Competitive utilization of glucose and glycerol by Escherichia coli

Forrest, Ian Simpson (1974) Competitive utilization of glucose and glycerol by Escherichia coli. PhD thesis, University of Glasgow.

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1) When glycerol is the sole source of carbon and energy in E.coli its utilization is regulated by the level of glycerokinase and the intracellular concentration of fructose-1,6-diphosphate, the negative modifier of glycerokinase activity. 2) E.coli 15224 exhibits diauxic growth on a mixture of glucose and glycerol in. simple defined media under aerobic conditions. 3) When cells have been trained to and are growing on glycerol, the addition of glucose represses, almost immediately, the synthesis of glycerokinase. Glycerol utilization falls by about 50% at the time of glucose challenge and continues to decrease for about 50 min, when it ceases altogether, and does not start again until glucose is exhausted. After glucose challenge, the glycerol carbon used continues to enter protein, nucleic acid and lipid with a slight increase in the proportion entering the last. The delay before glycerol utilization is completely shut off suggests that this process depends on some feature of the glucose phenotype which is not present in glycerol trained cells. This is supported by the observation that the addition of glucose to chloramphenicol treated cells depresses the rate of glycerol metabolism but never abolishes it. 4) In the above case total inhibition of glycerol utilization is an example of 'catabolite inhibition' (McGinnis & Paigen, 1969). In this case glycerol metabolism could be regulated by fructose-1,6-diphosphate concentration which, in turn, could reflect the rate of glucose metabolism. However fruetose-1,6-diphosphate inhibits glycerokinase of this strain to a maximum of 85%. Direct measurement shows that the intracellular concentration of fructose-1,6-diphosphate does not change significantly on glucose challenge. The fructose-1,6-diphosphate in cells growing on glycerol permits 31% of the glycerokinase activity to be utilized and, after glucose challenge, at least 26% of glycerokinase activity would be used if fructose-1,6-diphosphate was the only regulator. Thus total inhibition of glycerol metabolism cannot depend on glycerokinase inhibition by fructose-1,6-diphosphate. Glycerokinase is not inhibited by any other metabolite of glucose which has been tested. 5) The rate of free diffusion of glycerol into cells of E.coli is sufficient to account for a substantial proportion of the rate of glycerol utilization in the absence of glucose. Furthermore glucose does not affect the facilitated diffusion of glycerol. Thus glucose does not inhibit glycerol utilization by regulation of the rate of entry of glycerol into the cells. 6) The rate of glycerol utilization, the rate of glucose utilization and the ratios of glycerokinase and glucose PTS are all interrelated. The utilization of glucose and glycerol are mutually inhibitory, the degree of inhibition depending on the relative levels of glycerokinase and glucose PTS. However the rate of glycerol utilization depends on the rate of glucose utilization when the enzyme levels are constant. A model has been proposed in which glycerokinase and glucose PTS compete for a common element. This has been supported by observations, both in growing cells and washed cell suspensions, and it is speculated that the common element may involve the energy requirement of the first steps of glycerol and glucose utilization.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: W H Holms
Keywords: Biochemistry
Date of Award: 1974
Depositing User: Enlighten Team
Unique ID: glathesis:1974-74038
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Sep 2019 15:33
Last Modified: 23 Sep 2019 15:33

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