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Asp²² drives the protonation state of the Staphylococcus epidermidis glucose/H⁺ symporter.

ABSTRACT: The Staphylococcus epidermidis glucose/H⁺ symporter (GlcP_Se) is a membrane transporter highly specific for glucose and a homolog of the human glucose transporters (GLUT, SLC2 family). Most GLUTs and their bacterial counterparts differ in the transport mechanism, adopting uniport and sugar/H⁺ symport, respectively. Unlike other bacterial GLUT homologs (for example, XylE), GlcP_Se has a loose H⁺/sugar coupling. Asp²² is part of the proton-binding site of GlcP_Se and crucial for the glucose/H⁺ co-transport mechanism. To determine how pH variations affect the proton site and the transporter, we performed surface-enhanced IR absorption spectroscopy on the immobilized GlcP_Se We found that Asp²² has a pK_a of 8.5 ± 0.1, a value consistent with that determined previously for glucose transport, confirming the central role of this residue for the transport mechanism of GlcP_Se A neutral replacement of the negatively charged Asp²² led to positive charge displacements over the entire pH range, suggesting that the polarity change of the WT reflects the protonation state of Asp²² We expected that the substitution of the residue Ile¹⁰⁵ for a serine, located within hydrogen-bonding distance to Asp²², would change the microenvironment, but the pK_a of Asp²² corresponded to that of the WT. A167E mutation, selected in analogy to the XylE, introduced an additional protonatable site and perturbed the protonation state of Asp²², with the latter now exhibiting a pK_a of 6.4. These studies confirm that Asp²² is the proton-binding residue in GlcP_Se and show that charged residues in its vicinity affect the pK_a of glucose/H⁺ symport.

SUBMITTER: Seica AFS

PROVIDER: S-EPMC7650245 | biostudies-literature |

REPOSITORIES: biostudies-literature

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