Low- and high-affinity transport systems for citric acid in the yeast Candida utilis

Abstract

Citric acid-grown cells of the yeast Candida utilis induced two transport systems for citric acid, presumably a proton symport and a facilitated diffusion system for the charged and the undissociated forms of the acid, respectively. Both systems could be observed simultaneously when the transport was measured at 25°C with labelled citric acid at pH 3.5 with the following kinetic parameters: for the low-affinity system, Vmax, 1.14 nmol of undissociated citric acid s-1 mg (dry weight) of cells-', and Km, 0.59 mM undissociated acid; for the high-affinity system, Vmax, 0.38 nmol of citrate s-l mg (dry weight) of cells-', and Km, 0.056 mM citrate. At high pH values (above 5.0), the low-affinity system was absent or not measurable. The two transport systems exhibited different substrate specificities. Isocitric acid was a competitive inhibitor of citric acid for the high-affinity system, suggesting that these tricarboxylic acids used the same transport system, while aconitic, tricarballylic, trimesic, and hemimellitic acids were not competitive inhibitors. With respect to the low-affinity system, isocitric acid, L-lactic acid, and L-malic acid were competitive inhibitors, suggesting that all of these mono-, di-, and tricarboxylic acids used the same low-affinity transport system. The two transport systems were repressed by glucose, and as a consequence diauxic growth was observed. Both systems were inducible, and not only citric acid but also lactic acid and malic acid may induce those transport systems. The induction of both systems was not dependent on the relative concentration of the anionic form(s) and of undissociated citric acid in the culture medium. Undissociated citric acid entered the cells slowly by simple diffusion in glucose-grown cells. The permeability of the cells to undissociated acid by simple diffusion increased with pH, with the diffusion constant increasing 100-fold between pHs 3.0 and 5.0.

Transport of carboxylic acids in yeasts

“…Few tricarboxylate transporters have been characterized thus far in yeast. A low-and a high-affinity transport system subjected to glucose repression was identified in C. utilis (Cássio & Leão, 1991). The high-affinity transporter is specific for citrate and isocitrate and behaves as a proton symporter.…”

Characterization of an inducible citrate uptake system inPenicillium simplicissimum

“…If citrate efflux proceeded via an inverse citrate uptake system, abolishing the proton motive force should lead to an increased citrate efflux. This was observed, for instance, with Candida utilis [14]. However, uncouplers and sodium azide only decreased citrate uptake but did not increase citrate efflux.…”

Aspergillus niger Secretes Citrate to Increase Iron Bioavailability

“…Utilization of Fe(III) citrate as exogenous iron siderophore complex has been shown to take place in various other microbes that do not naturally secrete citrate (Frost and Rosenberg, 1973 ), and of the yeasts we tested, only C. jadinii was unable to grow when Fe(III) citrate was added to the medium, although it grew well with Fe(II)SO 4 , and even when no iron was added to the medium. Active citrate uptake in the asexual state of C. jadinii ( Candida utilis ) has been shown to be subject to glucose repression (Cassio and Leao, 1991 ), and it could be that the utilization of glucose in our experiments prevented C. jadinii from being able to utilize the Fe(III) citrate complex, whereas it is able to grow on other Fe(III) salts, such as Fe(III)Cl 3 (Thomas and Dawson, 1978 ). This would imply that C. jadinii is not able to deal with Fe(III) citrate via RIA, which is in contrast to S. cerevisiae (Haas, 2014 ).…”