Use of helper enzymes for ADP removal in infrared spectroscopic experiments: application to Ca2+-ATPase

Hydrolysis of Extracellular ATP by Vascular Smooth Muscle Cells Transdifferentiated into Chondrocytes Generates Pi but Not PPi

(1) Background: Tissue non-specific alkaline phosphatase (TNAP) is suspected to induce atherosclerosis plaque calcification. TNAP, during physiological mineralization, hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP_i). Since atherosclerosis plaques are characterized by the presence of necrotic cells that probably release supraphysiological concentrations of ATP, we explored whether this extracellular adenosine triphosphate (ATP) is hydrolyzed into the mineralization inhibitor PP_i or the mineralization stimulator inorganic phosphate (P_i), and whether TNAP is involved. (2) Methods: Murine aortic smooth muscle cell line (MOVAS cells) were transdifferentiated into chondrocyte-like cells in calcifying medium, containing ascorbic acid and β-glycerophosphate. ATP hydrolysis rates were determined in extracellular medium extracted from MOVAS cultures during their transdifferentiation, using ³¹P-NMR and IR spectroscopy. (3) Results: ATP and PP_i hydrolysis by MOVAS cells increased during transdifferentiation. ATP hydrolysis was sequential, yielding adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine without any detectable PP_i. The addition of levamisole partially inhibited ATP hydrolysis, indicating that TNAP and other types of ectonucleoside triphoshatediphosphohydrolases contributed to ATP hydrolysis. (4) Conclusions: Our findings suggest that high ATP levels released by cells in proximity to vascular smooth muscle cells (VSMCs) in atherosclerosis plaques generate P_i and not PP_i, which may exacerbate plaque calcification.

Reaction mechanism of adenylyltransferase DrrA from Legionella pneumophila elucidated by time-resolved fourier transform infrared spectroscopy

Modulation of the function of small GTPases that regulate vesicular trafficking is a strategy employed by several human pathogens. Legionella pneumophila infects lung macrophages and injects a plethora of different proteins into its host cell. Among these is DrrA/SidM, which catalyzes stable adenylylation of Rab1b, a regulator of endoplasmatic reticulum to Golgi trafficking, and thereby alters the function and interactions of this small GTPase. We employed time-resolved FTIR-spectroscopy to monitor the DrrA-catalyzed AMP-transfer to Tyr77 of Rab1b. A transient complex between DrrA, adenylylated Rab1b, and the pyrophosphate byproduct was resolved, allowing us to analyze the interactions at the active site. Combination of isotopic labeling and site-directed mutagenesis allowed us to derive the catalytic mechanism of DrrA from the FTIR difference spectra. DrrA shares crucial residues in the ATP-binding pocket with similar AMP-transferring enzymes such as glutamine synthetase adenylyltransferase or kanamycin nucleotidyltransferase, but provides the complete active site on a single subunit. We determined that Asp112 of DrrA functions as the catalytic base for deprotonation of Tyr77 of Rab1b to enable nucleophilic attack on the ATP. The study provides detailed understanding of the Legionella pneumophila protein DrrA and of AMP-transfer reactions in general.

Infrared spectroscopy of proteins

This review discusses the application of infrared spectroscopy to the study of proteins. The focus is on the mid-infrared spectral region and the study of protein reactions by reaction-induced infrared difference spectroscopy.

Toward a general method to observe the phosphate groups of phosphoenzymes with infrared spectroscopy

A general method to study the phosphate group of phosphoenzymes with infrared difference spectroscopy by helper enzyme-induced isotope exchange was developed. This allows the selective monitoring of the phosphate P-O vibrations in large proteins, which provides detailed information on several band parameters. Here, isotopic exchange was achieved at the oxygen atoms of the catalytically important phosphate group that transiently binds to the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a). [gamma-(18)O(3)]ATP phosphorylated the ATPase, which produced phosphoenzyme that was initially isotopically labeled. The helper enzyme adenylate kinase regenerated the substrate ATP from ADP (added or generated upon ATP hydrolysis) with different isotopic composition than used initially. With time this produced the unlabeled phosphoenzyme. The method was tested on the ADP-insensitive phosphoenzyme state of the Ca(2+)-ATPase for which the vibrational frequencies of the phosphate group are known, and it was established that the helper enzyme is effective in mediating the isotope exchange process.