Abstract
Elastase (EC 3.4.21.11) is normally involved in the degradation of elastin, and is believed to be the primary agent responsible for the induction of several degenerative connective tissue diseases. The primary subsite specificity of porcine pancreatic elastase (PPE) was mapped with a series of tetrapeptide amide substrates of the general formula Ac-Pro-Ala-Pro-X-NH₂, where X represented one of 7 amino acids. The kinetic results were compared to values predicted from molecular mechanics calculations (Presta, 1985) and previously reported kinetic studies. To further examine the interactions in the primary specificity subsite, three x-ray crystallographic studies were performed. First, 2.57 A resolution x-ray crystallographic data for the complex of Ac-Ala-Pro-Val-trifluoromethyl ketone with PPE were refined to R = 0.14. The inhibitor formed a stable, covalent, tetrahedral addition complex with the Oγ atom of the catalytic Ser-195. The second structure was the covalent enzyme-inhibitor complex of PPE with Ac-Ala-Pro-Val-difluoro-β-ketophenethylamide at 1.78 Å resolution (R = 0.16). As in the previous structure, the inhibitor formed a stable hemiketal intermediate with Oγ atom of Ser-195. An additional hydrogen bond between His-57N[epsilon] and one of the fluorines aided in stabilizing the complex. Both fluorinated inhibitors formed compounds which were structural mimics for the tetrahedral intermediate that is formed in the pathway for substrate hydrolysis. Thus, they can be regarded transition-state analogs. The last crystal structure was the complex of PPE with Cbz-Ala-Ile-boronic acid at 2.53 A resolution (R = 0.16). The mechanism of inhibition first involved a covalent bond with Oγ of Ser-195, followed by the loss of one of the boron hydroxyl atoms. The boron atom then becomes trigonal, but His-57 N[epsilon] makes a coordinate covalent bond (2.19 A) with the boron atom. The inclusion of His-57 in the mechanism of inhibition can explain the high potency of these inhibitors. The results from the synthesis and kinetics studies, in conjunction with those obtained from the crystallographic studies can give new information which can be used in the design of new medicinals targeted for elastase pathologies.
Takahashi, Lori Hina (1987). Kinetic and crystallographic analysis of porcine pancreatic elastase-ligand interactions. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -747644.