Biohydrogenation of unsaturated fatty acids takes place in rumen microorganisms

Desaturation of an acyl chain is a reaction widespread in Nature. The reverse process, namely the hydrogenation of double bonds, is found in only a few organisms. These organisms are commonly found in the rumens of cows, sheep and other ruminant animals. Linoleic acid, for example, can be hydrogenated by rumen flora (anaerobic bacteria and protozoa) to stearic acid by a series of reactions shown in Fig. 2.22.

First of all the substrate fatty acids must be released from leaf complex lipids by the action of acyl hydrolases (Section 3.5.3). The first reaction of the unesterified linoleic acid involves iso-merization of the ds-12,13 double bond to a trans-11,12 bond which is then in conjugation with the ds-9,10 double bond. The enzyme responsible for this isomerization has been partially purified from the cell envelope of Butyrivi-brio fibrisolvens and can act on a-linolenic as well as linoleic acid. Next, hydrogen is added across the cis-9,10 bond to form trans-vaccenic acid, which is further reduced to stearic acid. Analogous reactions occur with fatty acids other than

Biohydrogenation

Fig. 2.22 Biohydrogenation (by rumen micro-organisms).

CH3(CH2)16COOH

Fig. 2.22 Biohydrogenation (by rumen micro-organisms).

linoleic acid, but the positions of the cis and trans bonds will, of course, be different.

In spite of the high activity of rumen microorganisms (including the further breakdown of the fatty acids by oxidation) ruminants do not appear to suffer from essential fatty acid (EFA) deficiency. The amount of unchanged EFA passing through the rumen (up to 4% of dietary intake) is sufficient for the needs of the animal (Section 4.2.2.1). Hydrogenation can, however, be reduced by giving ruminants 'protected fats7, thereby enriching their tissues with polyunsaturated fatty acids (Section 4.1.2).

An interesting example of hydrogenation occurs in Bacillus cereus, which can reduce oleic to stearic acid. This reductase is induced by an increase in growth temperature and seems to be involved in the overall control of membrane fluidity (Section 6.5.9).

2.2.7 The biosynthesis of cyclic acids provided one of the first examples of a complex lipid substrate for fatty acid modifications

The only ring structures we shall discuss are the cyclopropanes and cyclopropenes.

The methylene group in cyclopropane acids originates from the methyl group of methionine in S-adenosyl methionine ('active methionine7). This is the same methyl donor involved in the formation of

10-methylene stearic acid and 10-methyl stearic acid from oleic acid (Section 2.2.2.5). The acceptor of the methyl group is likewise an unsaturated fatty acid. Thus, cis-vaccenic acid gives rise to lactoba-cillic acid, while oleic acid yields dihydrosterculic acid, the saturated derivative of sterculic acid (see Table 2.4). These reactions occur in a number of bacteria and in certain families of higher plants, e.g. Malvaceae and Sterculaceae.

When Law and his colleagues purified cyclopropane synthase from Clostridium butyricum, they found that the enzyme would catalyse the formation of cyclopropane fatty acids from 14C-labelled methionine only if phospholipids were added in the form of micellar solutions. They discovered that the real acceptor for the methylene group was not the unesterified monounsaturated fatty acid or its CoA or ACP thiolester, but phosphatidylethanola-mine - the major lipid of the organism (see Fig. 2.23).

The biosynthesis of cyclopropane and the related cyclopropene acids in higher plants has been studied by experiments with radioactive precursors. In this method, a supposed precursor for the compounds being studied is supplied to the plant and its incorporation into more complex molecules and/or conversion to products is studied at successive time intervals. The sequence in which the radiolabel appears in different compounds can be used to deduce the pathways by which they are made. Thus it has been shown that oleic acid gives rise to the cyclopropane derivative of stearic acid

Deduce Oleic Acid
Fig. 2.23 Formation of a cyclopropane fatty acid in Clostridium butyricum.

(dihydrosterculic acid). The latter can either be shortened by a-oxidation or desaturated to give sterculic acid (Fig. 2.24). Desaturation of the aoxidation product (dihydromalvalic acid) similarly yields malvalic acid (8,9-methylene-8-17:l).

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