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Many enzymes rely on small organic molecules called cofactors to assist in redox catalysis. Humans make some of these cofactors from scratch, but others--called vitamins--must be obtained through the diet. Authors demonstrate that pyrroloquinoline quinone (PQQ, shown) is required for the proper functioning of an enzyme that mice rely on to break down the amino acid lysine. PQQ's critical biochemical role, coupled with mammals' inability to synthesize it, qualifies PQQ as a vitamin, the authors suggest
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