Extremophilic Ligninolytic Enzymes

Abstract

Extremophilic microorganisms have developed a variety of physiological strategies that help them to survive on different ecological niche such as extreme temperature, pH, salt concentration and pressure. It has been demonstrated that these microorganisms produce extracellular isoenzyme capable to degrade the ligninocellulosic waste and other related compounds for their growth and survival. These are known as extremophilic ligninolytic enzyme. The extremophilic enzymes are considered superior than normal enzyme because they allow the performance of industrial processes even under adverse condition in which conventional proteins are completely denatured. The common extremophilic ligninolytic enzymes are manganese peroxidase (MnP), lignin peroxidase (LiP) and laccase. These enzymes predominantly have been reported in fungus but their occurrence and role for decolourisation and detoxification of various environmental pollutants also have been reported in bacteria and actinomycetes. Biochemically, MnP and LiP are glycosylated haem protein with molecular weight (MW) ranging from 38 to 62.5 kDa (MnP: 38–62.5 kDa; LiP: 38–46 kDa) while laccases are monomeric, dimeric and trimeric glycoprotein with MW range from 50 to 97 kDa. The optimum activity at pH range for MnP and LiP in fungus is 3.0–5.0 while in bacteria pH range for these enzymes ranges from pH 4.0 to 9.0. The optimum activity for laccase in fungus and bacteria are noted pH 4.0–10.0. The extremophilic activity of these enzymes is regulated due to presence of various salt bridge between amino acids to maintain their stability for catalytic function. Furthermore, the oxidation mechanism of these ligninolytic enzymes have revealed that MnP and lacasse require specific mediator (e.g. GST, tween 80, ABTS, HBT) while LiP does not require any mediator for oxidation of phenolics and non-phenolics compounds. The major biotechnological applications of these enzymes are decolourisation and detoxification of various lignin and ligninolytic waste. It has also scope for pulp biobleaching and ethanol production.