Methionine is an essential amino acid in humans; however, in plants and many microorganisms methionine is synthesized from both aspartic acid and cysteine. As part of this pathway, cystathionine γ-synthase (CGS; EC 220.127.116.11) catalyzes the reaction between O
-homoserine and l
-cysteine to produce l
-cystathionine and succinate. CGS is a transferase and acts in the committed step (the fifth overall) of the biosynthesis of l
-methionine. In bacteria this mechanism is performed by the enzyme MetB, which additionally plays roles in both selenoamino-acid metabolism and sulfur metabolism. MetB is covalently linked to the cofactor pyridoxal phosphate (PLP), similar to other members of the aspartate aminotransferase (AAT-I) superfamily of enzymes. Similar to other CGS enzymes, MetB forms a homotetramer, with each individual homodimer creating two active sites (Clausen et al.
The first CGS structure was solved for the Escherichia coli
enzyme covalently linked to the cofactor PLP (Clausen et al.
) and was followed by a structure of CGS from the plant Nicotiana tabacum
also covalently linked to PLP (Steegborn et al.
). Steegborn and coworkers also solved a series of structures bound to a variety of inhibitors as potential herbicides (Steegborn et al.
). With the exception of the inhibitor complexes, there are no structures of MetB bound to any ligand or ligand-like compound at high resolution.
is the third most common form of mycobacterial infection behind M. tuberculosis
and M. leprae
and mainly affects people from Africa, Australia and Southeast Asia (Walsh et al.
). Upon infection, this slow-growing mycobacterial infection produces painful ulcerated lesions known as Buruli ulcers. In the early stages of treatment Buruli ulcers can be treated with antibiotics; however, in later stages the ulcers have to be excised and in some cases lead to amputation (Nienhuis et al.
). To prevent or fight this painful disease, it is of interest to obtain structural information about potential drug targets from the organism.
MetB and other PLP-containing enzymes have been identified as prime drug targets for infectious disease organisms such as mycobacteria (Amadasi et al.
). In addition, mycobacteria prefer using methionine as a source of sulfur, adding to the significance of these targets (Wheeler et al.
). Here, we present two structures of the cystathionine γ-synthase MetB from M. ulcerans
covalently linked to PLP and bound to the buffer molecule HEPES.