Peptidases are classical objects of enzymology and structural studies. However, a few protein families with experimentally characterized proteolytic activity, but unknown catalytic mechanism and ...three‐dimensional structures, still exist. Using comparative sequence analysis, we deduce spatial structure for one of such families, namely, U40, which contains just one P5 protein from bacteriophage phi‐6. We show that this singleton sequence possesses conserved sequence motifs characteristic of lysozymes and is a distant homolog of lytic transglycosylases that cleave bacterial peptidoglycan. The structure of the P5 protein is therefore predicted to adopt the lysozyme‐like fold shared by T4, λ, C‐type, G‐type lysozymes, and lytic transglycosylases. Since previous biochemical experiments with P5 of phi‐6 have indicated that the purified enzyme possesses endopeptidase activity and not glycosidase activity, our results point to the possibility of a newly evolved molecular function and call for further experimental characterization of this unusual P5 protein.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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