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

Chemical Foundations of Life
Lipids
Fat-Soluble Vitamins, Steroids, and Other Lipids

Vitamins are Organic compounds whose trace amounts are essential for the normal functioning of Cells. Essential vitamins are those that the Organism cannot synthesize on its own; in their absence from the diet, The Cell perishes. (This fact formed The basis of an assay for the presence or absence of a given vitamin by cultivating Microorganisms in the test medium.) Naturally, Water-Soluble Vitamins, such as Vitamin C (ascorbic acid), are not Lipids. At the same time, vitamins A, E, K, and D are insoluble in water but soluble in organic Solvents, which is why they are classified as lipids. The Biological Role of Fat-soluble vitamins is not yet fully understood, with the exception of vitamin A, which is required to prevent night blindness (nyctalopia) in humans.

Interest in microorganisms and food products as sources of vitamins stems from the fact that water-soluble vitamins—such as thiamine, riboflavin, niacin (nicotinic acid), pantothenic acid, biotin, folic acid, and Choline—as well as lipid-soluble vitamins A, D, E, and K, are among those essential (or considered essential) for children and/or adults. Many of these compounds can be synthesized by various microorganisms. Yeasts, for example, produce ergosterol, a precursor to vitamin D₂ (calciferol), which is converted into it under the action of sunlight. The fat-soluble vitamin K is synthesized by microorganisms in the digestive tract of animals and humans; this example perfectly illustrates the mutual benefit provided by one organism to another (known as commensalism, discussed further in Chapter 13). It is well established that certain water-soluble vitamins are required to convert A number of Enzymes into their active form.

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FIG. 2.3. Some Examples of steroid structures.

Steroids constitute a class of biologically important lipids that share a common carbon Skeleton (Fig. 2.3, a). Notably, steroids include Hormones, which are exceptionally potent regulators of biological reaction rates; in human Tissues, hormones are effective at concentrations on the order of 10^-8 M. Currently, microorganisms are used only for relatively simple transformations that convert certain steroids into others of higher value. For instance, progesterone can be converted into cortisone in two steps: the first is microbiological and the second chemical (Fig. 2.3, b). Other similar examples are given in Table 12.15. The Structure of the starting progesterone is so complex that only a specific enzyme produced by one or more types of microorganisms can drive the required reaction with sufficient selectivity (i.e., with minimal formation of by-products). The well-known steroid Cholesterol (Fig. 2.3, c) is found almost exclusively in the membranes of animal tissues. Related steroids have been shown to have The ability to alter the permeability of cellular Plasma Membranes.

An important polymeric nutrient reserve for a number of Bacteria, including Alcaligenes eutrophus, is poly-β-hydroxybutyric acid (PHB), whose monomeric unit has the structure —OCH(CH₃)CH₂CO—. This polymer resides within the cells in the form of granules. When nutrients are scarce, the cell depolimerizes PHB to yield soluble β-hydroxybutyric acid, which is readily incorporated into metabolic pathways. The physical and mechanical properties of PHB make it suitable for manufacturing packaging Materials; considering that PHB is readily biodegradable, it is likely that this biopolymer will be produced on an industrial scale in the future.