The structure of truncated recombinant human bile salt-stimulated lipase reveals bile salt-independent conformational flexibility at the active-site loop and provides insights into heparin binding

Human bile salt-stimulated lipase (BSSL), which is secreted from the pancre as into the digestive tract and from the lactating mammary gland into human milk, is important for the effective absorption of dietary lipids. The dep endence of BSSL on bile acids for activity with water-insoluble substrates differentiates it from other lipases. We have determined the crystal struct ure of a truncated variant of human BSSL (residues 1-5.8) and refined it at 2.60 Angstrom resolution, to an R-factor of 0.238 and R-free of 0.275. Thi s variant lacks the C-terminal (alpha -helix and tandem C-terminal repeat r egion of native BSSL, but retains full catalytic activity. A short loop (re sidues 115-126) capable of occluding the active-site (the active site loop) is highly mobile and exists in two conformations, the most predominant of which leaves the active-site open for interactions with substrate. The bile salt analogue 3-[(3-cholamidopropyl)dimethylammoniol-1-propane sulfonic ac id (CHAPS) was present in the crystallisation medium, but was not observed bound to the enzyme. However, the structure reveals a sulfonate group from the buffer piperizine ethane sulfonic acid (PIPES), making interactions wit h Arg63 and His115. His115 is part of the active-site loop, indicating that the loop could participate in the binding of a sulphate group from either the glycosaminoglycan heparin (known to bind BSSL) or a bile acid such as d eoxycholate. Opening of the 115-126 active-site loop may be cooperatively l inked to a sulphate anion binding at this site. The helix bundle domain of BSSL (residues 319-398) exhibits weak electron density and high temperature factors, indicating considerable structural mobility. This domain contains an unusual Asp:Glu pair buried in a hydrophobic pocket between helices alp ha (H) and alpha (K) that may be functionally important. We have also solve d the structure of full-length glycosylated human BSSL at 4.1 Angstrom reso lution, using the refined coordinates of the truncated molecule as a search model. This structure reveals the position of the C-terminal helix, missin g in the truncated variant, and also shows the active-site loop to be in a closed conformation. (C) 2001 Academic Press.