Periplasmic-binding protein superfamily

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

Dwyer MA, HelUnga HW. Periplasmic binding proteins a versatile superfamily for protein engineering. Curr. Opin. Struct. Biol. 2004 14 495-504. 

Currently, five different molecular classes of mdr efflux pumps are known [5], While pumps of the the ATP-binding cassette (ABC) transporter superfamily are driven by ATP hydrolysis, the other four superfamilies called resistance-nodulation-division (RND), major facilitator superfamily (MFS), multidrug and toxic compound extrusion (MATE), and small multidrag resistance transporter (SMR) are driven by the proton-motive force across the cytoplasmic membrane. Usually a single pump protein is located within the cytoplasmic membrane. However, the RND-type pumps which are restricted to Gram-negative bacteria consist of two additional components, a periplasmic membrane fusion protein (MFP) which connects the efflux pump to an outer... 

Another superfamily is formed by bacterial di-heme CCP (with over 110 entries in PeroxiBase) that are periplasmic enzymes providing protection from oxidative stress. These homodimeric enzymes have a conserved tertiary structure containing two type-c hemes covalently attached to two predominantly a-helical domains via a characteristic binding motif. One heme acts as a low redox-potential center where H2O2 is reduced, and the other as a high redox-potential center that feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c [24]. In the crystal structure of the Geobacter sulfurreducens enzyme shown in Fig. 3.1g, the first heme appears as a bis-histidinyl-coordinated form (and... 

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