Solution NMR structures of oxidized and reducedEhrlichia chaffeensisthioredoxin: NMR-invisible structure owing to backbone dynamics

Abstract

Thioredoxins are small ubiquitous proteins that participate in a diverse variety of redox reactionsviathe reversible oxidation of two cysteine thiol groups in a structurally conserved active site. Here, the NMR solution structures of a reduced and oxidized thioredoxin fromEhrlichia chaffeensis(Ec-Trx, ECH_0218), the etiological agent responsible for human monocytic ehrlichiosis, are described. The overall topology of the calculated structures is similar in both redox states and is similar to those of other thioredoxins: a five-stranded, mixed β-sheet (β1–β3–β2–β4–β5) surrounded by four α-helices. Unlike other thioredoxins studied by NMR in both redox states, the1H–15N HSQC spectrum of reducedEc-Trx was missing eight additional amide cross peaks relative to the spectrum of oxidizedEc-Trx. These missing amides correspond to residues Cys35–Glu39 in the active-site-containing helix (α2) and Ser72–Ile75 in a loop near the active site, and suggest a change in backbone dynamics on the millisecond-to-microsecond timescale associated with the breakage of an intramolecular Cys32–Cys35 disulfide bond in a thioredoxin. A consequence of the missing amide resonances is the absence of observable or unambiguous NOEs to provide the distance restraints necessary to define the N-terminal end of the α-helix containing the CPGC active site in the reduced state. This region adopts a well defined α-helical structure in other reported reduced thioredoxin structures, is mostly helical in oxidizedEc-Trx and CD studies ofEc-Trx in both redox states suggests there is no significant difference in the secondary structure of the protein. The NMR solution structure of reducedEc-Trx illustrates that the absence of canonical structure in a region of a protein may be owing to unfavorable dynamics prohibiting NOE observations or unambiguous NOE assignments.