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Menaquinone (vitamin K2) biosynthesis: evidence that the Escherichia coli menD gene encodes both 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid synthase and alpha-ketoglutarate decarboxylase activities

The formation of 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC), the first identified intermediate in the menaquinone biosynthetic pathway, requires two reactions. They are the decarboxylation of alpha-ketoglutarate by an alpha-ketoglutarate decarboxylase, which results in the formation of succinic semialdehyde-thiamine PPi (TPP) anion, and the addition of the succinic semialdehyde-TPP anion to isochorismate carried out by the enzyme SHCHC synthase. Evidence is provided to support the conclusion that both enzymatic activities are encoded by an extended menD gene which is capable of generating a bifunctional 69-kDa protein. Consistent with the requirement for TPP in the decarboxylation of alpha-ketoglutarate, the translated amino acid sequence contains the characteristic TPP-binding motif present in all well-characterized TPP-requiring enzymes.