Outer sphere contributions within the thiol dioxygenase active site: a combined structure-function study

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TITLE. Outer sphere contributions within the thiol dioxygenase active site: a combined structure-function study PROJECT SUMMARY. Thiol dioxygenases (TDOs) are a subset of non-heme mononuclear iron oxygenases that catalyze the O2-dependent oxidation of thiol-bearing substrates to yield the corresponding sulfinic acid. Cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO) are the only known mammalian TDOs. These enzymes catalyze the oxidation of L-cysteine (CYS) and cysteamine (2-aminoethanthiol, CA) to produce cysteine sulfinic acid (CSA) and hypotaurine (HT), respectively. Ultimately, CSA and HT feed into the mammalian biosynthetic pathways for taurine (TAU). As the most abundant (sulfonic) amino acid derivative in the body, TAU plays a variety of essential functions including osmoregulation, conjugation substrate in the synthesis of bile salts, stabilization of skeletal muscle, maintenance of cardiac rhythm, and an essential neurotransmitter in embryonic central nervous system development. Despite having structurally similar substrates, CDO and ADO exhibit remarkable substrate-specificity; exhibiting no (or negligible) cross-reactivity. Significantly, changes in the cellular expression and/or activity of mammalian TDOs (CDO and ADO) have been correlated with the onset of various human diseases (cancer, neurodegenerative disorders, rheumatoid arthritis, as well as other metabolic disorders). An emerging subset of TDOs are referred to as N-terminal cysteinyl dioxygenases (NCOs). These enzymes are believed to function as physiologic O2-sensors by catalyzing the oxidation of protein N-terminal CYS-residues to initiate protein degradation. A similar function has recently been reported for mammalian ADO in controlling regulators of G protein signaling. Therefore ADO functions both as a small molecule TDO, specific for cysteamine (CA), as well as a NCO by regulating the stability of specific proteins in an O2-dependent fashion. A notable distinction within the NCO active site is the absence of a conserved sequence of spatially adjacent amino acids (Ser-His-Tyr) forming a hydrogen bonding network. This structural motif is highly conserved among structurally characterized TDOs and it is believed to regulate oxygen and thiol-substrate binding affinity at the TDO Fe-site. The absence of this conserved sequence among NCOs suggests a novel mechanism for O2-sensing in this class of enzymes. The central theme of the proposed activities is to investigate the role of outer-sphere interactions within the TDO active site that influence substrate efficiency and recognition. The experiments proposed involve a series of careful kinetic measurements to identify catalytically essential residues within the enzymatic active site. In parallel, spectroscopic and computational methods will be employed to develop a structural model for active site hydrogen-bonding interactions. Similar studies will be performed on members of the NCO class of enzymes to provide a valuable counterpoint to results obtained for TDOs. Potentially these studies will shed light on the mechanisms by which TDOs (and NCOs) regulate specificity of small molecule thiols, polypeptide substrates, and molecular oxygen.