Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 2 56The generally accepted mechanism of serine dependent lipase is shownsimplified in scheme 2.10:OHisSerOHisSerAspO -HNNOHR OR'OH HAspOHNNHOR OR'O -H HOxyanion holeOxyanion holeOHisSer-H 2 OOHisSerAspOH NNHOH OR OH+H 2 OAspOHNNHROOScheme 2.10: hydrolysis mechanism of serine lipases.The reaction proceeds via nucleophilic attack on the carbonyl groupof the substrate ester RCO 2 R' by the activated primary hydroxy group ofthe serine in the active site. The tetrahedral intermediate is stabilized byformation of hydrogen bonds between the negative charge on the esteroxygen and the amino acids of the oxyanion hole present in the proteinbackbone as shown in scheme 2.10.There are several examples of X-Ray crystal data which support thishypothesis 37 .The tetrahedral intermediate is then stabilized by elimination of R'OHleading to an activated ester termed acyl-enzyme. This is the centralintermediate in all four esterase-catalyzed reactions.In aqueous media the acyl group of the acyl enzyme is transferred tothe nuclephilic H 2 O, present in large excess; the equilibrium is shiftedtowards the hydrolysis product.In non aqueous systems the reverse reaction is possible and estersynthesis occurs (reaction b and c figure 2.10). Esterifications viaenzymatic acyl transfer (reaction d figure 2.10) are particularly attractive forsynthesis because no water is involved. If the acyl doner, which oftenserves also as solvent, is used in excess the acyl group from the acyl-