Abstract
Evolution of enzymes plays a crucial role in obtaining new biological functions for all life forms. Arsenate reductases (ArsC) are several families of arsenic detoxification enzymes that reduce arsenate to arsenite, which can subsequently be extruded from cells by specific transporters. Among these, the Synechocystis ArsC (SynArsC) is structurally homologous to the well characterized thioredoxin (Trx)-coupled ArsC family but requires the glutaredoxin (Grx) system for its reactivation, therefore classified as a unique Trx/Grx-hybrid family. The detailed catalytic mechanism of SynArsC is unclear and how the "hybrid" mechanism evolved remains enigmatic. Herein, we report the molecular mechanism of SynArsC by biochemical and structural studies. Our work demonstrates that arsenate reduction is carried out via an intramolecular thiol-disulfide cascade similar to the Trx-coupled family, whereas the enzyme reactivation step is diverted to the coupling of the glutathione-Grx pathway due to the local structural difference. The current results support the hypothesis that SynArsC is likely a molecular fossil representing an intermediate stage during the evolution of the Trx-coupled ArsC family from the low molecular weight protein phosphotyrosine phosphatase (LMW-PTPase) family.
Highlights
Arsenate reductases catalyze the reduction of arsenate to arsenite
Our current results provide the structural insights for understanding the hybrid mechanism of Synechocystis ArsC (SynArsC), and favor the hypothesis that SynArsC may represent an earlier stage during the enzyme evolution from LMW-PTPase to arsenate reductase
The Initial Stage of Arsenate Reduction—SynArsC contains a total of five cysteine residues, Cys8, Cys13, Cys35, Cys80, and Cys82, among which only the Cys8, Cys80, and Cys82 residues were identified essential for arsenate reduction activity [15, 17]
Summary
Arsenate reductases catalyze the reduction of arsenate to arsenite. Results: Structures of Synechocystis arsenate reductase in different reaction stages are determined and a hybrid catalytic mechanism is established. Two distinct ArsC families utilizing the glutathione (GSH)-glutaredoxin (Grx) redox pathway are represented by the Gram-negative bacteria Escherichia coli plasmid R773 ArsC and the yeast Saccharomyces cerevisiae ACR2p (or Leishmania major LmACR2) proteins (10 –14). These two families are structurally dissimilar, and are both unrelated to the Trx-coupled ArsC family [1, 5]. Our current results provide the structural insights for understanding the hybrid mechanism of SynArsC, and favor the hypothesis that SynArsC may represent an earlier stage during the enzyme evolution from LMW-PTPase to arsenate reductase
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