. Fourthly, the fraction of anaplerotic synthesis of oxaloacetate increases from 33% to 48% when increasing the dilution rate for glycerol supply, while 38% is detected when glucose is fed. Finally, the cultivation on glucose also allowed qualitative comparison with the flux ratio profile previously published for Pichia stipitis and S. cerevisiae grown on glucose in a chemostat culture at a dilution rate of 0.1 h−1. This provided a first indication that regulation of central carbon metabolism in P. pastoris and S. cerevisiae might be more similar to each other than to P. stipitis.