Food Lipids: Chemistry, Nutrition, and Biotechnology

pensations in that enthalpy lost by lipids undergoing transition is absorbed by membrane
proteins as they partition into the more fluid phase of the bilayer [14].
For complex mixtures of lipids found in biological membranes, at temperatures
above the phase transition, all component lipids are liquid crystalline, exhibiting
characteristics consistent with complete mixing of the various lipids. At temperatures
below the phase transition of the phospholipid with the highest melting temperature,
separation of the component into crystalline domains (lateral phase separation) can
occur. This ability of individual lipid components to adopt gel or liquid crystalline
arrangements has led to the suggestion that particular lipids in a biological membrane
may become segregated into a local gel state. This segregation could affect protein
function by restricting protein mobility in the bilayer matrix, or it could provide
packing defects, resulting in permeability changes. Exposure of plant food tissues to
refrigerator temperatures could thus induce localized membrane phase transitions,
upset metabolic activity, and create an environment that serves to reduce the quality
of the product [15].
Several compositional factors play a role in determining transition temperatures
of membranes. The longer the chain length in a phospholipid class, the higher the
transition temperature. Similarly, cation binding to PS membranes decreases the
phase transition temperature. However, the presence of cis double bonds on the
phospholipid fatty acids inhibits hydrocarbon chain packing in the gel state and
causes the phase transition to occur at a lower temperature. On the other hand, the
presence of the free fatty acid, oleic acid, had negligible effects on the bilayer phase
transition, whereas the free fatty acid, palmitic acid, increased the bilayer phase
transition temperature [16]. Differential effects on bilayer properties were also seen
by the incorporation of cholesterol, and these effects were dependent on the cholesterol
concentration [17]. In small amounts (�3 mol %), a softening of the bilayers
in the transition region occurred. However, higher cholesterol concentrations led to
a rigidification of the bilayer that was characterized as a liquid-ordered phase. This
phase is liquid in the sense that the molecules diffuse laterally as in a fluid, but at
the same time the lipid-acyl chains have a high degree of conformational order.
D. Membrane Lipid–Protein Interactions
Complete functioning of a biomembrane is controlled by both the protein and the
lipid, mainly phospholipid, components. In a bilayer membrane that contains a heterogeneous
distribution of both peripheral and integral proteins, there will be a certain
proportion of the phospholipids interacting with the protein component to give
the membrane its integrity at both the structural and the functional level. Thus, the
proportion of phospholipids in the bilayer interacting with protein at any one time
is dictated by protein density, protein type, protein size, and aggregation state of the
proteins.
The major structural element of the transmembrane part of many integral proteins
is the �-helix bundle and the disposition and packing of such helices determine
the degree of protein–lipid interactions. A single � helix passing through a bilayer
membrane has a diameter of about 0.8–1 nm, depending on side chain extension,
which is similar to the long dimension of the cross-section of a diacyl phospholipid
(�0.9–1.0 nm) [18]. In the absence of any significant lateral restriction of such an
individual peptide helix, the lateral and rotational motion of the peptide will be
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.