乙醯化程度對植物木聚醣於腸道降解之影響

Abstract

木聚醣為植物細胞壁之主要結構性多醣，主要由直鏈狀木醣單元組成，並常帶有側鏈或其他取代基團。乙醯化為植物木聚醣中常見之修飾形式，在調控植物生理功能方面扮演關鍵角色，亦為食品工業中常用以改良多醣理化特性之策略。雖然乙醯基取代廣泛存在於食物來源之多醣中，然而，目前僅有少數研究探討膳食纖維中乙醯基修飾與其對腸道微生物相之交互作用。乙醯基取代通常發生於木醣殘基之C-2與C-3位置，影響其與其他細胞壁成分之交互作用與可及性。乙醯化程度係以O-乙醯基與木醣基單元之莫耳比定義，決定了木聚醣之疏水性。乙醯化除調控木聚醣之理化性質外，亦會抑制木聚醣酶類之催化活性，進而影響其可降解性。鑒於植物木聚醣為膳食纖維之重要來源，廣泛分布於穀類與蔬菜中，故釐清其於結腸環境中之降解具有高度重要性。本研究旨在探討不同乙醯化程度木聚醣於結腸發酵過程中之可降解性，並評估其對腸道微生物相組成與代謝產物生成之影響。 山毛櫸木聚醣（beechwood xylan, BWX）經化學修飾後，其乙醯化程度分別達到0.0、0.5、1.1、1.6與1.9，依序標記為BWXDA0.0、BWXDA0.5、BWXDA1.1、BWXDA1.6與BWXDA1.9。針對上述五種樣品進行醣基組成與FTIR分析，以獲得其結構資訊，並接種人類糞便微生物群進行體外發酵試驗。醣組成與官FTIR分析結果顯示，樣品之醣組成與鍵結方式符合預期。研究結果顯示，乙醯化程度為影響木聚醣在人體結腸中可降解性之主要因素。隨著乙醯基取代程度的提升，氣體產量與培養基酸度皆下降。此外，乙醯化修飾重塑了腸道微生物組成，涵蓋門、科、屬與種層級。低乙醯化程度之受質（如BWXDA0.0與BWXDA0.5）顯著提升了Bacteroidota的相對豐富度，並降低Bacillota與Actinomycetota之豐富度，導致微生物多樣性下降；反之，較高乙醯化程度則促進Bacillota與Bacteroidota之間的平衡。在科層級中，乙醯化程度較低時，Bacteroidaceae與Lachnospiraceae呈現增加趨勢，Ruminococcaceae則相對減少。屬層級分析顯示，乙醯基修飾抑制了Bacteroides與Bifidobacterium的增殖，在Blautia、Fusicatenibacter與Agathobacter群體中亦呈現相同趨勢。在鑑定出的前十大物種中，Bacteroides xylanisolvens呈現獨特變化，於BWXDA0.5與BWXDA1.1組中相對豐富度上升。乙醯化亦顯著抑制短鏈脂肪酸（SCFA）之產生，顯示其不僅影響微生物組成，亦減弱代謝輸出。相關性分析指出，Bacteroides、Faecalibacterium、Escherichia-Shigella、Parabacteroides與Fusicatenibacter之相對豐富度與乙酸、丙酸及丁酸產量呈顯著相關，顯示上述菌屬可能在BWX與其乙醯化產物之代謝過程中扮演關鍵角色。綜合而言，乙醯化程度會改變木聚醣之微生物可利用性，進而重塑腸道微生物生態與代謝表現，為木聚醣於膳食干預與功能性食品設計上的應用提供依據。

Xylan, as a main constituent of plant cell walls, consists of chained xylose residues and multiple types of branching structures. Acetylation is a common modification in plant xylan and provides essential physiological functions. It is also a polysaccharide modification method used in the food industry. Although acetylated food components are commonly found in daily diets, few studies have correlated acetylation levels of acetyl dietary fiber and its impact on the gut microbial communities. Acetyl substitutions commonly occur at the C-2 and C-3 positions of the xylose residues. The degree of acetylation, defined by the mole ratio of O-acetyl units to xylosyl units, determines the hydrophobicity of xylan molecules. Acetylation not only influences the physical and chemical properties of xylan but also limits the activity of xylan-degrading enzymes. Considering that plant xylan is a source of dietary fiber, present in grains and vegetables, it is important to understand how it is degraded in the human colon. In this study, we aimed to investigate how varying degrees of acetylation influence xylan degradation in the colonic environment, shift the abundance of gut microbial populations, and alter metabolic outcomes. Beechwood xylan (BWX) was chemically modified to achieve degrees of acetylation of 0.0, 0.5, 1.1, 1.6, and 1.9, which were designated as BWXDA0.0, BWXDA0.5, BWXDA1.1, BWXDA1.6, and BWXDA1.9, respectively. These five samples were subjected to glycosyl composition and functional group analysis to obtain structural information, followed by in vitro fermentation inoculated with human fecal inoculum. Results from glycosyl and functional group analyses. Our results demonstrated that acetylation is a key factor in controlling xylan degradation in the human fecal cultures. As the degree of acetylation increased, both gas production and medium acidity during fermentation decreased. Changes in xylan acetylation altered gut microbial composition at the phylum, family, genus, and species levels. Substrates with lower degrees of acetylation significantly increased the relative abundance of Bacteroidota while reducing the abundance of Bacillota and Actinomycetota, leading to a decrease in microbial diversity. In contrast, higher acetylation levels resulted in a more balanced Bacillota/Bacteroidota ratio. At the family level, the lower degree of acetylation promoted the relative abundance of Bacteroidaceae and Lachnospiraceae, while Ruminococcaceae decreased. At the genus level, acetylation suppressed the relative abundance of Bacteroides and Bifidobacterium, as well as Blautia, Fusicatenibacter, and Agathobacter. Among the top 10 identified species, Bacteroides xylanisolvens displayed a unique trend, being specifically enriched in the BWXDA0.5 and BWXDA1.1 treatment groups. Acetylation of BWX significantly reduced the production of short-chain fatty acids during fermentation, indicating that acetyl modification not only affects microbial composition but also inhibits metabolic output. The relative abundances of Bacteroides, Faecalibacterium, Escherichia-Shigella, Parabacteroides, and Fusicatenibacter were strongly correlated with acetate, propionate, and butyrate production, suggesting that these taxa play key roles in the degradation of both native and acetylated BWX. These findings indicate that acetyl modification alters microbial utilization of xylan and consequently affects gut microbial composition, which may provide insights into the application of plant xylans in dietary fiber and functional food design.