Effective valorization of lignocellulosic biomass to ethanol by engineered yeast : 효모 발효를 통한 다양한 식물자원의 재활용 및 유용물질의 생산

국문 초록 (Abstract)

기후변화에 대응하여 CO2배출 및 미세먼지 저감화를 위해 새로운 바이오매스의 탐색을 통한 대체에너지 생산에 관한 연구가 지속적으로 이루어지고 있다. 본 학위 논문의 첫 번째 챕터에서는 이러한 다양한 식물자원들을 보다 효율적으로 활용하기 위해 리그노셀룰로직 바이오매스를 활용하여 유용물질을 생산한 다양한 연구들을 검토하였다. 따라서, 식물 자원의 종류, 재조합 된 효모균주의 사용 및 다양한 저해 성분들이 리그노셀룰로직 바이오매스의 생물전환에 영향을 미치는 요소들로 논의되었고, 이를 바탕으로 두 번째 챕터에서는, 실제 식물자원인 kenaf(Hibiscus cannabinus)를 활용하여 가수분해 및 동시당화 발효를 진행하였다. 본 연구를 통해 같은 작물이라도 품종에 따라 가수분해 결과가 크게 다르며, 이렇게 품종에 따라 상이한 당 함량과 acetate 함량이 발효에 큰 영향을 미치는 것으로 확인하였다. 또한 식물자원에 풍부한 xylose를 발효할 수 있도록 대사 공학적인 방법으로 재조합 된 균주를 사용함으로써 에탄올 생산성 향상에 크게 기여하였다. 더 나아가, acetate는 미생물 발효의 주요 억제 인자 중 하나인 것으로 분석되었고 pH level을 조절하여 acetate 저해를 극복함으로써 오탄당 발효를 효과적으로 촉진시켰다. 본 연구를 통해 리그노셀룰로스 바이오매스의 효과적인 생물전환에 있어 영향을 미치는 중요한 요인과 방법을 제시하고자 하였고, 바이오 에탄올 및 고부가가치물질 생산을 위한 신규 생물자원으로서의 kenaf의 가능성을 확인하였다.

다국어 초록 (Multilingual Abstract)

With growing interest in alternative fuels to minimize carbon and particle emissions, research continues on the production of lignocellulosic ethanol and on the development of suitable yeast strains. However, great diversities and continued technical advances in pretreatment methods for lignocellulosic biomass complicate the evaluation of developed yeast strains, and strain development often lags industrial applicability. In this review, recent studies demonstrating developed yeast strains with lignocellulosic biomass hydrolysates are compared. For the pretreatment methods, we highlight hydrothermal pretreatments (dilute acid treatment and autohydrolysis), which are the most commonly used and effective methods for lignocellulosic biomass pretreatment. Rather than pretreatment conditions, the type of biomass most strongly influences the composition of the hydrolysates. Metabolic engineering strategies for yeast strain development, the choice of xylose-metabolic pathway, adaptive evolution, and strain background are highlighted as important factors affecting ethanol yield and productivity from lignocellulosic biomass hydrolysates. A comparison of the parameters from recent studies demonstrating lignocellulosic ethanol production provides useful information for future strain development.

다국어 초록 (Multilingual Abstract)

Kenaf (Hibiscus cannabinus) is an annual fiber crop grown mainly in India and China. This crop is becoming a new bio-based energy source because of its fast growth rate, excellent CO2 absorption ability, and large productivity per unit area. In the present study, we evaluated 10 different cultivars of kenaf for their potential as biomass for cellulosic ethanol production. First, kenaf samples were hydrolyzed using dilute sulfuric acid, which is the most simple and cost-effective pretreatment method. Next, simultaneous saccharification and fermentation (SSF) of the hydrolysates were performed by wild-type and engineered xylose-fermenting yeast strains. Compositional analysis of the biomass, the hydrolysates, and the fermented products suggested that ethanol yield and productivity were significantly affected by a type of kenaf cultivars, which was not predictable based on the biomass compositions. Also, the ethanol production was maximized when the xylose fraction was utilized by engineered yeast under the control of pH to avoid acetate inhibition. Considering the sugar compositions and their fermentability, kenaf can be a promising energy-dedicated crop for cellulosic ethanol production.

목차 (Table of Contents)

• Ⅰ. Introduction 1
• Ⅱ. Chapter 1. Literature review - Xylose utilization in Saccharomyces cerevisiae during conversion of hydrothermally pretreated lignocellulosic biomass to ethanol 3
• Abstract of the thesis 4
• 1. Introduction 5
• 2. Main subjects 7
• 2.1 Hydrothermal pretreatments as the most commonly employed and effective methods for lignocellulosic biofuel production 7
• 2.2 Standard enzyme hydrolysis of lignocellulosic biomass using commercial enzyme cocktails 11
• 2.3 Engineered Saccharomyces cerevisiae for the utilization of xylose derived from hydrothermally pretreated lignocellulosic biomass 13
• 3. Conclusion 16
• 4. References 18
• Ⅲ. Chapter 2. Evaluation of Kenaf (Hibiscus cannabinus) as an alternative biomass for cellulosic ethanol production 23
• Abstract of the thesis 24
• 1. Introduction 25
• 2. Materials and Methods 27
• 2.1 Yeast strain and culture conditions 27
• 2.2 Lignocellulosic hydrolysate preparation 28
• 2.3 Yeast-mediated SSF (Simultaneous saccharification and fermentation) 28
• 2.4 Kenaf hydrolysate-like medium 29
• 2.5 Analytical methods 29
• 3. Results 31
• 3.1 Dilute acid pretreatment of kenaf 31
• 3.2 Saccharification of pretreated kenaf samples 34
• 3.3 SSF of kenaf hydrolysates 36
• 3.4 SSF of kenaf hydrolysates at elevated pH 37
• 4. Discussion 39
• 5. References 44
• Ⅳ. Conclusion 49