Role of potassium levels in pkBADH heterogeneity of NAD+ binding site

BADH-NAD+-K+ Complex Interaction Studies Reveal a New Possible Mechanism between Potassium and Glutamic 254 at the Coenzyme Binding Site

Betaine aldehyde dehydrogenase (BADH EC 1.2.1.8) catalyzes the irreversible oxidation of betaine aldehyde to glycine betaine using NAD⁺ as a coenzyme. Incubation of porcine kidney BADH (pkBADH) with NAD⁺ decreases the catalytic cysteine (C288) reactivity. Potassium ion increases the pkBADH affinity by the coenzyme. This work aimed to analyze pkBADH and NAD⁺ interaction in the presence and absence of K⁺ using ¹H NMR to identify the amino acids that interact with NAD⁺ and/or K⁺ to understand the regulation process of pkBADH-NAD⁺ complex formation mediated by the K⁺ ion and their impact on the substrate binding and catalysis. Nuclear magnetic resonance spectra of pkBADH were obtained in the presence and absence of NAD⁺ and K⁺. The results show a chemical shift of the signals corresponding to the catalytic glutamic that participates in the transfer of H⁺ in the reaction of the pkBADH-NAD⁺-K⁺ complex formation. Furthermore, there is a widening of the signal that belongs to the catalytic cysteine indicating higher rigidity or less grade of rotation of the structure, which is consistent with the possible conformations of C288 in the catalytic process; in addition, there is evidence of changes in the chemical environment that surrounds NAD⁺.

Spectroscopic analysis of coenzyme binding to betaine aldehyde dehydrogenase dependent on potassium

Glycine betaine is the main osmolyte synthesized and accumulated in mammalian renal cells. Glycine betaine synthesis is catalyzed by the enzyme betaine aldehyde dehydrogenase (BADH) using NAD⁺ as the coenzyme. Previous studies have shown that porcine kidney betaine aldehyde dehydrogenase (pkBADH) binds NAD⁺ with different affinities at each active site and that the binding is K⁺ dependent. The objective of this work was to analyze the changes in the pkBADH secondary and tertiary structure resulting from variable concentrations of NAD⁺ and the role played by K⁺ . Intrinsic fluorescence studies were carried out at fixed-variable concentrations of K⁺ and titrating the enzyme with varying concentrations of NAD⁺ . Fluorescence analysis showed a shift of the maximum emission towards red as the concentration of K⁺ was increased. Changes in the exposure of tryptophan located near the NAD⁺ binding site were found when the enzyme was titrated with NAD⁺ in the presence of potassium. Fluorescence data analysis showed that the K⁺ presence promoted static quenching that facilitated the pkBADH-NAD⁺ complex formation. DC data analysis showed that binding of K⁺ to the enzyme caused changes in the α-helix content of 4% and 12% in the presence of 25 mM and 100 mM K⁺ , respectively. The presence of K⁺ during NAD⁺ binding to pkBADH increased the thermal stability of the complex. These results indicated that K⁺ facilitated the pkBADH-NAD⁺ complex formation and suggested that K⁺ caused small changes in secondary and tertiary structures that could influence the active site conformation.

Effect of the drug cyclophosphamide on the activity of porcine kidney betaine aldehyde dehydrogenase

The enzyme betaine aldehyde dehydrogenase (BADH EC 1.2.1.8) catalyzes the synthesis of glycine betaine (GB), an osmolyte and osmoprotectant. Also, it participates in several metabolic pathways in humans. All BADHs known have cysteine in the active site involved in the aldehyde binding, whereas the porcine kidney enzyme (pkBADH) also has a neighborhood cysteine, both sensitive to oxidation. The antineoplastic and immuno-suppressant pre-drug cyclophosphamide (CTX), and its bioactivation products, have two highly oxidating chlorine atoms. This work aimed to analyze the effect of CTX in the activity of porcine kidney betaine aldehyde dehydrogenase. PkBADH was incubated with varying CTX concentration (0 to 2.0 mM) at 25 °C and lost 50 % of its activity with 2.0 mM CTX. The presence of the coenzyme NAD⁺ (0.5 mM) decreased 95% the activity in 2.0 mM CTX. The substrate betaine aldehyde (0.05 and 0.4 mM, and the products NADH (0.1-0.5 mM) and GB (1 and 10 mM) did not have an effect on the enzyme inactivation by CTX. The reducing agents, dithiothreitol and β-mercaptoethanol, reverted the pkBADH inactivation, but reduced glutathione (GSH) was unable to restore the enzyme activity. Molecular docking showed that CTX could enter at the enzyme active site, where its chlorine atoms may interact with the catalytic and the neighboring cysteines. The results obtained show that CTX inactivates the pkBADH due to oxidation of the catalytic cysteine or because it oxidizes catalytic and neighborhood cysteine, forming a disulfide bridge with a concomitant decrease in the activity of the enzyme.