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General and Practical Aspects of Membrane Protein Crystallization
Published in Hartmut Michel, Crystallization of Membrane Proteins, 1991
Since the first promising reports on the crystallization of membrane proteins44,45 many reports have appeared describing the crystallization of membrane proteins including the well-diffracting reaction center crystals from Rps. viridis46 and Rb. sphaeroides.47,48 However, most crystals are too small or too disordered for high-resolution X-ray work. In our laboratory alone about 50 different crystal forms have been obtained from various bacterial light-harvesting complexes and about 10 other membrane proteins. So far only the crystals of bacteriorhodopsin,42 porin, and maltoporin from Escherichia coli19,49 mammalian prostaglandin H synthase,50 porin from Rhodobacter capsulatus,51 photosystem I from a thermophilic cyanobacterium52 and the bacterial B800/850 light-harvesting complexes from Rps. acidophila53 and Rhodospirillum molischianum (Michel, to be published, see also Figure 3) might be good enough to yield high resolution structures in addition to the two known reaction center structures.1,2,3 The way from a first small crystal to a large well-diffracting crystal is usually long, painful, and frustrating. A long term commitment from both the scientist and the funding agency is needed.
Antibiotic uptake through porins located in the outer membrane of Gram-negative bacteria
Published in Expert Opinion on Drug Delivery, 2021
In a similar manner, substrate permeation is facilitated by an affinity to specific channels [71–73]. For example, the addition of malto-oligosaccharides to LamB (Maltoporin) from E. coli drives the sugar to enter the binding site and block the channel entirely for ions. The occupancy of the binding site with sugar was obtained by measuring the reduction in channel conductance [71]. The permeabilities obtained for malto-oligosaccharides using ion current noise agreed well with liposome swelling results, except in the case of sucrose [72]. Inspection of the structure revealed that sucrose has a binding site but is not able to permeate [73]. It is also possible to extend the noise analysis technique to antibiotic permeation [74]. However, the analysis requires a sufficiently strong binding of the drug to the channel, and the antibiotic in the binding site must sufficiently block the ion current. A further point to note is the inherent instability of antibiotics [61]. For example, in the case of ampicillin, a short exposure to basic pH to enhance the solubility can degrade large amounts of the compound giving raise to additional channel blockages [61].