Active transport of Ca²⁺ in bacteria: Bioenergetics and function

Summary

The bioenergetics of Ca²⁺ transport in bacteria are discussed with special emphasis on the interrelationship between transport and other cellular functions such as substrate oxidation by the respiratory chain and oxidative phosphorylation. The unusual polarity of Ca²⁺ movement provides an exceptional tool to compare active transport and other ATP requiring or generating processes since this ion is actively taken up by everted vesicles in which the coupling-factor ATPase is exposed to the external medium. As inferred from studies with everted vesicles, the active extrusion of Ca²⁺ by whole cells can be accomplished by substrate driven respiration, hydrolysis of ATP or as in the case ofStreptococcus faecalis by a nonhydrolytic unknown process which involves ATP directly. Substrate oxidation and the hydrolysis of ATP result in the generation of a pH gradient which can energize the Ca²⁺ uptake directly (Ca²⁺/H⁺ antiport) or via a secondary Na⁺ gradient (Ca²⁺/Na⁺ antiport). In contrast to exponentially growing cells sporulating Bacilli accumulate Ca²⁺ during the synthesis of dipicolinic acid. Studies involving Ca²⁺ transport provided evidence in support of the hypothesis that the Mg²⁺ ATPase fromEscherichia coli not only provides the driving force for various cellular functions but exerts a regulatory role by controlling the permeability of the membrane to protons. The different specificity requirements of various naphthoquinone analogs in the restoration of transport or oxidative phosphorylation, after the natural menaquinone has been destroyed by irradiation, has indicated that a protonmotive force is sufficient to drive active transport. However, in addition to the driving force (protonmotive force) necessary to establish oxidative phosphorylation, a specific spatial orientation of the respiratory components, such as the naphthoquinones, is essential for the utilization of the proton gradient or membrane potential or both. Finally evidence suggesting that intracellular Ca²⁺ levels might play a fundamental role in bacterial homeostasis is discussed, in particular the role of Ca²⁺ in the process of chemiotaxis and in conferring bacteria heat stability. A vitamin K-dependent carboxylation reaction has been found inEscherichia coli which is similar to that reported in mammalian systems which results in γ carboxylation of glutamate residues. Although all of the proteins containing γ-carboxyglutamate described so far are involved in Ca²⁺ metabolism, the role of these proteins inEscherichia coli is unknown and remains to be elucidated.