Crystal structure of bovine duodenase, a serine protease, with dual trypsin and chymotrypsin-like specificities

The three-dimensional structure of duodenase, a serine protease from bovine duodenum mucosa, has been determined at 2.4 Angstrom resolution. The enzym e, which has both trypsin-like and chymotrypsin-like activities, most close ly resembles human cathepsin G with which it shares 57% sequence identity a nd similar specificity. The catalytic Ser195 in duodenase adopts the energe tically favored conformation typical of serine proteinases and unlike the s trained state typical of lipase/ esterases. Of several meters in the active site of duodenase, the one associated with Ser214 is found in all serine p roteinases and most lipase/esterases. The conservation of the Ser214 residu e in serine proteinase, its presence in the active site, and participation in a hydrogen wafer network involving the catalytic triad (His57, Asp107, a nd Ser195) argues for its having an important role in the mechanism of acti on. It may be referred to as a fourth member of the catalJrtic triad. Duode nase is one of a growing family of enzymes that possesses trypsin-like and chymotrypsin-like activity. Not long ago, these activities were considered to be mutually exclusive. Computer modeling reveals that the S1 subsite of duodenase has structural features compatible with effective accommodation o f P1 residues typical of trypsin (Arg/Lys) and chymotrypsin (Tyr/Phe) subst rates. The determination of structural features associated with functional variation in the enzyme family may permit design of enzymes with a specific ratio of trypsin and chymotrypsin activities. Proteins 2000;41:8-16. (C) 2 000 Wiley-Liss, Inc.