Microorganisms and Enzymes Involved in Lignin Degradation Vis-à-vis Production of Nutritionally Rich Animal Feed: An Overview
Ramesh Chander Kuhad, Sarika Kuhar, Krishna Kant Sharma, Bhuvnesh Shrivastava
Lignocellulose Biotechnology Laboratory, Department of Microbiology, University of Delhi South Campus, Benito Juarez Road, New Delhi, 110021, India.
Lignocellulosics are the major structural component of woody and nonwoody plants and represent a major source of renewable organic matter. The plant cell wall consists of three major polymers: cellulose, hemicellulose, and lignin. Lignocellulose biomass, available in huge quantity, has attracted considerable attention as an alternate resource for pulp and paper, fuel alcohol, chemicals, and protein for food and feed using microbial bioconversion processes. The current industrial activity of lignocellulosic fermentation is limited because of the difficulty in economic bioconversion of these materials to value-added products. Lignin is degraded to different extents by variety of microorganisms including bacteria, actinomycetes, and fungi, of which wood-rotting fungi are the most effective, white-rot fungi in particular. White-rot fungi degrade wood by a simultaneous attack on the lignin, cellulose, and hemicellulose, but few of them are specific lignin degraders. The selective lignin degraders hold a potential role in economically bioconversion of plant residues into cellulose-rich materials for subsequent bioethanol and animal feed production. Different fungi adapt in accordance to conditions existing in the ecosystem and complete their task of carbon recycling of the lignified tissues, and some white-rot fungi have capability to completely mineralize it. It is known that white-rot fungi are able to perform lignin degradation by an array of extracellular oxidative enzymes, the best characterized of which are lignin peroxidase (LiP), manganese peroxidase (MnP), and laccase. However, the regulation of the production of individual enzymes and lignin degradation is a complex phenomenon. Unfortunately, even selected white-rot fungi take long in delignifying the lignocellulosic substrates. Therefore, it is necessary to improve these fungi for their ability to degrade lignin through various conventional and modern approaches. A considerable progress has been made in this direction during the past two decades; LiP, MnP, and laccase genes have been cloned, and an efficient Agrobacterium-mediated transformation system has been developed, which will eventually help in successful expression of the desired protein. This chapter presents an overview of diversity of lignin-degrading microorganisms and their enzymes especially in developing animal feed. In addition to that, advances in molecular approaches to enhance the delignification capability of microorganisms are also discussed.