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Food Lipids: Chemistry, Nutrition, and Biotechnology

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the C domain of the protein through hydrophobic interactions <strong>and</strong> ion pairing (82),<br />

<strong>and</strong> the binding does not induce a conformational change (83–85). Calcium ions<br />

activate <strong>and</strong> reduce the lag phase of human pancreatic lipase, particularly for mixed<br />

bile acid–lipase complexes (86).<br />

The structures of pancreatic lipase from other systems have been recently reported,<br />

including guinea pig (1) <strong>and</strong> horse (87). Overall, the nonhuman pancreatic<br />

lipases are structurally similar to that from humans but may differ in detail. For<br />

example, the guinea pig pancreatic lipase does not possess the lid that is typical of<br />

other lipases (1). Also, unlike the pancreatic lipases from humans <strong>and</strong> pigs, the horse<br />

enzyme is not glycosylated (87).<br />

2. Fungal Lipases<br />

The crystal structures of several fungal lipases have also been determined: for example,<br />

Rhizomucor miehei (88,89), G. c<strong>and</strong>idum (90,91), C. rugosa (92), C. antarctica<br />

(93), <strong>and</strong> Humicola lanuginosa (94). Although there are no obvious sequence<br />

similarities between pancreatic <strong>and</strong> fungal lipases, except for the Gly-X-Ser-X-Gly<br />

consensus sequence in the active site region (87,95), there are structural similarities.<br />

Fungal lipases are �/� proteins <strong>and</strong> have similar topologies based on a large central<br />

mixed �-pleated sheet pattern, mainly parallel, but the connectivities between str<strong>and</strong>s<br />

vary. The two lipases (see below) of G. c<strong>and</strong>idum have 554 amino acids <strong>and</strong> share<br />

about 85% sequence homology (13,96). Also, many fungal lipases possess the serine<br />

protease catalytic triad Ser ���His ���Asp in their active sites; however, glutamic<br />

acid is substituted for aspartic acid in the lipases from G. c<strong>and</strong>idum (90) <strong>and</strong> C.<br />

rugosa (92). Lipase II gene from G. c<strong>and</strong>idum has been cloned (13,97a), <strong>and</strong> the<br />

results of probing the active site by site-directed mutagenesis are consistent with<br />

X-ray crystallography data in that the Ser ���His ���Glu is the active site (97a).<br />

Fungal lipases also possess a lid that prevents access of the substrate to the<br />

active site, form a functional oxyanion hole, <strong>and</strong> have an interfacial binding site<br />

(Gly-X-Ser-X-Gly), but the motion of the lid differs between fungal <strong>and</strong> pancreatic<br />

lipases. Lid rearrangements by human pancreatic lipase (84,85), C. rugosa (92), <strong>and</strong><br />

G. c<strong>and</strong>idum (90) involve more than one loop. The lid is closed <strong>and</strong> covers the active<br />

site in G. c<strong>and</strong>idum <strong>and</strong> is open in C. rugosa; otherwise lipases from these two<br />

sources are very similar in sequence <strong>and</strong> structure. The three-dimensional structure<br />

of the extracellular lipase from Rhizopus delemar based on X-ray crystallographic<br />

coordinates is shown in Figure 2, with the lid partially open exposing the active site<br />

residues Ser 145 , Asp 204 , <strong>and</strong> His 257 (Fig. 2A) <strong>and</strong> residues 86–92 in the partially closed<br />

conformation (Fig. 2B) (97b).<br />

3. Bacterial Lipases<br />

Lipases from several Pseudomonas species have been crystallized, <strong>and</strong> preliminary<br />

X-ray crystallographic analyses of such species as P. cepacia (98), P. glumae (99),<br />

<strong>and</strong> P. fluorescens (100a) have been conducted. Their molecular masses, which are<br />

in the 30- to 35-kDa range, show extensive sequence homology to one another but<br />

little to those of other lipases; the common G-X-S-X-G sequence in the active site<br />

region is conserved, <strong>and</strong> activity appears at the substrate–water interface. On the<br />

other h<strong>and</strong>, lipases from Bacillus species have molecular weights at about 19 kDa,<br />

have the sequence A-X-S-X-G instead of the characteristic sequence, <strong>and</strong> do not<br />

exhibit interfacial activation (100b).<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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