Food Lipids: Chemistry, Nutrition, and Biotechnology

of the single-bonded carbon atoms. In contrast, the structure of a trans fatty acid is
relatively unaffected by the double bond [50]. The cis unsaturated bond thus interrupts
a succession of London–van der Waals forces between membrane fatty acids
by increasing the distance between adjacent fatty acids and lowers the membrane
crystallization temperature. The more unsaturated the fatty acid, the less it is able to
rotate around its carbon–carbon bonds and, consequently, the more it influences
membrane acyl packing. Some calculations suggest that an increase in the number
of double bonds provides diminishing returns in terms of the fatty acid’s influence
on van der Waals forces [99]. It is clear that acyl unsaturation can affect van der
Waals interactions through either changes in the length of association or by increasing
the distance of acyl separation. In addition, the position of a double bond in a
fatty acid also plays a role in the modification of membrane structure. Double bonds
at the methyl end of acyl chains are not particularly effective at modulating membrane
bulk properties due to the fact that they induce a lesser degree of acyl separation.
Interestingly, double bonds are typically first inserted at the �9 position, and
fatty acids with double bonds present only near the methyl end of the chain are
conspicuously absent from nature.
Cells take advantage of the large variety of fatty acid structures to maintain
consistent bulk membrane properties in the face of changing temperature and pressure.
Clearly, then, PUFAs are critical to cell function if viewed only from the perspective
of membrane structure. In light of this fact, it is interesting that animals are
largely incapable of producing PUFAs de novo.
2. Alterations in Membrane-Associated Enzyme Activity
Many membrane-associated enzymes are responsive to their fatty acid environment.
There is considerable evidence that membrane unsaturated fatty acid content can
modify the structure and therefore the functionality of membrane enzymes, and these
effects are often cited when dietary fatty acids modulate physiologic functions. The
direct action of membrane composition of membrane-associated enzyme activity is
often difficult to establish. However, an interesting line of experimentation that involves
measuring membrane-associated activities as a function of temperature has
provided compelling evidence that the degree of membrane unsaturation is critical
to cell function. Membrane-associated enzyme activity [100], molecular transport
[101], and the insertion of proteins into membranes [102] all have maximal activity
at temperatures just above the transition temperature of the membrane. Although
clearly this does not indicate that cells use the changes in membrane unsaturation to
modulate enzyme activities, it does provide strong evidence that diet-induced
changes in membrane composition could play an important role in the modification
of cell physiology.
For all of the difficulties in assigning a causal mechanism to changes in membrane-associated
enzyme activities, it is worth noting that the effects themselves are
largely incontrovertible. Dietary oils and fats have significant effects on many metabolic
activities, including the activity of membrane-associated enzymes. An increasing
body of literature has amassed concerning the modulation of membrane-bound
enzyme activities in both cell and reconstituted systems. Brenner [99] provides an
extensive review of the effects of unsaturated fatty acids enzyme kinetics.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.