SUMMARY: An approach has been made to the problem of how the synthesis of coenzymes is regulated. Two aspects of the problem have been studied, especially as they concern the synthesis of flavins by bacteria:

(1) How are coenzymes prevented from being synthesized as fast as amino acids or nucleic acid bases?

(2) How is coenzyme synthesis adjusted to the often changing physiological needs of bacteria?

Evidence is presented that flavins cannot inhibit the activity of enzymes in the flavin biosynthetic pathway of , but the amount of these enzymes can be made to vary by a factor of at least two. Repression might, therefore, account for the low rate of flavin synthesis. The possibility that repression rather than feedback inhibition also accounts for the low rate of synthesis of other coenzymes is discussed.

Flavin synthesis is not as precisely adjusted to the physiological needs of bacteria as are syntheses of major metabolites for the following reasons:

(1) Flavins are greatly overproduced by bacteria during exponential growth; the ratio of flavins excreted to flavins retained in the cells is between 0.8 and 8 for all strains and cultural conditions tested.

(2) Flavin synthesis is not tightly geared to growth; thus, flavin synthesis goes on uninterrupted for more than an hour when the growth rate of or is abruptly reduced from a rapid rate to zero; also growth goes on uninterrupted for over an hour when the flavin supply is abruptly cut off from rapidly growing lactic acid bacteria. Evidently the control mechanism in the flavin pathway is not very sensitive to physiological needs. This conclusion probably applies to other coenzymes as well.

Some incidental findings of interest from other points of view were:

(1) Although internal flavins can get out of , external flavins apparently cannot enter. This could account for the absence of flavinless mutants.

(2) After brief treatment with penicillin, becomes permeable to external flavins while remaining both impermeable to inulin and capable of synthesizing flavins.

(3) Less than 4 % of the intracellular flavins of are free, in a form that can be extracted with -butanol (5 %, v/v), toluene (0.05 %, v/v), cetyl trimethylammonium bromide (0.001 %, w/v) or distilled water. The remaining flavins are bound, in a form that can be extracted with trichloracetic acid (5 %, w/v). The intracellular concentration of free flavins in is estimated to be less than 4 x 10M.


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