Spectroscopic investigation of iron(III) cysteamine dioxygenase in the presence of substrate (analogs): implications for the nature of substrate-bound reaction intermediates

Binding Modes of Cysteamine Homologues Control Fe(III)–Thiolate Autoredox Rates in a Thiol Dioxygenase Model Complex

“…ADO achieves selectivity for cysteamine over cysteine by outer-sphere effects with nearby active site residues. 5,8 Outer-sphere effects also enable CDO selectivity for cysteine binding in the S-N mode. Interestingly, outer-sphere interactions in ADO are thought to be markedly weaker than those in CDO.…”

“…Interestingly, outer-sphere interactions in ADO are thought to be markedly weaker than those in CDO. 5 While ADO is indeed selective for cysteamine over cysteine for S dioxygenation, ADO-Fe(III) is still capable of binding cysteine. When ADO is treated with excess cysteine (1:10 ADO/cysteine), a low-spin ADO-Fe(III) center forms as detected by MCD and EPR spectroscopies.…”

“…4 In contrast, when ADO binds cysteamine or mercaptopropionate in a monodentate fashion, the ADO-Fe(III) center remains high-spin unless cyanide, a strong-field ligand, is also coordinated to the metal center. 4,5 Complex 2, our surrogate for the more reactive cysteinecontaining intermediate, contains low-spin Fe(III) and a cysteine-like substrate bound in a tridentate S-N-O mode. Collectively, these observations suggest that cysteine might bind to ADO-Fe(III) with higher denticity than cysteamine or mercaptopropionate given the discrepancy in Fe(III) spin states.…”

“…Enzymatic activity of thiol dioxygenases, such as cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), transform biological thiols into sulfinates through the activation of molecular oxygen. [1][2][3][4][5][6][7][8] Thiol dioxygenase active sites feature mononuclear non-heme Fe bound by three histidine residues forming a facial 3N binding mode (Figure 1A). The remaining three coordination sites on Fe are either occupied by three water molecules in the absence of substrate or by a combination of thiol substrate, water, or dioxygen.…”

“…1,11,12 There is growing interest in determining how thiol dioxygenases bind substrates to achieve selective thiol oxygenation as exemplified by recent studies focused on ADO. 4,5,7,13 The activity of ADO is somewhat unique among thiol dioxygenases because both cysteamine and peptides with N-terminal cysteine residues are oxygenated to sulfinates by the metalloenzyme. 14 Common structural features of cysteamine and N-terminal cysteine residues are thiol and amine groups making the S-N binding mode a plausible hypothesis.…”