The types of specificity that can be ascribed to lipases include:
1. Substrate specificity: The enzyme shows a different rate of lipolysis of various acylglycerols, tri-, di-, and monoacylglycerols, or various types of fatty acids.
2. Positional specificity: The enzyme catalyzes the release of fatty acids preferentially at the primary ester, secondary ester, or nonspecific at all esters.
3. Stereospecificity: The enzyme hydrolyzes the two primary esters (sn-1 or sn-3) at different rates (91).
Porcine pancreatic lipase has specificity for acylglycerols in the order: 1,3-triacylglycerol > 1(3)-monoacylglycerol > 2-monoacylglyercol (91). Eico-sapentaenoic and docosahexaenoic acids at the 1(3)-position are resistant to the action of lipase (92). The enzyme shows the same reactivity on palmitoyl, lino-leoyl, and linolenoyl groups at the 1(2)-position. However, triacylglycerols containing hydroperoxy linoleic or linolenic acids at the 1(3)-position are hydrolyzed at rates about two times higher than those of the unoxidized fatty acids in the corresponding positions (93). Lipolysis rates of the porcine pancreatic lipase on esters of saturated fatty acids increase with chain length until C9, which approaches the rates of the oleates. Substitutions or unsaturation, in particular at C2-C5, lead to increased resistance to hydrolysis (94). It has been demonstrated that human and rabbit gastric lipases have a stereospecificity for the sn-3 ester bond of trioctanoin or triolein (95). Pancreatic lipase cleaves the ester bonds in the sn-1 and sn-3-posi-tions without selectivity (96).
The Geotrichum candidum lipase is known to show a rather unique specificity for the hydrolysis of fatty acids with a cis-9 or cis,cis-9,12 unsaturations in preference to the trans-isomers or corresponding saturated fatty acids (97, 98). The isoform lipase B exhibits a very high degree of specificity for mono-unsaturated fatty acid esters of cis-9 double bonds, whereas lipase A hydrolyzes a wide variety of fatty acid esters (99, 100). Similar to pancreatic lipases, a majority of the microbial lipases attack preferentially the 1(3)-position of triacylglycerols, with exceptions, such as Corynebacterium acnes, Staphylococcal aureus, and Candida cylindracea lipases which hydrolyze all three positions (101-103). The isoforms III and IV from Geotrichum candidum have been shown to cleave the 2-position ester bond of triolein at nearly twice the rate of the cleavage of the 1(3)-position (104). Candida natarctica lipase A shows a preference for sn-2 when trioctanoin and triolein are used as substrates, whereas lipase B is selective for sn-3 (105). The Rhizopus arrhizus lipase shows stereospecificity with sn-1 preference toward trioleoylglycerols, while the Chromobacterium viscosum enzyme has sn-3 selectivity (105). It has been demonstrated that structural variations at the sn-2 position cause a change in the stereoselectivity. The preference of Chromobacterium viscosum lipase is shifted from sn-3 to sn-1 when the sn-2 acylester is replaced by a nonester alkyl chain (106).
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