ABSTRACT
Lipases are a class of water-soluble enzymes that act on water-insoluble substrates and are stable in both polar and nonpolar environments [1,2]. The first two published crystallographic structures of lipases were those of Rhizomucor miehei lipase [3] and human pancreatic lipase [4]. The elucidation of the structure of lipases, in the early part of this decade, has allowed us to better understand their function. Lipases can act both in bulk solution and at interfaces; however, their activity is greatly enhanced at interfaces. This phenomenon is referred to as “interfacial activation.” Crystallographic studies of lipases have shown that interfacial activation results partly from a conformational change in the enzyme upon interfacial binding [5,6]. This conformational change involves a loop (lid) movement: A helical fragment of about 20 amino acids, called the “lid” or “flap,” seals or closes the active site, preventing substrate molecules from entering. Upon interfacial binding, however, this lid is displaced by a conformational change and the active site becomes exposed to solvent and substrate [5,6]. The structure and activity of a cutinase from Fusarium solani, which lacks a lid, lend support for this mechanism of lipase interfacial activation [7,8]. The crystal structure of this cutinase [7] shows that the protein folding pattern is very similar to that of R. miehei lipase [9]. Cutinases are α/β proteins and their active site is composed of the catalytic triad Asp-His-Ser [7]. However, this enzyme lacks a “lid” covering the active site; hence, the active site is exposed to solvent and has a performed oxyanion hole [8]. Moreover, this cutinase is not activated at oil-water interfaces.
