Crystal structure of goat lactoperoxidase at 3.5 A resolution

Department of Biophysics, All India Institute of Medical Sciences, New Delhi
â—‹Amit K. Singh E.S. Ethayathulla Nagendra Singh Sujata Sharma Tej P. Singh

Lactoperoxidase is a heme containing enzyme that catalyzes the inactivation of various micro-organisms. The crystal structure of goat lactoperoxidase was determined as part of the programme of structural and functional studies of antimicrobial secretory glycoproteins such as lactoperoxidases and lactoferrins.The structure has been refined to an R-factor of 0.177. The protein is crystallized in space group P1 with two molecules in the unit cell. Thus, the structure contains two identical crystallographically independent molecules. Their conformations are also formed essentially identical. Both molecules contain one calcium ion in each with pentagonal bipyramidal geometries. The structure has also revealed a number of halide binding sites. The structure contains two cis-prolines. It is essentially an alpha-helical protein with a little beta structure. The central core region contains the heme binding site which is composed of five alpha-helices. The heme porphyrin ring is considerably distorted from planarity. Heme is covalently linked to protein via Asp94 and glu242. A proximal ligand to the heme iron atom has been identified as His336 which in turn is hydrogen bonded to Asn421. On the distal side of the heme group, His95 and Arg239 are likely to participate directly in the catalytic mechanism in a manner analogous to the distal histidine and arginine of the non-homologous enzyme cytochrome C peroxidase.