These structural changes seem to have been caused mainly by
the bulkiness of the amino acid side-chains. As shown in Fig. 2,
among the four alterations, the S to G alteration generates the
greatest change in bulkiness. As seen in the orientation of the Nterminal
end of the S111G model (Fig. 3), this single mutation
presumably generated an unexpectedly large effect on the global
conformation of the enzyme. It is strongly suggested that the
conformation around His 124 was also altered, as described above.
The N to S mutationwas the second largest change in the bulkiness
(Fig. 2). Presumably, N85S and E135Q also have a less exposed His at
124 as compared to the other variants, except for S111G (Fig. 3).
These conformational characteristics can also well explain relatively
low enzymatic activities of these variants. In the case of
E135Q, however, the change in bulkiness was small. Thus, it is not
likely to be the major cause of the conformational change observed
for E135Q. Change in polarity may have been the cause in this case.
Although the variant QS also contains the S to G mutation at 111, the
N to S mutation at 85 and the E to Q mutation at 135, the changes in
the global conformation and the conformation of the CAS (colored
blue) were expected to be rather small. In QS, the four altered
amino acid residues were from those at the corresponding positions
in non-carnivorous plants. Thus, Gly (111), Ser (85), Gln (135)
and Ala (115) may have functioned together in sustaining the