Investigation of alkaline and thermal stability of alpha-l-arabinofuranosidase produced by directed evolution

Abstract

Alpha-L-arabinofuranosidase (Abf) is a type of glycoside hydrolase that cleaves the α-L-arabinofuranosidic bonds in the polysaccharides with arabinose. There are many ranges of biotechnological application fields of this enzyme such as pulp and paper industry. The purpose of this study is to identify Geobacillus vulcani GS90 by 16S rRNA analysis and to investigate the alkaline and thermal stability of Abf and its mutants produced by error-prone PCR. During this study, firstly, partial 16S rDNA gene was amplified by using universal primers, cloned and sequenced by Sanger method. The partial 16S rDNA sequence was analyzed by BLAST and phylogenetic tree was constructed. Secondly, abf and its mutants were cloned and 73 mutants were screened for functional analysis in terms of total proteins. After purification of Abf and its functional mutant enzymes, they were analyzed in terms of stability and activity against three different conditions. They were 70oC- pH 5.0, 71oC- pH 5.0 and 70oC- pH 9.6. It was detected that G. vulcani 3S-1 was the closest strain of G. vulcani GS90. In addition, it could be deduced that L307S and Q90H/L307S mutants were more stable than Abf at 71oC- pH 5.0 and less stable at 70oC- pH 9.6. According to SWISS MODEL analysis, the surrounding residues of 90th and 307th amino acid constructed no hydrogen bonds in Abf, two hydrogen bonds in Q90H/L307S and three hydrogen bonds in L307S. Moreover, it was detected that these mutants had both a longer β-strain and more number of β-strains than Abf. Finally, the predicted solvent accessibilities of Abf, Q90H/L307S and L307S were investigated and it was deduced that relocations of R447 and some asparagines could have affected the alkaline stability of them.