The effect of NBD-Cl in nucleotide-binding of the major subunit alpha and B of the motor proteins F1FO ATP synthase and A1AO ATP synthase

Undesirable effects of chemical inhibitors of NAD(P)+ transhydrogenase on mitochondrial respiratory function

NAD(P)⁺ transhydrogenase (NNT) is located in the inner mitochondrial membrane and catalyzes a reversible hydride transfer between NAD(H) and NADP(H) that is coupled to proton translocation between the intermembrane space and mitochondrial matrix. NNT activity has an essential role in maintaining the NADPH supply for antioxidant defense and biosynthetic pathways. In the present report, we evaluated the effects of chemical compounds used as inhibitors of NNT over the last five decades, namely, 4-chloro-7-nitrobenzofurazan (NBD-Cl), N,N'-dicyclohexylcarbodiimide (DCC), palmitoyl-CoA, palmitoyl-l-carnitine, and rhein, on NNT activity and mitochondrial respiratory function. Concentrations of these compounds that partially inhibited the forward and reverse NNT reactions in detergent-solubilized mouse liver mitochondria significantly impaired mitochondrial respiratory function, as estimated by ADP-stimulated and nonphosphorylating respiration. Among the tested compounds, NBD-Cl showed the best relationship between NNT inhibition and low impact on respiratory function. Despite this, NBD-Cl concentrations that partially inhibited NNT activity impaired mitochondrial respiratory function and significantly decreased the viability of cultured Nnt^-/- mouse astrocytes. We conclude that even though the tested compounds indeed presented inhibitory effects on NNT activity, at effective concentrations, they cause important undesirable effects on mitochondrial respiratory function and cell viability.

Relevance of the conserved histidine and asparagine residues in the phosphate-binding loop of the nucleotide binding subunit B of A₁A₀ ATP synthases

The nucleotide binding sites in A-ATP synthases are located at the interfaces of subunit A and B, which is proposed to play a regulatory role. Differential binding of MgATP and -ADP to subunit B has been described, which does not exist in the related α and B subunits of F-ATP synthases and V-ATPases, respectively. The conserved phosphate loop residues, histidine and asparagine, of the A-ATP synthase subunit B have been proposed to be essential for γ-phosphate interaction. To investigate the role of these conserved P-loop residues in nucleotide-binding, subunit B residues H156 and N157 of the Methanosarcina mazei Gö1 A-ATP synthase were separately substituted with alanine. In addition, N157 was mutated to threonine, because it is the corresponding amino acid in the P-loop of F-ATP synthase subunit α. The structures of the subunit B mutants H156A, N157A/T were solved up to a resolution of 1.75 and 1.7 Å. The binding constants for MgATP and -ADP were determined, demonstrating that the H156A and N157A mutants have a preference to the nucleotide over the wild type and N157T proteins. Importantly, the ability to distinguish MgATP or -ADP was lost, demonstrating that the histidine and asparagine residues are crucial for nucleotide differentiation in subunit B. The structures reveal that the enhanced binding of the alanine mutants is attributed to the increased accessibility of the nucleotide binding cavity, explaining that the structural arrangement of the conserved H156 and N157 define the nucleotide-binding characteristics of the regulatory subunit B of A-ATP synthases.