Different activation energies in glucose uptake in Saccharomyces cerevisiae DFY1 suggest two transport systems.
The analysis of initial glucose uptake in Saccharomyces cerevisiae at 25 degrees, 20 degrees, 15 degrees and 10 degrees C by computer-assisted nonlinear regression analysis predicts two transport systems. The first demonstrates Michaelis-Menten kinetics and the second shows first order behaviour. The activation energies of these two systems were calculated by the Arrhenius equation at four different growth phases, namely early exponential (EE), middle exponential (ME2), late exponential (LE) and early stationary (ES) with 2% glucose in the batch medium. The activation energies calculated from the V(m) values in EE, ME, LE and ES growth phases were 15.8 +/- 1.7, 13.5 +/- 1.0, 15.1 +/- 0.8 and 13.5 +/- 0.7 kcal/mol. These values are in agreement with activation energies calculated for the first mechanism, facilitated diffusion, which is the mechanism deduced from countertransport experiments. The activation energies derived for the second transport system from the first order rate constants in cells grown to EE, ME2, LE and ES were 8.0 +/- 2.1, 8.1 +/- 1.3, 9.6 +/- 3.0 and 7.5 +/- 2.6 kcal/mol. These values are still significantly higher than for free diffusion of glucose in water and lower as predicted for passage of glucose through the lipid phase. Therefore, we assume in addition to carrier-mediated facilitated diffusion the entrance of glucose into the cell through a pore.
Reinhardt
C
C
Department of Pharmacology and Toxicology, Philipps-University Marburg, Germany.
Völker
B
B
Martin
H J
HJ
Kneiseler
J
J
Fuhrmann
G F
GF
D001692:Biological Transport; D005947:Glucose / Q000378:metabolism*; D007700:Kinetics; D008956:Models, Chemical; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D013696:Temperature; D013816:Thermodynamics
D009004:Monosaccharide Transport Proteins; D005947:Glucose