Several serotypes of Escherichia coli produce protein toxins closely related to Shiga toxin (Stx) from Shigella dysenteriae serotype 1. These Stx-producing E. coli cause outbreaks of hemorrhagic colitis and hemolytic uremic syndrome in humans, with the latter being more likely if the E. coli produce Stx2 than if they only produce Stx1. To investigate the differences among the Stxs, which are all AB 5 toxins, the crystal structure of Stx2 from E. coli O157:H7 was determined at 1.8-Å resolution and compared with the known structure of Stx. Our major finding was that, in contrast to Stx, the active site of the A-subunit of Stx2 is accessible in the holotoxin, and a molecule of formic acid and a water molecule mimic the binding of the adenine base of the substrate. Further, the A-subunit adopts a different orientation with respect to the B-subunits in Stx2 than in Stx, due to interactions between the carboxyl termini of the B-subunits and neighboring regions of the A-subunit. Of the three types of receptor-binding sites in the B-pentamer, one has a different conformation in Stx2 than in Stx, and the carboxyl terminus of the A-subunit binds at another. Any of these structural differences might result in different mechanisms of action of the two toxins and the development of hemolytic uremic syndrome upon exposure to Stx2.

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The structure of the complex between the serine protease Streptomyces griseus protease B (SGPB) and the third domain of the Kazal-type ovomucoid inhibitor from turkey has been solved at 1.8-A resolution and refined to a conventional R factor of 0.125. As others have reported previously for analogous complexes of proteases and protein inhibitors, the inhibitor binds in a fashion similar to that of a substrate; it is not cleaved, but there is a close approach (2.7 A) of the active site nucleophile Ser-195 O gamma to the carbonyl carbon of the reactive peptide bond of the inhibitor. Contrary to the structural reports regarding the other enzyme-inhibitor complexes, we conclude that there is no evidence for a significant distortion of this peptide bond from planarity. The mechanism of inhibition can be understood in terms of the equilibrium thermodynamic parameters Ka, the enzyme-inhibitor association constant, and Khyd, the equilibrium constant for inhibitor hydrolysis. These thermodynamic parameters can be rationalized in terms of the observed structure.