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

Abstract

The temperature dependence of the active monosaccharide transport across the cell membrane of the yeastRhodotorula gracilis has been studied between 0 and 55°C withd-xylose as the transported substrate: (i) Between 0 and 10°C there is virtually no transport. (ii) The initial velocity of transport increases exponentially from 15 to 30°C (ΔE=32±2 kcal/mol). (iii) At 30°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 (“ΔE=−15 kcal/mol”). (iv) In the temperature range of 50–55°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°C, to the crystalline: liquid-crystalline phase change; the second one, around 30°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°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.