Energy coupling of membrane transport and efficiency of sucrose dissimilation in yeast

Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS)

“… 213 Remarkably, two of the suppressor mutants regained H + -coupled sucrose transport through acidic residues now located in TM7 and TM11, which are positioned parallel to the naturally occuring position of the acidic residues that were mutated in TM1 and TM4. 213 In the melibose transporter MelB, both Na + and H + compete for binding to the same ion-binding site, but due to their respective affinities and physiological concentrations, Na + is thought to be the primary driving ion. 214 Interestingly, however, some sugar epimers can be coupled to either Na + or H + , whereas others can only use Na + .…”

“…For example, similar to DHA1 members, in the disaccharide sugar porter Mal11, it was possible to retain H + coupling by relocating the acidic residues to a different helix. 109 , 213 Furthermore, the structure of the DHA1 member YajR, 305 in particular, indicates that substrate occlusion might still be achieved by local gating helices from one of the two bundles in some cases.…”

“…It was only when all three residues were neutralized that H + and substrate cotransport uncoupled, with no impairment to the substrate binding site because the system is fully active in downhill (counterflow) transport, i.e., mutation of all three residues completely abolished H + -coupled transport, but still enabled facilitated diffusion of the substrate. It was shown recently that it was possible to evolve yeast strains to grow in a medium only containing sucrose as a carbon source complemented with the MalII triple mutant . Remarkably, two of the suppressor mutants regained H + -coupled sucrose transport through acidic residues now located in TM7 and TM11, which are positioned parallel to the naturally occuring position of the acidic residues that were mutated in TM1 and TM4 .…”

“…It was shown recently that it was possible to evolve yeast strains to grow in a medium only containing sucrose as a carbon source complemented with the MalII triple mutant . Remarkably, two of the suppressor mutants regained H + -coupled sucrose transport through acidic residues now located in TM7 and TM11, which are positioned parallel to the naturally occuring position of the acidic residues that were mutated in TM1 and TM4 . In the melibose transporter MelB, both Na + and H + compete for binding to the same ion-binding site, but due to their respective affinities and physiological concentrations, Na + is thought to be the primary driving ion .…”

“…Owing to the large variety of MFS-folds and substrate gating rearrangements, within the large family sugar porter and DHA subfamilies, we expect other MFS members will demonstrate conformational changes and transport behavior that are more similar to one another than these examples highlighted here. For example, similar to DHA1 members, in the disaccharide sugar porter Mal11, it was possible to retain H + coupling by relocating the acidic residues to a different helix. , Furthermore, the structure of the DHA1 member YajR, in particular, indicates that substrate occlusion might still be achieved by local gating helices from one of the two bundles in some cases.…”