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Magnesium, manganese and mutual depletion systems in halophilic bacteria

1986; Oxford University Press; Volume: 39; Issue: 1-2 Linguagem: Inglês

10.1111/j.1574-6968.1986.tb01854.x

1574-6968

Eveline Médicis,

Photosynthetic Processes and Mechanisms

The multi-ionic equilibria between enzymes, substrates and monovalent and divalent cations are related in such a way that a change in concentration of one element modifies the repartition of all the concentrations of the other elements, leading to a mutual depletion system. The pyruvate kinase reaction is a good application of the mutual depletion model: this cytoplasmic enzyme utilizes magnesium (Mg) and potassium (K) as cofactors and reacts with free phosphoenolpyruvate and MgADP, substrates involved in the binding of protons, K+ and Mg2+. Pyruvate kinase from Vibrio costicola, a moderately halophilic eubacterium, obeys the mutual depletion system and is competitively inhibited by physiological concentrations of potassium ions. This effect is relieved by manganese which forms more stable complexes than magnesium. Pyruvate kinase from Halobacterium cutirubrum cannot be described unambiguously by the mutual depletion model. Cytoplasmic concentrations of potassium ions are elevated in halophilic bacteria and may thus inhibit the formation of the divalent cation complexes necessary in the enzymatic machinery of halophilic bacteria. Accordingly, the contents of the most abundant divalent cation, Mg2+, and of the trace element manganese, Mn2+, are higher in the halophilic bacteria, V. costicola, Halobacterium volcanii, and H. cutirubrum, and their increase is proportional to the ionic strength of the extracellular media. The Mn2+ increase is more marked than the Mg2+ increase, although the Mn2+ content is about two orders of magnitude lower than the Mg2+ content.