Structural Pathway for Water Permeation through Sodium-Glucose Cotransporters

Abstract

The accuracy of the Microenvironment Modulated_Screened Coulomb Potential (MM_SCP) 1 to predict pKa values for titratable residues in biological macromolecules is analyzed. This self-consistent approach calculates electrostatic energetics using a modified form of the SCP-continuum solvent model 2 , which is modulated by the hydrophobicity of the local environment around the titratable group. The analysis has been applied to 345 ionizable groups (Asp, Glu, His, Cys, Tyr, Lys, N-and C-termini) in 59 proteins belonging to multiple structural classes. Among these residues, 82% were predicted with errors < 1 unit), although half of residues with large shifts were correctly predicted (error <1 unit). Further analysis showed that the MM_SCP accounts for the pKa shifts of ionizable groups in hydrophobic microenvironment 3 and treats interaction energies very well. Most errors originate from steric clashes or improper handling of certain interaction pairs (e.g., ionic H-bonds). A feature in the MM_SCP was used to identify steric clashes between the titratable residue and other parts of the sequence; after local minimization the calculated pKa improved by half a unit in more than half the cases. Other methods to improve the predictions will be discussed, such as optimizing the H-bond network, or using refined reference pKa values 4 which improves the root mean square deviations by at least 0.1 unit for 25 proteins.

Structural Determinants of Water Permeation through the Sodium-Galactose Transporter vSGLT

“…This pathway allows water to permeate the inward-facing state. This observation was later confirmed by other groups (15,25,26).…”

“…A subset of these residues (Y87, F424, and Q428) were recently identified as lining the constriction zone based on direct observation of simulations (25), and Y87 and F424 were proposed to form part of the extracellular gate along with M73, based on visualization of the static x-ray structure (21). Using grand canonical Monte Carlo simulations of the transporter, Sasseville et al also identified Y87, Q425, and Q428, as well as A259, F479, and M483, as components of a constriction zone that acted as a barrier to water permeation through vSGLT (15). It is not surprising that some of these amino acids have bulky hydrophobic side chains capable of occluding and drying pathways in particular rotameric states, whereas others are charged or polar and form interactions between adjacent TM helices to hold the outer gate shut.…”

“…In addition to its role as a passive water channel, it has been observed that SGLT1 undergoing normal transporter turnover energized by a Na þ or glucose gradient can move water from a region of low osmolyte concentration to a region of high osmolyte concentration. Two general classes of model have been developed to explain this observation: molecular-based models involving the cotransport of water (4,(11)(12)(13)(14), or an unstirred-boundary-layer model, which posits the existence of local osmotic gradients due to the build-up of sugar near the membrane (5,6,9,15).…”

“…The water-filled cavities contain a substantial number of water molecules, and the inward movement of ion and substrate moves these water molecules with them via a number of potential mechanisms nicely outlined by Zeuthen and MacAulay (17). Contrary to the cotransport models, the unstirredboundary-layer hypothesis suggests that SGLTs are not able to actively pump water molecules, but rather that water movement occurs passively due to the establishment of a local osmolyte concentration gradient (5,6,9,15). In this model, SGLT activity increases the intracellular concentration of Na þ and sugar at the membrane due to slow intracellular diffusion while at the same time depleting the extracellular concentration.…”

Whey protein hydrolysates enhance water absorption in the perfused small intestine of anesthetized rats

“…The ratio of sodium to glucose has been examined with the goal of optimizing the speed of water absorption. 1) The mechanism underlying the promotion of water absorption by SGLT1 has not been adequately clarified, and the hypotheses regarding this mechanism are roughly divided into two types: (1) those involving passive water transport driven by the local osmotic gradient formed by the transport of the substrates, 2,3) and (2) those involving active water transport coupled with the transport of the substrates at the molecular level by SGLT1, which acts like a water pump. 4,5) Substrates that are transported by other transporters, such as amino acid transporters and peptide transporters, would also promote water transport in both mechanisms.…”