Augmentation of protein-derived acetic acid production by heat-alkaline-induced changes in protein structure and conformation

Abstract

a b s t r a c tWaste-derived acetic acid (HAc) is an attractive feedstock for microbe-mediated biofuel production. However, fermentative conversion of HAc from waste-activated sludge (WAS) has low yield because of the high concentration of proteins not readily utilizable by microorganisms without prior hydrolysis. We investigated a combined technology for HAc augmentation during sludge protein fermentation. The maximal HAc yield increased over two-fold, reaching 0.502 ± 0.021 g/g protein (0.36 ± 0.01 g COD/g COD, 52% of the total volatile fatty acids) when synthetic sludge protein was heated at 120 C for 30 min, treated at pH 12 for 24 h, and fermented at pH 9 for 72 h. Comprehensive analysis illustrated that the heat-alkaline pretreatment significantly induced protein fragmentation, simultaneously increasing the efficiency of protein biohydrolysis (from 35.5% to 85.9%) by inducing conformational changes indicative of protein unfolding. Consequently, the native a-helix content was decreased from 67.3% to 32.5% by conversion to an unordered shape, whose content increased from 27.5% to 45.5%; disulfide bonds were cleaved, whereas the main SeS stretching pattern was altered from gauche-gauche-gauche to gauchegauche-trans, consequently causing increased protein susceptibility to proteolytic hydrolysis (76.3% vs. 47.0%). Economic analysis indicated that anaerobic fermentation with appropriate heat-alkaline pretreatment is a cost-effective approach for waste conversion to energy sources such as HAc.

Energy use and carbon footprints differ dramatically for diverse wastewater-derived carbonaceous substrates: An integrated exploration of biokinetics and life-cycle assessment

“…The sludge characteristics are similar to those presented in our previous work 34 . The collected sludge was initially filtered using a 0.45 mm mesh to remove small particles, washed twice with distilled water, and finally stored at −4 °C for further respirometry tests.…”

Production of volatile fatty acids from slaughterhouse blood by mixed-culture fermentation

“…In the fed-batch reactors, the addition rate of hydrolysed blood with trypsin was much lower than that of the semi-continuous reactors; therefore, its activity reduction rate was probably higher than the amount of trypsin addition avoiding the negative effects observed in the semicontinuous reactors. Therefore, for (semi)-continuous operation, the other pre-treatment methods should be used to facilitate the rate and extent of VFA production, such as heat-alkaline pre-treatment [25] and ultraviolet pretreatment [26]. The microbial community analysis proved the importance of the Firmicutes phylum and its Clostridiales order in the transformation of proteins into VFA at high concentrations, consistent with the results of bacteria community composition analysis of digester using pig blood as substrate [5].…”

Stepwise pH control to promote synergy of chemical and biological processes for augmenting short-chain fatty acid production from anaerobic sludge fermentation

“…Sludge samples containing the seed inoculum were mixed to afford feed sludge that was then transferred into six identical 2.5-L fermentation reactors at 8.6 g/L (transferred volume = 2 L) to test different methods of fermentative pH control. The test period was at least 20-day long, a typical duration for SCFA production by anaerobic sludge fermentation (Wu et al 2010, Yuan et al 2006, and it was assumed that the degree of hydrolysis can be maximised within the first 72 h under high-pH conditions (Wang et al 2016). Consequently, during the first three days (Phase I) of the experimental process, five fermentation reactors (R1-R5) were maintained at a constant pH of 11 by addition of 2 M NaOH.…”

“…Sludge and biogas samples were collected from reactors every 24 h using previously described procedures (Cai et al 2004, Wang et al 2016 and subjected to chemical analysis (for details, see Section 2.6). In addition, sludge samples collected on days 3 (Phase I end) and 25 (Phase II end) were also used to analyse microbial populations in relation to hydrolysis, acidification, and methanogenesis.…”

“…Alkaline pH is believed to enhance the anaerobic conversion of sludge into SCFAs, e.g., Yuan et al (2006) documented that SCFA production at alkaline pH (10-11) significantly surpassed that observed at acidic or neutral pH. Additionally, Wang et al (2016) revealed that alkaline sludge pretreatment (pH 12) causes the fragmentation and denaturation of proteins, improving their susceptibility to subsequent proteolytic cleavage and hydrolysis and hence augmenting SCFA production via anaerobic fermentation at pH 9. Moreover, Zheng et al (2013) reported that alkaline fermentation hampers methanogenesis and benefits SCFA accumulation.…”