# Production of Lactate by Metabolically Engineered Scheffersomyces stipitis

## TL;DR

Scientists engineered a yeast to produce lactate from sugars, achieving high yields using lignocellulosic biomass.

## Contribution

A novel metabolic engineering approach in Scheffersomyces stipitis for efficient lactate production from xylose and glucose.

## Key findings

- Engineered strain produced 19.27 g/L lactate from 50 g/L xylose after 74 hours.
- Increasing temperature to 32 °C improved lactate yield by 30%.
- Ethanol production was suppressed on xylose in the engineered strain.

## Abstract

Lactate is a valuable compound used in food, chemical, and pharmaceutical industries. High-value, optically pure L- or D-lactate can be synthesized microbially via specific dehydrogenases. The non-conventional yeast Scheffersomyces stipitis, which is known for fermenting both hexoses and pentoses, is a promising host for biochemical production from lignocellulosic biomass but does not naturally produce lactate. In this study, we engineered S. stipitis to produce lactate by expressing two codon-optimized bacterial L-lactate dehydrogenase genes under the control of strong native promoters. The engineered strain produced 7.42 g/L (0.46 g/g yield) and 11.67 g/L (0.58 g/g yield) lactate from glucose and xylose, respectively. The highest titer, 19.27 g/L (0.52 g/g yield), was achieved from 50 g/L xylose after 74 h. Increasing the fermentation temperature from 28 °C to 32 °C improved yield by 30%, while a neutralizing agent further enhanced yield by 25% and prevented lactate degradation following carbon depletion. Although the wildtype strain produced a significant amount of ethanol on both glucose and xylose, the engineered strain produced ethanol as a side product exclusively on glucose and not on xylose. This phenomenon could be advantageous for biotechnological applications and may reflect shifts in gene expression depending on the carbon source or even on the presence of lactate.