A New Mechanism for Anaerobic Unsaturated Fatty Acid Formation inStreptococcus pneumoniae

Abstract

The anaerobic pathway for unsaturated fatty acid synthesis was established in the 1960s in Escherichia coli. The double bond is introduced into the growing acyl chain by FabA, an enzyme capable of both the dehydration of ␤-hydroxydecanoyl-acyl carrier protein (ACP) to trans-2-decenoyl-ACP, and the isomerization of trans-2 to cis-3-decenoyl-ACP. However, there are a number of anaerobic bacteria whose genomes do not contain a fabA homolog, although these organisms nonetheless produce unsaturated fatty acids. We cloned and biochemically characterized a new enzyme in type II fatty acid synthesis from Streptococcus pneumoniae that carries out the isomerization of trans-2-decenoyl-ACP to cis-3-decenoyl-ACP, but is not capable of catalyzing the dehydration of ␤-hydroxy intermediates. This tetrameric enzyme, designated FabM, has no similarity to FabA, but rather is a member of the hydratase/isomerase superfamily. Thus, the branch point in the biosynthesis of unsaturated fatty acids in S. pneumoniae occurs following the formation of trans-2-decenoyl-ACP, in contrast to E. coli where the branch point takes place after the formation of ␤-hydroxydecanoyl-ACP.

The complete genome sequence ofLactobacillus bulgaricusreveals extensive and ongoing reductive evolution

“…It is not clear whether L. bulgaricus would be able to produce unsaturated fatty acids, however, because a FabA or FabM homologue is missing (40). Failure to produce acetyl-CoA or the inability to produce unsaturated fatty acids may explain the need for Tween 80 in laboratory media for L. bulgaricus.…”

Streptococcus mutans: a new Gram-positive paradigm?

“…In fact, there is a near reversal of the proportions of saturated and unsaturated membrane fatty acids, with unsaturated fatty acids occupying nearly 60 % of the membrane composition during acidic growth (Fozo & Quivey, 2004;Quivey et al, 2000). Around this time, the fabM gene, encoding an enzyme with trans-2-cis-3-decenoyl isomerase activity, was identified and characterized in Streptococcus pneumoniae (Marrakchi et al, 2002) and shown to be responsible for the formation of unsaturated fatty acids. Parallel studies, conducted with S. mutans, showed that following mutation of the fabM gene, the organism could no longer produce unsaturated fatty acids (Fozo & Quivey, 2004).…”

“…In addition, inactivation of fabM resulted in a substantial reduction in the ability of S. mutans to cause caries in rats (Fozo et al, 2007). This was a departure from the E. coli paradigm of fatty acid synthesis, in which unsaturated fatty acids arise by the combined action of two enzymes, FabA, a dehydratase that forms unsaturated fatty acids from C 10 : hydroxy-fatty acid intermediates and FabB, the condensing enzyme that is capable of elongating unsaturated fatty acids (Marrakchi et al, 2002).…”

Synthesis and Biological Activity of Novel

“…This can, at least in part, be attributed to the diastereomeric mixture used in the reaction and the formation of E-and Z-isomers. Since the products were inseparable by flash chromatography, the mixture was reacted further in the oxidation of the vinylic borane (18) with Me 3 NO to give aldehyde (19) in 35% yield over two steps (corrected for the dr of 10:3 of alkene (15)). Aldehyde (19) was the only diastereoisomer that was isolated and subsequently reacted in a Pinnick oxidation to give carboxylic acid (20) in 91% yield.…”

“…In contrast to FabZ, FabA is a bifunctional enzyme and also catalyzes the isomerization of trans-2-acetyl-ACP to cis-3-acetyl-ACP, essential for unsaturated fatty acid synthesis. FabA is only present in Gram-negative bacteria and it is believed that Gram-positive bacteria, which also produce unsaturated fatty acids, utilize other isomerases such as FabM and FabZ 1 [18,19]. FabZ is the primary dehydratase in the FAS II pathway and simple inhibitors have been reported that show good in vitro activity for Helicobacter pylori [20].…”