Pyrroloquinoline quinone (PQQ) has received considerable attention due to its numerous important physiological functions. PqqA is a precursor peptide of PQQ with two conserved residues: glutamate and tyrosine. After linkage of the C␥ of glutamate and C⑀ of tyrosine by PqqE, these two residues are hypothesized to be cleaved from PqqA by PqqF. The linked glutamate and tyrosine residues are then used to synthesize PQQ. Here, we demonstrated that the pqqF gene is essential for PQQ biosynthesis as deletion of it eliminated the inhibition of prodigiosin production by glucose. We further determined the crystal structure of PqqF, which has a closed clamshell-like shape. The PqqF consists of two halves composed of an N-and a C-terminal lobe. The PqqF-N and PqqF-C lobes form a chamber with the volume of the cavity of ϳ9400 Å 3 . The PqqF structure conforms to the general structure of inverzincins. Compared with the most thoroughly characterized inverzincin insulin-degrading enzyme, the size of PqqF chamber is markedly smaller, which may define the specificity for its substrate PqqA. Furthermore, the 14-amino acid-residue-long tag formed by the N-terminal tag from expression vector precisely protrudes into the counterpart active site; this N-terminal tag occupies the active site and stabilizes the closed, inactive conformation. His-48, His-52, Glu-129 and His-14 from the N-terminal tag coordinate with the zinc ion. Glu-51 acts as a base catalyst. The observed histidine residue-mediated inhibition may be applicable for the design of a peptide for the inhibition of M16 metalloproteases.Pyrroloquinoline quinone (PQQ), 4 an aromatic orthoquinone, has been recognized as the third class of redox cofactors in addition to the well known cofactors, nicotinamide (NAD(P) ϩ ) and flavin (FAD, FMN) (1). PQQ was first identified from methanol dehydrogenase in methylotrophic bacteria (2), and several bacterial dehydrogenases, such as glucose dehydrogenases, quinate dehydrogenase, and alcohol dehydrogenase, were later found to be quinoproteins (3-5). Recently, sugar oxidoreductase in the basidiomycete Coprinopsis cinerea (6), 2-keto-D-glucose dehydrogenase from Pseudomonas aureofaciens (7) and pyranose dehydrogenase from C. cinerea (8) were all characterized as novel PQQ-dependent enzymes. Free PQQ has been identified in a wide variety of foods (9) and milk (10). PQQ is an essential nutrient for proper growth and development in mice (11). The strong-antioxidant capacity of PQQ protects living cells and biomolecules from oxidative stress in vivo and in vitro (12, 13). Furthermore, PQQ exerts a protective effect against ultraviolet irradiation-induced human dermal fibroblast senescence in vitro (14) and suppresses the serum low density cholesterol level to prevent various diseases (15). In addition, PQQ may improve skin conditions and slow the progression of osteoarthritis (16).The chemical structure of PQQ was determined in 1980 (17). Since then gene clusters involved in the synthesis of PQQ from different bacteria that range from 4 genes ...
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