Bacterial Biosynthesis of Menaquinones

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikimic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thiamin pyrophosphate-dependent condensation between 2-oxoglutarate and chorismic acid. The reaction catalyzed by o-succinylbenzoate synthetase is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxoglutarate, then addition of the succinyl moiety to isochorismate, followed by removal of the pyruvoyl side chain and the hydroxyl group of isochorismate.

o-Succinyl benzoate forms a coenzyme A thioester, which then undergoes dehydration and ring closure to naphthohydroquinone-3-carboxylate. This reacts with polyprenyl pyrophosphate, with the loss of carbon dioxide. The immediate product of the enzymic reaction, desmethylmenaquinol, is oxidized to desmethylmenaquinone, which is methylated to yield menaquinone (Meganathan, 2001)

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