Catabolite interconversion of glucose transport systems in the yeast Candida wickerhamii

Abstract

Candida wickerhamii IGC 3244 growing in glucose medium transported glucose by facilitated diffusion (at 25°C and pH 5, the K s value was 1.7 mM and the V max value was 1.6 mmol/h per g dry wt.), while cells grown under derepressed conditions produced a glucose proton symport (at 25°C and pH 5, the K s value was 0.18 mM and the V max value was 1.8–1.9 mmol/h per g dry wt.). In each case, the Lineweaver-Burk plot of initial uptake rates was linear, indicating the presence of a single system. In buffer with 2% glucose, the symport suffered catabolite inactivation while the facilitated diffusion system emerged concomitantly in such a way that the combined V max remained nearly constant. During the conversion process, the Linewaver-Burk plots were biphasic, indicating the transitory co-existence of the two systems. A model is proposed that envisions the proton symport as composed of the facilitated diffusion system in association with (an)other transport protein(s), the latter being sensitive to carbon catabolite repression and inactivation.