A model of the monosaccharide uphill transporting cell membrane system in yeast

Some physiological observations on the uptake of D-glucose and 2-deoxy-D-glucose by starving and exponentially-growing yeasts

Some methods for measuring the uptake of sugars by yeasts were investigated critically. A study was made of the effects of starvation of Pichia pinus, Candida utilis, Saccharomyces cerevisiae and Rhodosporidium toruloides on their uptake of D-glucose and 2-deoxy-D-glucose. Marked changes in the rates of uptake of these sugars occured during 10 h of starvation, including (a) an immediate increase of up to 75% above that for growing cells and (b) a continuous decline to as little as 4%. Each yeast behaved differently. The rates did not remain constant during the periods of starvation often used for studies on the transport of sugars into yeasts. For Pichia pinus, there were striking differences, associated with starvation, between the transport of 2-deoxy-D-glucose and D-glucose, despite evidence that the two sugars enter this yeast by means of the same carrier. Some physiological explanations for these findings are discussed.

[Transport and utilization of alditols in the yeast Rhodotorula gracilis glutinis (author's transl)]

The obligate aerobic yeast Rhodoturula gracilis was found to take up the alditols D-glucitol, D-mannitol, ribitol, xylitol, D-arabinitol, L-arabinitol and erythritol by means of a constitutive mobile membrane carrier. This uptake involved active transport, that is, it was dependent on the supply of metabolic energy, leading to the accumulation of alditols inside the cells. The accumulation ratio (intracellular concentration to extracellular concentration, Si/SO) was much lower for alditols than for monosaccharides. As for monosaccharides, this ratio decreased with increasing extracellular concentration, even to values below 1. The kinetic data showed that the carrier system for alditols was identical to that for monosaccharides, though it had a much lower affinity and maximum velocity for alditols. Hence the uptake of alditols was blocked in the presence of monosaccharides. Only ribitol and L-arabinitol were catabolized following enzyme induction. The other alditols were not broken down.

Temperature dependence of the energy-linked monosaccharide transport across the cell membrane of Rhodotorula gracilis

The temperature dependence of the active monosaccharide transport across the cell membrane of the yeast Rhodotorula gracilis has been studied between 0 and 55 degrees C with D-xylose as the transported substrate: (i) Between 0 and 10 degrees C there is virtually no transport. (ii) The initial velocity of transport increases exponentially from 15 to 30 degrees C (deltaE equal to 32 plus or minus 2 kcal/mol). (iii) At 30 degrees C a sharp "break" occurs in the Arrhenius plot and with increasing temperature the transport becomes inactivated, with a positive slope of the corresponding straight line ("deltaE equal to minus 15 kcal/mol"). (iv) In the temperature range of 50-55 degrees C, both the transport and the metabolic activity cease. In order to account for the abrupt changes of the membrane permeability, we attempted to ascribe them to phase transitions in the membrane structure: the first one, between 10 and 15 degrees C, to the crystalline: liquid-crystalline phase change; the second one, around 30 degrees C, to a change from highly ordered (low entropy) to less ordered (high entropy) membrane structure. Whereas the former phase transition is reversible, the latter appears to be irreversible. Arrhenius plots of the cell respiration exhibit a "break" at 30 degrees C, as well. However, at higher temperatures there is no thermal inactivation of the respiratory activity. The importance of a proper organization of the cell membrane constituents for the efficient transport function is discussed.