Structural features of lipoprotein lipase. Lipase family relationships, binding interactions, non-equivalence of lipase cofactors, vitellogenin similarities and functional subdivision of lipoprotein lipase.

A structural homology between lipoprotein lipase, pancreatic lipase and hepatic lipase is known and indicates that all three lipases are members of a common protein family. Lipoprotein lipase and pancreatic lipase utilize small protein co-factors, apolipoprotein C-II and co-lipase, respectively, but comparisons reveal no homology between the co-factor molecules. Hence, they do not show the same relationship as their target enzymes. Neither do screenings detect any extensive similarities between lipoprotein lipase, serine hydrolases, or apolipoproteins. Scannings against data bank proteins show that a 105-residue segment of lipoprotein lipases exhibits a 35-40% residue identity with a sub-region of Drosophila vitellogenins. One fifth of the conserved amino acid residues (8 of 40) are glycine, a pattern which is typical of distantly related forms of protein families. This supports a true relationship between large segments of Drosophila vitellogenins and lipases. Physiological and functional aspects of the vitellogenin/lipoprotein lipase similarities are given. The region concerned is entirely within the N-terminal domain of lipoprotein lipase and constitutes the segment where the similarity to hepatic and pancreatic lipases is most pronounced. Within this lipase region a 10-residue putative lipid-binding site exists for which further similarities have been found to the otherwise not closely related lingual/gastric lipases, prokaryotic lipases and lecithin-cholesterol acyltransferase. Another segment in lipoprotein lipase, where the heparin-binding site has been mapped, exhibits a correlation between strength of heparin binding and extent of basic residues among members of the lipase family. It further exhibits weak similarities with the 'Zn-finger' DNA-binding segment of steroid hormone receptors and may indicate convergence in a binding interaction. Thus, a functional subdivision of lipoprotein lipase into different segments can be distinguished.