Properties of the sugar carrier in baker's yeast

A total of 16 hexoses and pentoses were investigated with respect to transport intoSaccharomyces cerevisiae cells. All monosaccharides were transported across the cytoplasmic membrane but only those with an equatorial hydroxyl group in positions 1 and 4 of theC1 chair conformation and those with an equatorial hydroxyl group in position 2 and an equatorial −CH₂OH group in position 5 of the1C chair conformation reached an equilibrium distribution in the entire cell water volume. Other monosaccharides reached a distribution in only 20–66% of the intracellular water. The two groups of sugars are apparently transported by different carriers (either in parallel or in series), each of them showing countertransport and an apparent activation energy of 6,700–7,800 cal/mol. The carrier transporting the perfectly distributing sugars (Group 1) is affected by uranyl nitrate but not by 2,4-dinitrophenol, the other carrier (Group 2) is apparently not susceptible to uranyl ions but is influenced by 2,4-dinitrophenol. The space of distribution of the Group 1 sugars is reduced in hypertonic media in accordance with changes of intracellular water, that of the Group 2 sugars is altered only very slightly. The carriers differ in their kinetic parametres (mobility of the loaded carriers, maximum rate of transport). There is only a very indistinct competition for transport between representatives of the two groups. Preincubation with d-galactose induces the formation or unmasking of a transport system whereafter even the Group 2 sugars reach equilibrium in the entire cell water. Part I. Fol. microbiol. 10: 30, 1965.