Microbial inorganic pyrophosphatases

Abstract

Nearly all of the PPases studied so far are synthesized constitutively. PPases of Staphylococcus aureus and M. lysodeikticus are inductive, synthesized only in a medium containing PP(i). Recent discoveries have suggested that PPase might have an important role not only in the regulation of macromolecular synthesis and growth, but also in evolutionary events by affecting the accuracy by which DNA molecules are copied during chromosome duplication. The production of PPase is stimulated 1.5- to 3-fold in E. coli as a result of partial inhibition of DNA synthesis. Possibly inhibition of DNA synthesis affects PPase synthesis indirectly, by blocking cell division, for example. PPase has been shown to act as a stimulator for protein synthesis by removing PP(i), the potent inhibitor of aminoacyl-tRNA synthetases. Furthermore, a novel mechanism has been presented according to which PP(i), and thus also PPase, might regulate ρ-termination activity in the transcription of E. coli. Distribution of PPase to soluble and membrane fractions has a great importance in directing enzyme functions to specific reaction pathways. Intrinsic to the enzymatic hydrolysis of PPase is the fact that the substrate (MgPP(i)2-) is complex of effectors. Free pyrophosphate is a strong competitive inhibitor and Mg2+ is an activator. In a solution containing Mg2+ and PP(i) several different moleclar species occur, and their concentrations can be calculated if pH, total concentrations of Mg2+ and PP(i), and the stability constants for each molecular species are known. The complexity of the reaction mixture makes the critical examination of kinetic results very difficult, because it is not possible to change the concentration of only one of the reaction components at a time. Hence, it has not been convincingly shown with any PPases that the sigmoidicity of the substrate saturation curve is due to homotropic interactions, and heterotropic effects have been established with certainty only for PPase from S. cerevisiae. Adenylate nucleotides regulate PPase activity by competing with PP(i) for Mg2+, so the activity of PPases might be adjusted to anabolic and catabolic reactions via the relation between the adenylate charge and Mg2+. Several bacterial PPases are stabilized by reductants in vitro. In the case of Streptococcus faecalis it has been shown that this stabilization reflects regulation at the activity level, with the ratio of reduced glutathione to oxidized glutathione as an effector in vivo.