Kinetics and pH-dependence of glycine-proton symport in Saccharomyces cerevisiae

Interactions between intracellular pH (pHi) and H+-coupled transmembrane transport of glycine have been studied by means of 31P-NMR, using both aerobic and 'energy starved' cells of the yeast Saccharomyces cerevisiae. The general features of glycine transport in the yeast strain used (NCYC 239) are similar to those already reported for Saccharomyces carlsbergensis and S. cerevisiae, there being two kinetically distinct glycine uptake systems, with pH-independent K1/2 values near 14 and 0.4mM, respectively, but pH-dependent maximal velocities. Glycine transport itself has no measurable effect on pHi in aerobic cells, and only a marginal effect in energy-starved cells, but changes of pHi, imposed by extracellular addition of butyric acid, strongly influence glycine transport. Indeed, the dependence of glycine influx (in energy-starved cells) upon cytoplasmic H+ concentration appears to be third order, showing Hill slopes of 2.7-3.0. A crucial kinetic role for cytoplasmic pH in glycine transport is further indicated by a proportionality between the decline of flux and the decline of pHi produced by various metabolic inhibitors and uncouplers. Extracellular pH (pHo), by contrast, has only a weak effect on glycine influx, showing a Hill slope of 0.5. The major observations can be accommodated by a simple cyclic carrier scheme, in which 2 or more protons are transported along with glycine, but only one extracellular proton binding site dissociates in the testing range, with a pK near 5.5. The model requires a finite membrane potential, which must be somewhat sensitive to both pHi and pHo, and accommodates the discrepancy between measured net proton flux (one per glycine) and the kinetically required proton flux (two or more per glycine) by shunting through other proton-conducting pathways in the yeast membrane.