Pyrroloquinoline Quinone PQQ and 6Hydroxydopa topa Quinone

A cofactor known to be present in methylogenic bacteria and other microorganisms, PQQ was considered at first to be a cofactor for mammalian copper amine oxidases and other copper enzymes. Its essential nature led some workers to consider PQQ an undiscovered vitamin. This, however, turned out not to be the case and PQQ as a cofac-tor has now been relegated to the world of microorganisms.

What at first was thought to be PQQ in copper oxidases turned out to be a cofactor with quinone properties that was derived by modifying a tyrosine residue in the enzyme (Figure 13). The synthesis of 6-hydroxy (topa) quinone, a derivative of tyrosine, requires copper in an apparently autocatalytic reaction. Though rare and limited, this most unusual biochemical reaction opens a new chapter on cofactors by showing that some enzymes have a limited but specific capacity

H II

O OH

PQQ 6-Hydroxydopa (topa) quinone

Figure 13 Comparison of structures of pyrroloquinoline quinone (PQQ) with topa quinone. Cofactor on the right has been identified in mammalian systems.

to synthesize cofactors on their surface through modification of existing amino acid side chains.

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