Substrate and endproduct regulation of cellobiose uptake by Candida wickerhamii |
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Authors: | I Spencer-Martins N van Uden |
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Institution: | (1) Laboratory of Microbiology, Gulbenkian Institute of Science, 2781 Oeiras Codex, Portugal |
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Abstract: | Summary Cellobiose-grown cells of Candida wickerhamii transported cellobiose as glucose by a glucose-proton symport after previous hydrolysis of the disaccharide by an exocellular -glucosidase. Both the symport and the -glucosidase were subject to glucose-induced repression and inactivation while glucose also acted as a competitive inhibitor of the enzyme (K
i
0.3 mM). Under conditions of glucose repression glucose was transported by facilitated diffusion. Cellobiose acted as a competitive inhibitor of the latter (K
i
75 mM) and is possibly a low-affinity substrate, while it inhibited non-competitively the glucoseproton symport (K
i
80 mM). The affinity of cellobiose for the cell-bound -glucosidase was much higher (K
m
4.2 mM) than for the purified enzyme as reported by others (K
m
67–225 mM). Ethanol reversibly inhibited the two glucose transport systems with exponential non-competitive kinetics. The minimum inhibitory concentrations were about 3% and 4% (w/v) for facilitated diffusion and proton symport while the respective exponential inhibition constants were 0.58 l mol-1 and 1.65 l mol-1. Ethanol affected the -glucosidase in a complex way, a major effect was deviation from Michaelis-Menten kinetics for ethanol concentrations higher than 4% (w/v), the Hill coefficient increasing up to 1.8 at 6% (w/v) ethanol. |
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