Structure of recombinant human rheumatoid arthritic synovial fluid phospholipase A₂ at 2.2 Å resolution

Abstract

PHOSPHOLIPASES A₂ (PLA₂s) may be grouped into distinct families of proteins that catalyse the hydrolysis of the 2-acyl bond of phospholipids and perform a variety of biological functions. The best characterized are the small (relative molecular mass ˜ 14,000) calcium-dependent, secretory enzymes of diverse origin, such as pancreatic and venom PLA₂s¹. The structures and functions of several PLA₂s are known². Recently, high-resolution crystal structures of complexes of secretory PLA₂s with phosphonate phospholipid analogues have provided information about the detailed stereochemistry of transition-state binding³, confirming the proposed catalytic mechanism of esterolysis⁴. By contrast, studies on mammalian nonpancreatic secretory PLA₂s (s–PLA₂s) have only recently begun; s–PLA₂s are scarce in normal cells and tissues but large amounts are found in association with local and systemic inflammatory processes and tissue injury in animals and man^5–7. Such s–PLAs have been purified from rabbit and rat inflammatory exudate^8,9, from synovial fluid from patients with rheumatoid arthritis^10,11 and from human platelets¹¹. Cloning and sequencing shows that the primary structure of the human s-PLA₂^11,12 has about 37% homology with that of bovine pancreatic PLA₂¹³ and 44% homology with that of Crotalus atrox PLA₂¹⁴. The human s-PLA₂ is an unusually basic protein, yet contains most of the highly conserved amino-acid residues and sequences characteristic of the PLA₂s sequenced so far^1,15. Here we report the refined, three-dimensional crystal structure at 2.2 Å resolution of recombinant human rheumatoid arthritic synovial fluid PLA₂. This may aid the development of potent and specific inhibitors of this enzyme using structure-based design.