BLBS102-c09 BLBS102-Simpson March 21, 2012 11:15 Trim: 276mm X 219mm Printer Name: Yet to Come
9 Enzymes in Food Processing 193hydrolase family of enzymes that are characterized by a partic-
ularα-helix andβ-structure topology, that is, a fan-like pattern
of the centralβ-sheets, and also include esterases, thioesterases,
and several other intracellular hydrolases. The catalytic triad
of theα/βhydrolase proteins consists of a nucleophilic serine
residue, an aspartic or glutamic acid residue, and a histidine
group and differs from the catalytic triad in serine proteases
only in the sequence in which the amino acids in the triad
occur (Wang and Hartsuck 1993, Wong and Schotz 2002).
Homologous lipases exhibit subtle differences from one another,
and four major types are distinguished based on structural,
functional, or sequence homologies: (i) the consensus sequence
GxSxG around the active site serine, where x can be any amino
acid, (ii) those with the strand-helix motif also around the active
site serine; (iii) those with a buried active site covered by a region
known in different lipases as active site loop, lid domain, or flap,
and (iv) those with the order of the catalytic triad residues Ser...
Asp/Glu...His (Carriere et al. 2000, Svendsen 2000, Hui and
Howles 2002).Lipase Specificity, Mechanism of Action, and Some
General PropertiesThere are several categories of lipase specificity. Fatty acid-
specific lipases tend to prefer specific fatty acids or classes
of fatty acids (e.g., short-chain, polyunsaturated, etc.). Some
lipases exhibit positional specificity and are regioselective in
discriminating between the external, primary (sn-1 andsn-3 po-
sitions), and internal, secondary (sn-2 position), ester bonds.
For example, several lipases of microbial origin, as well as gas-
tric and phospholipases, show a preference for the external ester
bonds; some exhibit partial stereospecificity, that is, a preference
for eithersn-1 orsn-3 position of TG, and some others exhibit
enantioselectivity and can differentiate between enantiomers of
chiral molecules (Anthonsen et al. 1995, Hou 2002).
The most accepted kinetic model of lipase activity is that
the lipase exists in both the active and inactive conformation
in solution. In aqueous solutions, the inactive conformation is
predominant, thus the enzyme activity is low. However, at the
lipid–water interface, the active conformation is favored, and
water-insoluble substrates are hydrolyzed by the enzyme. The
active forms of the enzyme in solution that are adsorbed at the
interface may have different conformations (Martinelle and Hult
1995). Interfacial activation of lipases is well documented. Ex-
cess substrate induces a drastic increase in lipase activity, while
esterases display a hyperbolic, Michaelis–Menten relationship
between the enzyme activity and substrate concentration, lipases
show a sigmoidal response (Anthonsen et al. 1995). Binding of
lipase to the emulsified substrate at the lipid–water interface elic-
its a conformational change that greatly enhances its catalytic
activity: the lid domain flips backwards to expose the active site;
as well, theβ5 loop orients in a way to enhance access of the
substrate to the enzyme’s active site.
Lipases are water-soluble proteins; nonetheless, their sub-
strates are water-insoluble. Pancreatic lipases have the following
features: temperature optima around 37◦C and thermal stabilities
up to approximately 50◦C (Gjellesvik et al. 1992), and pH op-tima in the range of 6.5–8.5 and pH stabilities between pH 6 and
10 (Wang and Hartsuck 1993). The molecular weights range be-
tween 45 and 100 kDa (Wong and Schotz 2002, Wang and Hart-
suck 1993), and they are anionic with isoelectric points typically
ranging from 4.5 to 7 under physiological conditions (Armand
2007). They are inhibited by organophosphorous compounds
like di-isopropyl fluorophosphates via irreversible phosphoryla-
tion of active site serine. High concentrations of both cationic
(e.g., quaternary ammonium salts) and anionic (e.g., SDS) sur-
factants also inhibit lipase activity. Other inhibitors of lipases
include metal ions (e.g., Fe^2 +/3+and mercury derivatives). Cal-
cium and zinc ions may either activate lipases or produce no
effect (Patkar and Bjorkling 1994, Anthonsen et al. 1995).Conventional Sources of Lipases—Mammalian,
Microbial, and PlantThe pancreas and serous glands of ruminants, especially young
calf and lamb, and the pancreas of pigs have served as important
sources of lipases for flavor development in dairy products for
quite some time now. In subsequent years, microbial lipases
have received most of the attention in lipase research and
have been considerably exploited in numerous applications
as a consequence of the relative ease of their production and
genetic manipulation, as well as the wider range in properties.
Microbial lipases constitute the most significant commercially
produced lipases.
Lipases have also been sourced from plants. However, there
is a dearth of information on plant lipases compared to mam-
malian and microbial lipases. Plant lipases thus far characterized
include the triacylglyceride lipases, various phospholipases, as
well as glycolipase and sulpholipase. The appeal of plant li-
pases derives from their unique capacity to cleave all three fatty
acids from TGs (e.g., as shown by oat lipase) and their dis-
tinct substrate specificities and abilities to synthesize “structured
lipids” (as demonstrated with rape-seed lipase that esterifies fatty
acids to primary alcohols exclusively). Several distinctive fea-
tures of plant lipases have been imputed to phospholipases, such
as the use of phospholipase D from cabbage to synthesize phos-
phatidylglycerols for use as an artificial lung surfactant. In food
processing and preservation, an understanding of the properties
of plant lipases is crucial for maintaining quality and freshness,
particularly in oilseeds, cereals, and oily fruits, and for the pro-
duction of high-quality edible oils (Mukherjee and Hills 1994,
Mukherjee and Kiewit 2002).
Phospholipases are a broad class of enzymes catalyzing
mainly water-insoluble substrates and are involved in diges-
tive, regulatory, and signal transduction pathways, among oth-
ers. Their nomenclature (phospholipase A 1 ,A 2 ,B,C,andD)
is based on which phospholipid ester bond they attack. The
majority of these enzymes require Ca^2 +for (maximal) catal-
ysis, have many disulfide bonds, and are relatively small with
molecular weights ranging from 13 to 30 kDa, but there are
few above 80 kDa (Lopez-Amaya and Marangoni 2000a). The
amino acid sequences of most phospholipases are totally differ-
ent from those of TG lipases (Svendsen 2000). Like lipase activ-
ity, phospholipase activity has also been associated with the