Membrane-bound proteins sugar transporters

For both hexoses and pentoses, the mono-, di-, and oligosaccharides and their derivatives are taken up via specific membrane-bound transport proteins. Few detailed studies of sugar uptake by the clostridia have been reported, but observations made to date indicate that the transport mechanisms are similar to those found in a wide variety of other bacteria (Mitchell 1998). A large number of genes for different types of transporters have been found in the genome of C. acetobutylicum ATCC 824 (Nblling et al. 2001). 

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... 

FIGURE 11-43 Structure of the lactose transporter (lactose permease) of E. coli. (a) Ribbon representation viewed parallel to the plane of the membrane shows the 12 transmembrane helices arranged in two nearly symmetrical domains shown in different shades of blue. In the form of the protein for which the crystal structure was determined, the substrate sugar (red) is bound near the middle of the membrane where it is exposed to the cytoplasm (derived from PDB ID 1 PV7). (b) The structural changes postulated to take place during one transport... 

Owing to observations on certain competitive and reciprocal antiport relations between sugar and amino acid transport, Alvarado and Crane and Alvarado have recently postulated a new polyfunctional, mobile carrier system. involved in the uphill transport of sugars, neutral amino acids and basic amino acids in the small intestine that consists of a mosaic of fixed, specific membrane sites which acquire mobility as a result of deformations of the mobile membrane resulting in local, transient engagements of the two protein surfaces, thus allowing bound substrates to be alternately exposed to the extra- and intercellular fluids. ... 

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