Signori Lorenzo, Passolunghi Simone, Ruohonen Laura, Porro Danilo, Branduardi Paola
University of Milano Bicocca, Piazza della Scienza 2, 20126 Milan, Italy.
Microb Cell Fact. 2014 Apr 8;13(1):51. doi: 10.1186/1475-2859-13-51.
The yeast Kluyveromyces marxianus features specific traits that render it attractive for industrial applications. These include production of ethanol which, together with thermotolerance and the ability to grow with a high specific growth rate on a wide range of substrates, could make it an alternative to Saccharomyces cerevisiae as an ethanol producer. However, its ability to co-ferment C5 and C6 sugars under oxygen-limited conditions is far from being fully characterized.
In the present study, K. marxianus CBS712 strain was cultivated in defined medium with glucose and xylose as carbon source. Ethanol fermentation and sugar consumption of CBS712 were investigated under different oxygen supplies (1.75%, 11.00% and 20.95% of O2) and different temperatures (30°C and 41°C). By decreasing oxygen supply, independently from the temperature, both biomass production as well as sugar utilization rate were progressively reduced. In all the tested conditions xylose consumption followed glucose exhaustion. Therefore, xylose metabolism was mainly affected by oxygen depletion. Loss in cell viability cannot explain the decrease in sugar consumption rates, as demonstrated by single cell analyses, while cofactor imbalance is commonly considered as the main cause of impairment of the xylose reductase (KmXR) - xylitol dehydrogenase (KmXDH) pathway. Remarkably, when these enzyme activities were assayed in vitro, a significant decrease was observed together with oxygen depletion, not ascribed to reduced transcription of the corresponding genes.
In the present study both oxygen supply and temperature were shown to be key parameters affecting the fermentation capability of sugars in the K. marxianus CBS712 strain. In particular, a direct correlation was observed between the decreased efficiency to consume xylose with the reduced specific activity of the two main enzymes (KmXR and KmXDH) involved in its catabolism. These data suggest that, in addition to the impairment of the oxidoreductive pathway being determined by the cofactor imbalance, post-transcriptional and/or post-translational regulation of the pathway enzymes contributes to the efficiency of xylose catabolism in micro-aerobic conditions. Overall, the presented work provides novel information on the fermentation capability of the CBS712 strain that is currently considered as the reference strain of the genus K. marxianus.
马克斯克鲁维酵母具有一些特定特性，使其在工业应用中颇具吸引力。这些特性包括乙醇生产，以及耐热性和在多种底物上以高比生长速率生长的能力，这可能使其成为酿酒酵母作为乙醇生产者的替代选择。然而，其在限氧条件下共发酵戊糖和己糖的能力尚未得到充分表征。
在本研究中，马克斯克鲁维酵母CBS712菌株在以葡萄糖和木糖作为碳源的限定培养基中培养。研究了CBS712在不同氧气供应（氧气含量为1.75%、11.00%和20.95%）和不同温度（30°C和41°C）下的乙醇发酵和糖消耗情况。通过降低氧气供应，与温度无关，生物量产量和糖利用率均逐渐降低。在所有测试条件下，木糖消耗均在葡萄糖耗尽后发生。因此，木糖代谢主要受氧耗竭影响。单细胞分析表明，细胞活力丧失无法解释糖消耗速率的下降，而辅因子失衡通常被认为是木糖还原酶（KmXR）-木糖醇脱氢酶（KmXDH）途径受损的主要原因。值得注意的是，当在体外测定这些酶活性时，随着氧耗竭观察到显著下降，这并非归因于相应基因转录的减少。
在本研究中，氧气供应和温度均被证明是影响马克斯克鲁维酵母CBS712菌株糖发酵能力的关键参数。特别是，观察到木糖消耗效率降低与参与其分解代谢的两种主要酶（KmXR和KmXDH）的比活性降低之间存在直接相关性。这些数据表明，除了由辅因子失衡决定氧化还原途径受损外，该途径酶的转录后和/或翻译后调控也有助于微需氧条件下木糖分解代谢的效率。总体而言，本研究提供了关于目前被视为马克斯克鲁维酵母属参考菌株的CBS712菌株发酵能力的新信息。
