Characterization of a Neutral and Thermostable Glucoamylase from the Thermophilic Mold Thermomucor indicae-seudaticae: Activity, Stability, and Structural Correlation

Prokaryotic and eukaryotic diversity in hydrothermal continental systems

The term extremophile was suggested more than 30 years ago and represents microorganisms that are capable of developing and living under extreme conditions, these conditions being particularly hostile to other types of microorganisms and to humankind. In terrestrial hydrothermal sites, like hot springs, "mud pools", solfataras, and geysers, the dominant extreme conditions are high temperature, low or high pH, and high levels of salinity. The diversity of microorganisms inhabiting these sites is determined by the conditions of the environment. Organisms belonging to the domains Archaea and Bacteria are more represented than the one belonging to Eukarya. Eukarya members tend to be less present because of their lower tolerance to higher temperatures, however, they perform important ecosystem processes when present. Both prokaryotes and eukaryotes have morphological and physical adaptations that allow them to colonize extreme environments. Microbial mats are complex associations of microorganisms that help the colonization of more extreme systems. In this review, a characterization of prokaryotic and eukaryotic organisms that populate terrestrial hydrothermal systems are made.

Cloning of a novel thermostable glucoamylase from thermophilic fungus Rhizomucor pusillus and high-level co-expression with α-amylase in Pichia pastoris

BACKGROUND

Fungal amylase, mainly constitute of fungal α-amylase and glucoamylase, are utilized in a broad range of industries, such as starch hydrolysis, food and brewing. Although various amylases have been found in fungi, the amylases from Aspergillus dominate the commercial application. One of main problems exist with regard to these commercial use of amylases is relatively low thermal and acid stability. In order to maximize the efficiency of starch process, developing fungal amylases with increased thermostability and acid stability has been attracting researchers' interest continually. Besides, synergetic action of glucoamylase and α-amylase could facilitate the degradation of starch. And co-expressing glucoamylase with α-amylase in one host could avoid the need to ferment repeatedly and improves cost-effectiveness of the process.

RESULTS

A novel fungal glucoamylase (RpGla) gene encoding a putative protein of 512 amino acid residues was cloned from Rhizomucor pusillus. BLAST analysis revealed that RpGla shared highest identity of 51% with the Rhizopus oryzae glucoamylase (ABB77799.1). The fungal glucoamylase RpGla was expressed in Pichia pastoris (KM71/9KGla) with maximum activity of 1237 U ml(-1). The optimum pH and temperature of RpGla were pH 4.0 and 70 °C, respectively. Fungal α-amylase (RpAmy) gene was also cloned from R. pusillus and transformed into KM71/9KGla, resulted in recombinant yeast KM71/9KGla-ZαAmy harboring the RpGla and RpAmy genes simultaneously. The maximum saccharogenic activity of KM71/9KGla-ZαAmy was 2218 U ml(-1), which improved 79% compared to KM71/9KGla. Soluble starch hydrolyzed by purified RpGla achieved 43% glucose and 34% maltose. Higher productivity was achieved with a final yield of 48% glucose and 47% maltose catalyzed by purified enzyme preparation produced by KM71/9KGla-ZαAmy.

CONCLUSIONS

A novel fungal glucoamylase and fungal α-amylase genes were cloned from Rhizomucor pusillus. The two enzymes showed good thermostability and acid stability, and similar biochemical properties facilitated synergetic action of the two enzymes. A dramatic improvement was seen in amylase activity through co-expressing RpGla with RpAmy in Pichia pastoris. This is the first report of improving activity through co-expression glucoamylase with α-amylase in P. pastoris. Besides, fungal glucoamylase and α-amylase from R. pusillus were shown as promising candidates for further application in starch hydrolysis.