Glycerol Utilization by Facilitated Diffusion Coupled to Phosphorylation in Bacteria
Escherichia coli and probably most other bacteria entrap glycerol by the tandem action of a cytoplasmic membrane protein which catalyzes facilitated diffusion and an ATP-dependent kinase subject to feedback inhibition. The facilitator protein behaves as though it provides an aqueous channel with an effective pore diameter of about 0.4 nm. The kinase is also not highly specific, since, in addition to glycerol, the enzyme can phosphorylate dihydroxyacetone and Lglyceraldehye. Under physiological conditions, however, the two proteins appear to function as a complex, and together they impose a more stringent substrate specificity.
Whereas wild-type E. coli can grow at a maximal rate on glycerol at concentrations well below 0.05 mM, mutants lacking the facilitator require at least 5 mM of the compound to achieve full growth rate. Utilization of glycerol as sole carbon and energy source by wild-type cells is rate-limited by the action of fructose-1,6-bisphosphate as a noncompetitive inhibitor. Utilization of glycerol in the presence of glucose is prevented at least in part by increased concentration of dephosphorylated factor IIIGlc of the phosphoenolpyruvate phosphotransferase system. This effect seems to be exerted either on the kinase alone or on both the kinase and the facilitator.
Glycerol is bactericidal to mutants synthesizing high levels of a kinase which is insensitive to feedback control by fructose-1, 6-bisphosphate. The cells are killed by the copious production of methylglyoxal from the elevated pool of dihydroxyacetone phosphate. In contrast, glycerol is bacteriostatic to mutants blocked in the dehydrogenation of sn-glycerol 3-phosphate.
It is suggested that the evolution of a concentrative mechanism for the uptake of glycerol in bacterial and other kinds of cells is prevented by the high intrinsic permeability of biological membranes to the compound, a property which would make active transport a Sisyphean process.
KeywordsCatabolite Repression Glycerol Kinase Glycerol Uptake Glycerol Utilization Concentrative Mechanism
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