Substrate specificity of pancreatic lipase: Influence of the structure of fatty acids on the reactivity of esters

Abstract

Various esters of monoenoic, polyenoic, methyl-branched, and ω-cyclohexyl or ω-phenyl substituted acids and also esters of aliphatic acids of various chain lengths were hydrolysed by porcine pancreatic lipase, and their maximal rates of lipolysis were compared to the rates of the corresponding oleates. 1. 1. Substituents or unsaturation on Carbon 2–5 led to a relative resistance of the esters against lipase. Cis and trans unsaturation and the one triple bond tested inhibited lipolysis by comparable degrees. Introduction of additional but more remote double bonds, as in marine polyenoic acids, introduced no additional resistance. Between unsaturation at Δ 2,3 and Δ 5,6, the resistance does not depend in a coherent manner on the actual position of the double bond. Δ 2,3-acids or Δ 4,5-acids may in fact be slightly better substrates than Δ 5,6-acids. 2. 2. Compared to a monomethyl-branched acid, multiple branching introduced further resistance. Cyclohexylacetic acid was almost completely resistant. Inhibition by ω-phenyl groups disappeared gradually as the chain was lengthened to carbon 6. The ω-phenyl octanoates had an abnormally high rate of lipolysis, two to three times that of oleate. 3. 3. Lipolysis rates of esters of straight-chain saturated acids increased from C 2 to a maximum at C 4, decreased suddenly at C 5 and then increased until around C 9 the rates of the oleates were approached. 4. 4. Formates were attacked by lipase at rates comparable to those of oleates. 5. 5. It is concluded that the resistance of unsaturated or branched acids is due to steric hindrance during the formation of the activated complex. This hindrance disappears largely for structures after C 5. The saturated C 4-chain seems to be optimally adaptable to the enzyme. The adsorption of the enzyme to the interphase should be viewed as a process preceding and separate from the formation of the substrate-enzyme complex. In this complex the esters are fixed to the enzyme in a two-dimensional orientation, probably along the axis carbonyl carbon-ether oxygen of the carboxyl ester group. Three-point fixation is a possibility. It is suggested that fixation and activation of the substrate is achieved by hydrogen bonding to both carbonyl and ether oxygen, and that the leaving alkoxy group is received by a labile hydrogen of the enzyme.