Conformational changes during the reaction cycle of plasma membrane Ca2+-ATPase in the autoinhibited and activated states.

Plasma membrane Ca2+-ATPase (PMCA) transports Ca2+ by a reaction cycle including phosphorylated intermediates. Calmodulin binding to the C-terminal tail disrupts autoinhibitory interactions, activating the pump. To assess the conformational changes during the reaction cycle, we studied the structure of different PMCA states using a fluorescent probe, hydrophobic photolabeling, controlled proteolysis and Ca2+-ATPase activity. Our results show that calmodulin binds to E2P-like states, and during dephosphorylation, the hydrophobicity in the nucleotide-binding pocket decreases and the Ca2+ binding site becomes inaccessible to the extracellular medium. Autoinhibitory interactions are disrupted in E1Ca and in the E2P ground state whereas they are stabilized in the E2·Pi product state. Finally, we propose a model that describes the conformational changes during the Ca2+ transport of PMCA.

E2P-like states of plasma membrane Ca2+­ATPase characterization of vanadate and fluoride-stabilized phosphoenzyme analogues.

The plasma membrane Ca2+­ATPase (PMCA) belongs to the family of P-type ATPases, which share the formation of an acid-stable phosphorylated intermediate as part of their reaction cycle. The crystal structure of PMCA is currently lacking. Its abundance is approximately 0.1% of the total protein in the membrane, hampering efforts to produce suitable crystals for X-ray structure analysis. In this work we characterized the effect of beryllium fluoride (BeFx), aluminium fluoride (AlFx) and magnesium fluoride (MgFx) on PMCA. These compounds are known inhibitors of P-type ATPases that stabilize E2P ground, E2·P phosphoryl transition and E2·Pi product states. Our results show that the phosphate analogues BeFx, AlFx and MgFx inhibit PMCA Ca2+­ATPase activity, phosphatase activity and phosphorylation with high apparent affinity. Ca2+­ATPase inhibition by AlFx and BeFx depended on Mg2+ concentration indicating that this ion stabilizes the complex between these inhibitors and the enzyme. Low pH increases AlFx and BeFx but not MgFx apparent affinity. Eosin fluorescent probe binds with high affinity to the nucleotide binding site of PMCA. The fluorescence of eosin decreases when fluoride complexes bind to PMCA indicating that the environment of the nucleotide binding site is less hydrophobic in E2P-like states. Finally, measuring the time course of E → E2P-like conformational change, we proposed a kinetic model for the binding of fluoride complexes and vanadate to PMCA. In summary, our results show that these fluoride complexes reveal different states of phosphorylated intermediates belonging to the mechanism of hydrolysis of ATP by the PMCA.

A metabolic control analysis approach to introduce the study of systems in biochemistry: the glycolytic pathway in the red blood cell.

Metabolic control analysis (MCA) is a promising approach in biochemistry aimed at understanding processes in a quantitative fashion. Here the contribution of enzymes and transporters to the control of a given pathway flux and metabolite concentrations is determined and expressed quantitatively by means of numerical coefficients. Metabolic flux can be influenced by a wide variety of modulators acting on one or more metabolic steps along the pathway. We describe a laboratory exercise to study metabolic regulation of human erythrocytes (RBCs). Within the framework of MCA, students use these cells to determine the sensitivity of the glycolytic flux to two inhibitors (iodoacetic acid: IA, and iodoacetamide: IAA) known to act on the enzyme glyceraldehyde-3-phosphate-dehydrogenase. Glycolytic flux was estimated by determining the concentration of extracellular lactate, the end product of RBC glycolysis. A low-cost colorimetric assay was implemented, that takes advantage of the straightforward quantification of the absorbance signal from the photographic image of the multi-well plate taken with a standard digital camera. Students estimate flux response coefficients for each inhibitor by fitting an empirical function to the experimental data, followed by analytical derivation of this function. IA and IAA exhibit qualitatively different patterns, which are thoroughly analyzed in terms of the physicochemical properties influencing their action on the target enzyme. IA causes highest glycolytic flux inhibition at lower concentration than IAA. This work illustrates the feasibility of using the MCA approach to study key variables of a simple metabolic system, in the context of an upper level biochemistry course. © 2018 International Union of Biochemistry and Molecular Biology, 46(5):502-515, 2018.

Aluminum inhibits the plasma membrane and sarcoplasmic reticulum Ca2+-ATPases by different mechanisms.

Aluminum (Al3+) is involved in the pathophysiology of neurodegenerative disorders. The mechanisms that have been proposed to explain the action of Al3+ toxicity are linked to changes in the cellular calcium homeostasis, placing the transporting calcium pumps as potential targets. The aim of this work was to study the molecular inhibitory mechanism of Al3+ on Ca2+-ATPases such as the plasma membrane and the sarcoplasmic reticulum calcium pumps (PMCA and SERCA, respectively). These P-ATPases transport Ca2+ actively from the cytoplasm towards the extracellular medium and to the sarcoplasmic reticulum, respectively. For this purpose, we performed enzymatic measurements of the effect of Al3+ on purified preparations of PMCA and SERCA. Our results show that Al3+ is an irreversible inhibitor of PMCA and a slowly-reversible inhibitor of SERCA. The binding of Al3+ is affected by Ca2+ in SERCA, though not in PMCA. Al3+ prevents the phosphorylation of SERCA and, conversely, the dephosphorylation of PMCA. The dephosphorylation time courses of the complex formed by PMCA and Al3+ (EPAl) in the presence of ADP or ATP show that EPAl is composed mainly by the conformer E2P. This work shows for the first time a distinct mechanism of Al3+ inhibition that involves different intermediates of the reaction cycle of these two Ca2+-ATPases.