| Summary: | Cytosolic pyruvate is an essential metabolite in lactic acid production during microbial fermentation. However, under aerobiosis, pyruvate is transported to the mitochondrial matrix by the mitochondrial pyruvate carrier (MPC) and oxidized in cell respiration. Previous reports using <i>Saccharomyces cerevisiae</i> or <i>Aspergillus oryzae</i> have shown that the production of pyruvate-derived chemicals is improved by deleting the <i>MPC1</i> gene. A previous lactate-producing <i>K. phaffii</i> strain engineered by our group was used as a host for the deletion of the <i>MPC1</i> gene. In addition, the expression of a bacterial hemoglobin gene under the alcohol dehydrogenase 2 promoter from <i>Scheffersomyces stipitis</i>, known to work as a hypoxia sensor, was used to evaluate whether aeration would supply enough oxygen to meet the metabolic needs during lactic acid production. However, unlike <i>S. cerevisiae</i> and <i>A. oryzae</i>, the deletion of Mpc1 had no significant impact on lactic acid production but negatively affected cell growth in <i>K. phaffii</i> strains. Furthermore, the relative quantification of the <i>VHb</i> gene revealed that the expression of hemoglobin was detected even in aerobic cultivation, which indicates that the demand for oxygen in the bioreactor could result in functional hypoxia. Overall, the results add to our previously published ones and show that blocking cell respiration using hypoxia is more suitable than deleting Mpc for producing lactic acid in <i>K. phaffii</i>.
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