Another unusual type of citric acid cycle enzyme in Helicobacter pylori: the malate : quinone oxidoreductase

The only enzyme of the citric acid cycle for which no open reading frame (O RF) was found in the Helicobacter pylori genome is the NAD-dependent malate dehydrogenase. Here, it is shown that in this organism the oxidation of ma late to oxaloacetate is catalyzed by a malate:quinone oxidoreductase (MQO). This flavin adenine dinucleotide-dependent membrane-associated enzyme dona tes electrons to quinones of the electron transfer chain. Similar to succin ate dehydrogenase, it is part of both the electron transfer chain and the c itric acid cycle. MQO activity was demonstrated in isolated membranes of H. pylori, The enzyme is encoded by the ORF HP0086, which is shown by the fac t that expression of the HP0086 sequence from a plasmid induces high MQO ac tivity in mqo deletion mutants of Escherichia coli or Corynebacterium gluta micum. Furthermore, this plasmid was able to complement the phenotype of th e C. glutamicum mqo deletion mutant. Interestingly, the protein predicted t o be encoded by this ORF is only distantly related to known or postulated M QO sequences from other bacteria. The presence of an MQO shown here and the previously demonstrated presence of a 2-ketoglutarate:ferredoxin oxidoredu ctase and a succinyl-coenzyme A (CoA):acetoacetyl-CoA transferase indicate that H. pylori possesses a complete citric acid cycle, but one which deviat es from the standard textbook example in three steps.