Читать книгу Biodiesel Technology and Applications - Группа авторов - Страница 28

Transesterification of oils for biodiesel production is done using either chemical or enzymatic catalyst [108]. An enzymatic catalyst is used at first place due to their normal reaction conditions, reusability, easy products separation, and production of high-quality product. There is less energy consumption in enzyme catalysis as it occurs at a low temperature as compared to chemical catalysis requiring high energy consumption [109, 110]. Further, enzymatic catalysis is environment-friendly as there is no wastewater production and produces pure biodiesel as compared to chemical catalysis [107]. Among enzymatic catalysts, lipase with excellent biochemical and physiological properties is most commonly used to catalyze the transesterification process. Lipases play their role in several industrial processes like alcoholysis, acidolysis, amynolysis, and hydrolysis reactions but their leading role in biodiesel production is considered very important [108–111]. The use of lipase in biodiesel production is proved to be beneficial due to its characteristics like high efficiency, convert FFAs completely into methyl/ethyl esters, reaction specificity, require low temperature, minimum energy consumption, and fewer side products [109]. Lipases belong to class “hydrolases” as they carry out hydrolyses of triglycerides producing glycerol and fatty acids from it in an oil-water interface [110]. A general reaction for biodiesel production using lipase is as follows:

Lipases work on specific substrates and carry out catalysis of heterogeneous reactions in water-soluble as well as insoluble systems. Further, lipases have the properties like chemo-specificity, region-specificity, and stereo-specificity [111]. When classification is made based on region-specificity, there come three classes of lipases: 1) non-specific lipases, 2) 1,3-specific lipases, and 3) fatty acid-specific lipases. Non-specific lipases have ability to attach with all the possible positions of triglycerides to give FFAs and glycerol. The intermediates of the reaction, diglycerides, and monoglycerides do not accumulate in the reaction as they are instantly hydrolysed into fatty acids and glycerol [112]. 1,3-specific lipases are specific for the 1 and 3 positions of triglycerides and remove fatty acids from these positions. 1,3-specific lipases carry out the conversion of triglyceride to diglycerides much faster than diglyceride to monoglyceride [113]. Fatty acid-specific lipases carry out hydrolysis of a specific type of esters which have double bonded long chains of fatty acids in cis position between C-9 and C-1. Hydrolysis of esters with unsaturated fatty acids occur slowly and such class of lipases is not much common [114]. All the hydrolytic enzymes including lipases have common folding pattern involve in a hydrolytic activity called α/β hydrolase fold which is made up of a β sheet of eight strands (one of which is antiparallel while remaining seven strands are parallel) connected by α helices. Histidine residue, catalytic acid residue and Nucleophilic residue are present in α/β hydrolase fold. Pentapeptide sequence (Gly-X-Ser-X-Gly) which is a highly conserved in most of the lipases involved in the construction of ‘nucleophilic elbow’ which is a typical β-turn-α motif having active nucleophilic serine residue between a β strand and an α-helix. Catalytic triad made up of amino acids like histidine, serine, and aspartic acid or glutamic acid build the active site of lipases. The same catalytic triad is seen in serine proteases predicting common catalytic mechanism in them. Amphiphilic α helix peptide sequence forms a lid or flap which covers the active site of lipase and has a structural variability depending upon the lipase source organism. Changes in the structure of the lid are responsible for the activation/inactivation of lipases [114]. Changes in the conformation of lipase structure as well as the quality and quantity of interface being used in the reaction are responsible for the activation of lipase. When the lipase enzyme meets the oil/water interface there occur some changes in lipase structure that results in its activation. For the activation of lipase first, the lid opens to uncover the active site of lipase upon its contact with the ordered interface [115]. Due to this restructuring of lipase, electrophilic region is created around serine residue present in active site, lid hydrophilic side which was exposed in native form now partly buried inside the polar cavity and hydrophobic side of lid completely exposed, thus creating a non-polar surface around the active site for efficient attachment of lipid interface with it [115].