Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, P.O. Box 5003, N-1432, Aas, Norway ; VTT Technical Research Centre of Finland, P.O. Box 1000, Espoo, 02044 VTT Finland ; Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 27, Helsinki, 00014 Finland.
Departamento de Biotecnologia, Escola de Engenharia de Lorena, Universidade de São Paulo, 12602-810 Lorena, SP Brasil.
Biotechnol Biofuels. 2014 Oct 15;7(1):153. doi: 10.1186/s13068-014-0153-3. eCollection 2014.
Sugar cane internodes can be divided diagonally into four fractions, of which the two innermost ones are the least recalcitrant pith and the moderately accessible pith-rind interface. These fractions differ in enzymatic hydrolyzability due to structural differences. In general, cellulose hydrolysis in plants is hindered by its physical interaction with hemicellulose and lignin. Lignin is believed to be linked covalently to hemicellulose through hydroxycinnamic acids, forming a compact matrix around the polysaccharides. Acetyl xylan esterase and three feruloyl esterases were evaluated for their potential to fragment the lignocellulosic network in sugar cane and to indirectly increase the accessibility of cellulose.
The hydrolyzability of the pith and pith-rind interface fractions of a low-lignin-containing sugar cane clone (H58) was compared to that of a reference cultivar (RC). Acetyl xylan esterase enhanced the rate and overall yield of cellulose and xylan hydrolysis in all four substrates. Of the three feruloyl esterases tested, only TsFaeC was capable of releasing p-coumaric acid, while AnFaeA and NcFaeD released ferulic acid from both the pith and interface fractions. Ferulic acid release was higher from the less recalcitrant clone (H58)/fraction (pith), whereas more p-coumaric acid was released from the clone (RC)/fraction (interface) with a higher lignin content. In addition, a compositional analysis of the four fractions revealed that p-coumaroyl content correlated with lignin, while feruloyl content correlated with arabinose content, suggesting different esterification patterns of these two hydroxycinnamic acids. Despite the extensive release of phenolic acids, feruloyl esterases only moderately promoted enzyme access to cellulose or xylan.
Acetyl xylan esterase TrAXE was more efficient in enhancing the overall saccharification of sugar cane, compared to the feruloyl esterases AnFaeA, TsFaeC, and NcFaeD. The hydroxycinnamic acid composition of sugar cane fractions and the hydrolysis data together suggest that feruloyl groups are more likely to decorate xylan, while p-coumaroyl groups are rather linked to lignin. The three different feruloyl esterases had distinct product profiles on non-pretreated sugar cane substrate, indicating that sugar cane pith could function as a possible natural substrate for feruloyl esterase activity measurements. Hydrolysis data suggest that TsFaeC was able to release p-coumaroyl groups esterifying lignin.
甘蔗节间可对角分为 4 个部分，其中最内层的两个部分是最难降解的髓心和适度可接近的髓心-皮界面。这些部分在酶解方面存在差异，这是由于结构上的差异造成的。一般来说，植物纤维素的水解受到其与半纤维素和木质素的物理相互作用的阻碍。木质素被认为通过羟基肉桂酸与半纤维素共价连接，在多糖周围形成一个紧密的基质。乙酰木聚糖酯酶和 3 种阿魏酰酯酶被评估其潜在的作用，即打断甘蔗中的木质纤维素网络，并间接增加纤维素的可及性。
与对照品种（RC）相比，低木质素甘蔗克隆（H58）的髓心和髓心-皮界面部分的水解率和整体水解率被进行了比较。乙酰木聚糖酯酶提高了所有 4 种底物中纤维素和木聚糖水解的速度和整体产率。在所测试的 3 种阿魏酰酯酶中，只有 TsFaeC 能够从髓心和界面部分释放对香豆酸，而 AnFaeA 和 NcFaeD 则从这两个部分释放阿魏酸。从木质素含量较低的克隆（H58）/部分（髓心）中释放出更多的对香豆酸，而从木质素含量较高的克隆（RC）/部分（界面）中释放出更多的阿魏酸。此外，对 4 个部分的成分分析表明，对香豆酰基含量与木质素相关，而阿魏酰基含量与阿拉伯糖含量相关，这表明这两种羟基肉桂酸的酯化模式不同。尽管酚酸的释放量很大，但阿魏酰酯酶只是适度地促进了纤维素或木聚糖的酶解。
与阿魏酰酯酶 AnFaeA、TsFaeC 和 NcFaeD 相比，乙酰木聚糖酯酶 TrAXE 更有效地提高了甘蔗的整体糖化效率。甘蔗部分的羟基肉桂酸组成和水解数据表明，阿魏酰基更有可能修饰木聚糖，而对香豆酰基则与木质素相连。三种不同的阿魏酰酯酶在未经预处理的甘蔗基质上有不同的产物谱，这表明甘蔗髓心可能是阿魏酰酯酶活性测定的天然底物。水解数据表明 TsFaeC 能够释放与木质素酯化的对香豆酰基。
