substrate (Brzozowski et al., 1991; Derewenda et al., 1992). This was proposed to be
the functional basis of ‘interfacial activation’, the phenomenon whereby lipases are
inactive in the absence of lipid-water interfaces, and become active in their presence
(Brzozowski et al., 1991; Derewenda et al., 1992). This postulate led to the notion
that there were two basic states for the lid, namely ‘open’ and ‘closed’. The Rd lipase
model largely agreed with these interpretations of lipase structure and function.
However, it expanded appreciation of the nature of interfacial inactivation. As crys-
tallized, the Rd lipase asymmetric unit contained two lipase molecules. In one of
these the lid was down, atop the active site (Figure 3A). In the other molecule,
the lid was in a conformation intermediate between closed and open (Figure 3B),
and appeared to be stabilized there by detergent molecules added during crystalliza-
tion. This implies that there is much more conformational fluidity to the lid structure
than a simple case of ‘open in lipid, closed in water’. Rather, it appears that the nature
of the substrate, and the presence and chemical features of other molecules in the
4.4 Crystallization and determination of three-dimensional structure 75
Figure 3B. (See color plate, page XVIII).