A dynamic C-terminal segment in the Mycobacterium tuberculosis Mn/Fe R2lox protein can adopt a helical structure with possible functional consequences.

Mycobacterium tuberculosis R2-like ligand-binding oxidase (MtR2lox) belongs to a recently discovered group of proteins that are homologous to the ribonucleotide reductase R2 proteins. MtR2lox carries a heterodinuclear Mn/Fe cofactor and, unlike R2 proteins, a large ligand-binding cavity. A unique tyrosine-valine cross link is also found in the vicinity of the active site. To date, all known structures of R2 and R2lox proteins show a disordered C-terminal segment. Here, we present two new crystal forms of MtR2lox, revealing an ordered helical C-terminal. The ability of alternating between an ordered and disordered state agrees well with bioinformatic analysis of the protein sequence. Interestingly, ordering of the C-terminal helix shields a large positively charged patch on the protein surface, potentially used for interaction with other cellular components. We hypothesize that the dynamic C-terminal segment may be involved in control of protein function in vivo.