Enzyme Shuffles Its Tail For Catalysis

Researchers have found an unusual case in which an enzyme seems to adopt two different secondary structures to modulate its catalytic activity (Nat. Chem. Biol. 2014, DOI: 10.1038/nchembio.1694). The phosphatidylinositol mannosyltransferase known as PimA is a membrane-associated enzyme that initiates the biosynthesis of components essential to the virulence of the tuberculosis bacterium. When Marcelo E. Guerin of the University of the Basque Country, in Spain; Pedro M. Alzari of the Pasteur Institute, in Paris; and coworkers crystallized PimA, they found the enzyme forms a tetramer that includes two different conformational states. The conformations are similar except in the enzyme’s N-terminal domain. In one conformation, two α-helices are folded neatly, whereas in the other conformation, the structure is extended and partially disordered. According to the researchers, the structures “reveal an exceptional flexibility of the protein along the catalytic cycle, including β-strand-to-α-helix and α-helix-to-β-strand transitions.” The structural changes, which are set off by interactions of PimA with phospholipids in the bacterial cell membrane, may cause changes in catalytic activity that help the enzyme carry out its cellular function, but the details of this structure-function connection have yet to be determined.