ИСТИНА

My PhD research project focused on membrane-bound inorganic pyrophosphatases (mPPases). mPPases are integral membrane proteins that hydrolyze pyrophosphate (PPi) and transport H+ and/or Na+ ions across membranes thereby forming ion gradients. mPPases are found in bacteria, archaea, protists and plants. mPPases are potential drug targets against parasitic diseases like malaria. Furthermore, mPPases have been shown to improve stress resistance in plants. mPPases can be divided into different subfamilies based on their K+ requirements and ion pumping specificities. This thesis consists of four articles in which I studied the evolution and functional properties of different mPPase subfamilies. In the first article we discovered that previously identified Na+-transporting PPases (Na+-PPases) are in fact able to transport H+ ions at low (< 5 mM) Na+ concentrations. The emergence of the H+ transport activity was surprisingly not accompanied with a decrease in the Na+ transport efficiency, suggesting that the two ions do not directly compete for the same ion translocation mechanism. Further enzyme kinetic and mutational analyses led to the identification of two distinct Na+ binding sites controlling the PPi hydrolysis and ion transport specificity in Na+- PPases. In the second article we focused on a group of mPPases that is phylogenetically distant from other subfamilies. To investigate whether these enzymes are functional mPPases, divergent mPPases from Chlorobium limicola and Cellulomonas fimi were cloned and characterized. Despite the sequence divergence, these enzymes were identified as functional mPPases that transport H+ ions and are regulated by Na+. We concluded that the group of divergent mPPases forms a new subfamily—the Na+- regulated H+-PPases. In the third article we investigated the evolutionary path from Na+-PPases to mPPases that can transport both Na+ and H+ ions (Na+,H+-PPases). Ten new enzymes were characterized and classified into subfamilies based on their ion pumping abilities. The first group of Na+,H+-PPases was named “Na+-regulated Na+,H+- PPases” as their H+ transport was inhibited by Na+. The second group of Na+,H+- PPases was named “true Na+,H+-PPases” because the Na+ concentration did not affect their ability to transport H+. Furthermore, we found that the two differentially regulated groups of double-pumping enzymes have evolved separately. In the fourth article we showed that the K+/Lys cationic center, which determines the K+ dependence of mPPases, is conserved among all mPPase subfamilies. Our mutational analysis revealed that the K+/Lys center has an important role in PPi hydrolysis and enhances Na+ ion binding. Furthermore, our results suggested that substrate inhibition is a result of the allosteric inter-subunit regulation of mPPases and that the K+/Lys center is part of the regulation mechanism.