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of all structures of the inactive unphosphorylated protein kinases (see review [17]).
Page 219
The various displacements of the conserved upper domain of the catalytic cores of various kinases
documented by crystallographic work suggest that this is the important underlying mechanism of
catalysis. Analysis of crystal contacts of various kinases is however required to define the extent of
displacement due to the lattice forces. In the case of cAPK, the displacement as observed for mammalian
cAPK in the cubic crystal form is due to the intermolecular interaction in the lattice [20]. Analysis of the
two crystal structures of the cell cycle-controlling kinases clearly shows two binding modes of ATP. In
the inactive state without cyclin, ATP binding of its triphosphate moieties is different from that in the
active form with cyclin bound. The major difference is the re-arrangement upon cyclin binding of the
conserved Lys33-Glu51 pair, which is responsible for the binding of the α and β phosphates of ATP.
III. Crystallographic Analysis of Substrate Specificities of Individual Kinases
The most important contribution of subsequent crystallographic studies has been the confirmation of the
structural homology extending through the members of this family of enzymes. The crystal structures of
CDK-2 [1], ERK-2 [5], twitchin [2], insulin receptor kinase [3], phosphorylase kinase, CK-1 [6], along
with structure of calcium/calmodulin-dependent protein kinase I [8] provide solid proof for the structural
conservation of the catalytic core in the family. This is further confirmed by the recent structure of the
active complex of CDK2/cyclin, which shows that Lys33-Glu51 pair is at a distance of 3.0 Å [7] as
predicted in the model of CDK-2 based on the cAPK structure [21]. The structure of the complex has
also confirmed the binding of cyclin to helix C and to the upper lobe, demonstrating the mechanism of
activation by cyclin that results in bridging the invariant residues into the common network of distances
required by structural homology of the protein kinase catalytic core.
The crystallographic analysis of the structural homology of protein kinases can now be carried out using
structures of various kinases to find a common search model to be used in molecular replacement
methods (J.M. Sowadski and R. Karlsson private communication). The structures of various kinases
have been used as search models to solve the structure of the cAPK using cAPK diffraction data. The
best search model consists of fragments of the catalytic core excluding the activation loop, inserts, and
upper lobe due to rotational motion observed in each structure. A structure solution has been found for
several protein kinases using this selected model as shown in Figure 2. One of the most critical aspects
of this analysis is the presence of the structurally
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