Journal: Biomolecules

Article Title: Cystathionine-β-synthase: Molecular Regulation and Pharmacological Inhibition

doi: 10.3390/biom10050697

Figure Lengend Snippet: The proposed mechanism of AOAA’s action on human CBS. ( A ) Two representative intermediates of the CBS reaction mechanism. The cofactor is bound to the ε-amino group of Lys119, thus forming an internal aldimine. The interaction with the substrate (e.g., L-cysteine) forms an external aldimine followed by internal rearrangement and eventually the regeneration of the enzyme-bound PLP. The interaction of AOAA stop the catalytic cycle through irreversibly binding the formyl moiety of PLP, thus preventing the regeneration of enzyme-bound PLP. ( B ) Docking simulation: interactions of the tentative PLP-AOAA complex in the catalytic pocket of CBS (Part B is reproduced by permission from ).

Article Snippet: The first report in which AOAA was identified as an inhibitor of a PLP-dependent enzyme is a paper by Wallach who (working at the Upjohn Company) in 1961, described the inhibitory effect of AOAA on the catalytic activity of γ-aminobutyric acid (GABA) aminotransferase (GABA-T) in vitro and in vivo [ ].

Techniques: Binding Assay

Journal: Biomolecules

Article Title: Cystathionine-β-synthase: Molecular Regulation and Pharmacological Inhibition

doi: 10.3390/biom10050697

Figure Lengend Snippet: Multiple modes of AOAA’s action on H 2 S producing pathways and other transaminases in cancer cells. AOAA suppresses cellular H 2 S levels by directly inhibiting CBS and CSE activity, by suppressing H 2 S formation through the 3-MST pathway via inhibition of cysteine amino transferase (CAT), and by inhibiting the non-enzymatic formation of H 2 S from vitamin B6 or PLP. In addition, AOAA also inhibits a variety of transaminases (including GOT1, a key enzyme of the malate/aspartate shuttle). In a cancer cell, these combined effects of AOAA may produce synergistic inhibition of cellular bioenergetics, resulting in an impairment of cancer cell proliferation and viability. By inhibiting CBS-derived and 3-MST-derived H 2 S, AOAA suppresses mitochondrial electron transport and cancer cell bioenergetics by preventing the donation electrons at complex II, by suppressing the H 2 S-induced direct stimulation of ATP synthase and by lifting the H 2 S-mediated inhibition of intramitochondrial adenylate cyclase (this latter effect is not shown on this scheme). The malate-aspartate shuttle translocates electrons that are produced in glycolysis across the semipermeable inner membrane of the mitochondrion to support oxidative phosphorylation. These electrons enter the electron transport chain at Complex I. The shuttle system is required because the mitochondrial inner membrane is impermeable to NADH (a primary reducing equivalent of the electron transport chain). In humans, the cytoplasmic enzyme (GOT1) is one of the key enzymes in the malate shuttle: it catalyzes the interconversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate using PLP as a cofactor. By inhibiting GOT, AOAA reduces the transfer of electron donors to the mitochondria, thereby providing an additional mode for the suppression of cancer cell bioenergetics. Finally, many tumors up-regulate their metabolism through glutaminolysis. In this process, glutamine is taken up into the cells, and it rapidly deaminated by deaminases to yield glutamate (the uptake and the conversion is not shown in the current scheme). In turn, glutamate (Glu) is converted by alanine aminotransferase (ALT) enzymes, in particular by glutamate pyruvate transaminase 2 (GPT2) to α-ketoglutarate and enters the TCA cycle. Because AOAA inhibits ALT/GPT2, this process is inhibited, and the tumor cells become deprived from an important metabolic fuel. The current figure is a modified version of a figure that was reproduced by permission from .

Article Snippet: The first report in which AOAA was identified as an inhibitor of a PLP-dependent enzyme is a paper by Wallach who (working at the Upjohn Company) in 1961, described the inhibitory effect of AOAA on the catalytic activity of γ-aminobutyric acid (GABA) aminotransferase (GABA-T) in vitro and in vivo [ ].

Techniques: Activity Assay, Inhibition, Derivative Assay, Produced, Membrane, Phospho-proteomics, Modification