# Enzymatic Synthesis of Biobased Polyesters and Polyamides
## 5 Lipases  
Lipases (triacylglycerollipases, triacylglycerolacyl hydrolases, E.C. 3.1.1.3) are enzymes which catalyze the hydrolysis of water-insoluble triglycerides with long-chain fatty acids to di-glycerides, mono-glycerides and glycerol with release of free fatty acids in aqueous solution (Scheme 8). In organic synthesis, lipases can be used to catalyze other reactions in non-aqueous media, for example, esterification, transesterification, interestrification, amidation, transamidation, aminolysis, aldol condensation and Michael addition [155; 156; 157; 158; 159]. Generally, lipases possess high catalytic reactivity in nonpolar organic solvents with log \(P\) (logarithm of partition coefficient) of more than 1.9 [160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172]. Examples of suitable organic solvents for lipases are benzene (2), toluene (5), diphenyl ether (4.05), hydrocarbons like cyclohexane (3.2) and \(n\)-hexane (3.5), and so on [163]. Lipases also function in some green solvents such as ionic liquids and supercritical CO\({}_{2}\)[164; 165; 166; 167; 168]. Despite their different sources and diverse structures, all lipases possess a very similar \(\alpha/\beta\) hydrolase fold (Scheme 9). The \(\alpha/\beta\) hydrolase fold consists of a \(\beta\)-sheet core of five to eight parallel strands (only the second \(\beta\) strand shows an antiparallel orientation to the others) connected on both sides by \(\alpha\)-helices, forming a \(\alpha/\beta/\alpha\) sandwich-like shape [169; 170; 171; 172]. Lipases and other enzymes including esterases, proteases, dehalogenases, epoxide hydrolases and peroxididases which exhibit similar structural features, belong to the \(\alpha/\beta\) hydrolase family [169; 173]. It is generally acknowledged that the specificity, selectivity and catalytic reactivity of an enzyme depend on its active site, the region that undergoes the binding of substrate molecules and the occurrence of enzymatic reactions. The active site of an enzyme consists of amino acid residuals that form temporary bonds with the substrate (binding site) and other amino acid residues that catalyze the corresponding reaction of that substrate (catalytic site). As for lipases, the active site is situated inside a pocket, which is located above the central \(\beta\)-sheet of the protein [174]. Although the active sites of lipases have different shapes, sizes, depths of the pockets, and physicochemical characteristics of their amino acids [175], the binding sites display highly homologous amino acid sequences [171], and the active site of lipases consists of a highly conserved catalytic triad: a nucleophilic residue (serine), a histidine base and a catalytic acidic residue (aspartic or glutamic acid, usually aspartic acid) (Scheme 9). In addition, many lipases exhibit a lid, a surface loop that is a lipophilic \(\alpha\)-helical domain in the polypeptide chain and covers the active sites [171; 176]. The lid controls the access of substrate molecules to the catalytic center of lipase. In the presence of a lipid-water interface, the lid opens the active center and thus the active site becomes accessible. In this case, a large hydrophobic surface of the enzyme is revealed, which activates the enzyme. However, without the lipid-water interface, the lid is in a closed confirmation. As a consequence, the active center is not accessible and the enzyme is inactive. Figure 9: (a) The \(\alpha/\beta\)-sheet core of the enzyme. The \(\alpha/\beta\)-sheet core of the enzyme. The general catalytic mechanism of lipases is illustrated in Scheme 10, which involves an acylation step followed by a deacylation step [171; 174]. At the acylation step, the hydroxyl group of the catalytic serine is activated by transferring a proton among the aspartate, histidine, and serine residues of the catalytic triad, rendering an increase of the nucleophilicity of the hydroxyl residue of the serine.