Biochemical Engineering Fundamentals Part 1 - Bailey J., Ollis D. 1989

Chemical Foundations of Life
Lipids
Fatty Acids and Related Lipids

Lipids are defined as biological compounds that are soluble in non-polar Solvents (such as benzene, chloroform, ether, etc.) and practically insoluble in Water. It follows from this definition that lipids can have diverse chemical structures and perform various biological Functions. Their relatively low solubility in aqueous media is the reason why lipids are found predominantly in non-aqueous biological phases, particularly in cellular and organelle membranes. Lipids include fats, which serve as reserves of polymeric biological fuel, as well as several important biological mediators. Lipids are also components of more complex compounds, such as Lipoproteins and lipopolysaccharides, which are likewise located mainly in biological Cell membranes and the outer envelopes of certain Viruses.

Saturated Fatty acids are relatively Simple Lipids with the general formula СН₃(СН₂)_nСООН. During Biosynthesis, the hydrocarbon chain of fatty acids is built from identical two-carbon monomer units; therefore, fatty acids can be regarded as non-information-bearing Biopolymers with a terminal carboxyl group. In biological systems, n typically takes even values from 12 to 20.

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FIG. 2.1. Some stable aggregated states of fatty acids in water.

Unsaturated fatty acids are formed by replacing a saturated (—С—С—) bond with a double bond (—С=С—). For example, oleic acid is an unsaturated analogue of stearic acid (n=16):

The hydrocarbon chain imparts hydrophobic properties to these compounds, whereas the carboxyl group is highly hydrophilic. Therefore, when a fatty acid is located at the air-water interface, a small amount of it forms an oriented monomolecular layer (monolayer) in which the polar carboxyl groups are bound by water, while the hydrocarbon chains are directed toward the air phase (Fig. 2.1). This phenomenon underlies the MECHANISM OF ACTION of detergents, which are salts of fatty acids. The formation of a soap monolayer significantly reduces the surface tension at the air-water interface, which dramatically enhances the ability of the solution to wet and clean contaminated surfaces.

Hydrophobic-hydrophilic lipid molecules of this type exhibit very low solubility; increasing the solution concentration beyond the limit required to form a monolayer leads to the aggregation of the excess solute into relatively large ordered structures called micelles (Fig. 2.1). The driving force behind this process is the reduction in the overall Free energy of the system during micelle formation from solution. A particular micellar Structure is dictated by an increase in the number of energetically favorable contacts between groups of the same degree of Hydrophobicity (hydrophilicity) and a corresponding decrease in interactions between hydrophobic and hydrophilic groups. It is known that analogous interactions between the hydrophobic and hydrophilic segments of a single biopolymer are responsible for the existence of the polymer chain in a single preferred conformation. We will briefly examine this behavior of DNA and Proteins later.

Fats, which perform the vital function of intracellular fuel, are esters formed by the Condensation of fatty acids with glycerol:

In an alkaline medium upon heating, fats and other lipids discussed in this section are hydrolyzed into glycerol and fatty acid salts (soaps)—this is precisely how soaps were first obtained from animal fats. This reaction, which is the reverse of the fat synthesis scheme given above, takes place in the digestive tract of animals at BODY Temperature AND is catalyzed by specialized fat-splitting Enzymes; microorganisms also produce such enzymes, whose role is to hydrolyze certain fats into smaller fragments that can subsequently penetrate The Cell through cell membranes.

In structure (though not in function), phosphoglycerides closely resemble fats. In the molecules of the latter, a phosphoric acid residue replaces one of the terminal fatty acid residues. The resulting compound possesses both hydrophilic and hydrophobic residues within its molecule; hence, at sufficiently high concentrations, phosphoglycerides are also capable of forming micelles. If a partition with a small aperture is placed in a phospholipid solution, a planar bimolecular layer (bilayer) about 70 Å (7∙10^-7 cm) thick can form within the aperture (Fig. 2.2, a). Biological Plasma Membranes typically contain significant amounts of Phospholipids and other polar lipids. Furthermore, plasma membranes feature a bimolecular layer clearly visible in electron micrographs (Fig. 2.2, b) of approximately the same thickness as the spontaneously formed double phosphoglyceride layer depicted in Fig. 2.2, a. For this reason, synthetic lipid bilayer membranes serve as a convenient model for studying the fundamental features of delicate Introduction/36.html">Biological Membranes.

In A number of physical properties, lipid bilayer membranes resemble cell membranes. Both membrane types possess high electrical resistance and substantial electrical capacitance, which largely accounts for the impermeability of natural membranes to electrically charged substances, such as phosphorylated compounds. This membrane property, in turn, enables Cells to store reserves of charged nutrients and metabolic intermediates, as well as to maintain a high concentration gradient of small cations (Н⁺, К⁺, Na⁺, etc.) inside and outside the cell.

Other membrane components and their role in the exchange of substances between the cell and its environment will be discussed in Sections 2.5 and 5.7. The barriers and transport channels for biochemically important compounds determine which substances among those involved in the complex network of enzyme-catalyzed processes will enter the cell, which will remain inside, and which will be expelled. The functions of regulating substance transport are essential for the normal viability of any cell; they also play an important role in technological processes where cells act as catalysts.

FIG. 2.2. a — spontaneous formation of a stable phosphoglyceride bilayer in the aperture of a partition placed in a vessel containing a lipid solution; b — in appearance, the spontaneously formed double layer resembles analogous Cell Membrane Structures clearly discernible in electron micrographs. [Fig. 2.2, b is reproduced with permission from: Robertson J. B., Membrane Models: Theoretical and Real, In The Nervous system, vol 1: The Basic Neurosciences, Tower B. D. (ed.), p. 43, Raven Press, New York.]

By adding small amounts of various substances, one can selectively alter the permeability of both model bimolecular and plasma membranes to specific ions. In particular, it has been established that certain Antibiotics and other cation-complexing compounds can significantly enhance passive ion transport in both membrane types. More complex processes account for the increased permeability of living cell walls following mild chemical or thermal Treatment. This cell property is successfully utilized in microbiological production processes for intermediates of cellular METABOLISM, as well as for reducing the nucleic acid content in cell biomass used as animal feed.