Direct structural observation of an acyl-enzyme intermediate in the hydrolysis of an ester substrate by elastase

The method of X-ray crystallographic cryoenzymology has been used to determine the crystal structure of a kinetically significant species on the reaction pathway of a crystalline enzyme. The structure of a specific acyl-enzyme intermediate in the elastase-catalyzed hydrolysis of the N-carbobenzoxy-L-alanine p-nitrophenyl ester has been determined and refined against X-ray diffraction data at 2.3-A resolution. The difference Fourier electron density map clearly shows electron density for the trapped acyl-enzyme. The acyl-enzyme was formed at -26 degrees C and was stabilized at -55 degrees C during data collection, taking advantage of the glass transition in protein dynamics that occurs at around -50 degrees C.