Читать книгу Biodiesel Technology and Applications - Группа авторов - Страница 29

Lipases interact with ester bonds of their substrate like acylglycerols to catalyze the reactions of hydrolysis, synthesis, and transesterification. Triglycerides, which are insoluble and long chained fatty acids, are precisely catalyzed by lipases [113]. Lipase carries out triglyceride oil transesterification with methanol in three reversible steps with the first step for conversion of triglycerides to diglycerides followed by the second step of diglycerides to monoglycerides conversion, and finally, monoglycerides convert into glycerol molecules. Here, each conversion step produces one FAME molecule; hence, a total of three FAME molecule are produced from one triglyceride [116]. Two models are mainly under discussion to describe the kinetics mechanism for esterification reactions, Michaelis-Menten kinetics and Ping Pong Bi Bi model. Lipase catalyzed esterification mainly elaborated by Ping Pong Bi Bi mechanism which is a bi-substrate reaction that releases two products. It involves following steps: 1) acyl-donor donate their acyl group to the enzyme resulting in the formation of acyl-enzyme complex, 2) release of the water molecule as a product, 3) binding of acyl acceptor with the enzyme complex, and 4) release of ester [117, 118]. Many researchers made some modifications in this model depending upon inhibiting factors [118]. The catalytic activity begins with the transient tetrahedral intermediate formation with a negatively charged carbonyl oxygen atom. The reaction between the hydroxyl group oxygen present in nucleophilic serine residue of lipase enzyme and activated carbonyl carbon of the substrate involved building this transient tetrahedral intermediate. The intermediate thus formed is stabilized by its interaction with two peptide NH groups. After that nucleophilic hydroxyl group of water react with the carbonyl carbon of acyl-enzyme complex resulting in the formation of acyl product and enzyme is released for further catalysis [119].