Molecular and biotechnological aspects of xylanases

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.     

Ecological and biotechnological aspects of lichens

Lichens and the partners from three different kingdoms are both taxonomically and physiologically a very diverse group, which makes them interesting from both ecological and biotechnological points of view. A lichen is a mutual ecophysiological innovation in many extreme environments in which symbiosis seems to protect the partners. Lichen’s ability to grow in harsh environments can be advantageous, resulting in important ecological niches, or disadvantageous when lichens occupy and cause biodeterioration of cultural monuments. Recently, new candidate compounds for drugs, UVB protection, and antifreeze proteins for frozen foods were discovered. Lichens were also found to have potential in bioplastic degradation and prevention of desertification. Nevertheless, there is still large potential for further industrial screening and research on lichen products. Due to improved culture techniques of isolated symbionts, increased knowledge of their secondary metabolism and improved methods for solubilizing lichen metabolites, the screening and activity tests can be implemented more easily today than in the past.     

Glucoamylases: structural and biotechnological aspects

Glucoamylases, one of the main types of enzymes involved in starch hydrolysis, are exo-acting enzymes that release consecutive glucose units from the non-reducing ends of starch molecules. Glucoamylases are microbial enzymes, present in bacteria, archaea, and fungi but not in plants and animals. Structurally, they are classified in family 15 of glycoside hydrolases and characterised by the invariable presence of a catalytic domain with (α/α)₆-fold, often bound to a non-catalytic domain of diverse origin and function. Fungal glucoamylases are biotechnologically very important as they are used industrially in large amounts and have been extensively studied during the past 30 years. Prokaryotic glucoamylases are of biotechnological relevance for being generally thermophilic enzymes, active at elevated temperatures.     

Kinetics for the inhibition of acetylcholinesterase from the electric eel by some organophosphates and carbamates

The inhibition kinetics for some organophosphates (paroxon, diisopropylfluorophosphate, sarin, VX, soman and soman isomers) and carbamates (physostigmine, neostigmine, pyridostigmine and carbaryl) in the reaction with acetylcholinesterase from electric eel have been studied. Dissociation constants and rate constants for the irreversible step were determined. The great differences in inhibitory power of the organophosphates were almost entirely due to differences in affinity. A possible correlation between affinity and bonding rate is discussed.     

Systematic and biotechnological aspects of halophilic and halotolerant actinomycetes

More than 70 species of halotolerant and halophilic actinomycetes belonging to at least 24 genera have been validly described. Halophilic actinomycetes are a less explored source of actinomycetes for discovery of novel bioactive secondary metabolites. Degradation of aliphatic and aromatic organic compounds, detoxification of pollutants, production of new enzymes and other metabolites such as antibiotics, compatible solutes and polymers are other potential industrial applications of halophilic and halotolerant actinomycetes. Especially new bioactive secondary metabolites that are derived from only a small fraction of the investigated halophilic actinomycetes, mainly from marine habitats, have revealed the huge capacity of this physiological group in production of new bioactive chemical entities. Combined high metabolic capacities of actinomycetes and unique features related to extremophilic nature of the halophilic actinomycetes have conferred on them an influential role for future biotechnological applications.     

Microbiological aspects of soil structure

Summary A simple experimental model has been devised to study the effects of organic amendments on aggregate stability and microbial activity in the soil. In the two soils investigated the different organic materials used all produced increases in aggregate stability but significant differences were observed both in the magnitude and time of the increase attributable to individual treatments. Microbial activity, as assessed by visual techniques, was broadly correlated with the changes in aggregate stability though the different treatments tended to produce their own characteristic patterns of colonization. It was generally difficult to relate changes in aggregate stability with the activity of specific micro-organisms but in the case of samples treated with glucose and cellulose, yeasts were the dominant organisms. These have subsequently been shown to produce substantial quantities of an extracellular polysaccharide which is effective in bringing about aggregation.     

The main hygienic aspects of biotechnological processes

It was shown that the present technologies and implementation at the majority of biotechnological plants do not completely meet the hygienic requirements. Under such conditions, there is a complex of unfavourable factors which can have a negative effect on the production areas and environment and consequently on the workers and nearby population. The biological and, at some plants, chemical factors are the leading in the complex. The main aspects of the studies aimed at increasing the safety of biotechnological production, improving the labor conditions and decreasing the disease incidence in the personnel are described in general.     

Updates on naringinase: structural and biotechnological aspects

Naringinases has attracted a great deal of attention in recent years due to its hydrolytic activities which include the production of rhamnose, and prunin and debittering of citrus fruit juices. While this enzyme is widely distributed in fungi, its production from bacterial sources is less commonly known. Fungal naringinase are very important as they are used industrially in large amounts and have been extensively studied during the past decade. In this article, production of bacterial naringinase and potential biotechnological applications are discussed. Bacterial rhamnosidases are exotype enzymes that hydrolyse terminal non-reducing α-l-rhamnosyl groups from α-l-rhamnose containing polysaccharides and glycosides. Structurally, they are classified into family 78 of glycoside hydrolases and characterized by the presence of Asp567 and Glu841 in their active site. Optimization of fermentation conditions and enzyme engineering will allow the development of improved rhamnosidases for advancing suggested industrial applications.     

Microbiological and engineering aspects of biohydrogen production

Dramatically rising oil prices and increasing awareness of the dire environmental consequences of fossil fuel use, including startling effects of climate change, are refocusing attention worldwide on the search for alternative fuels. Hydrogen is poised to become an important future energy carrier. Renewable hydrogen production is pivotal in making it a truly sustainable replacement for fossil fuels, and for realizing its full potential in reducing greenhouse gas emissions. One attractive option is to produce hydrogen through microbial fermentation. This process would use readily available wastes as well as presently unutilized bioresources, including enormous supplies of agricultural and forestry wastes. These potential energy sources are currently not well exploited, and in addition, pose environmental problems. However, fuels are relatively low value products, placing severe constraints on any production process. Therefore, means must be sought to maximize yields and rates of hydrogen production while at the same time minimizing energy and capital inputs to the bioprocess. Here we review the various attributes of the characterized hydrogen producing bacteria as well as the preparation and properties of mixed microflora that have been shown to convert various substrates to hydrogen. Factors affecting yields and rates are highlighted and some avenues for increasing these parameters are explored. On the engineering side, we review the potential waste pre-treatment technologies and discuss the relevant bioprocess parameters, possible reactor configurations, including emerging technologies, and how engineering design-directed research might provide insight into the exploitation of the significant energy potential of biomass resources.