STRUCTURE OF THE CHITINASE FROM JACK BEAN
Hahn, M. 1, Hennig, M 2, Schlesier, B. 3 and Höhne, W. 1
1 Institut für Biochemie der Charité,
Humboldt-Universität, Monbijoustr.2, 10117 Berlin, Germany
2 F. Hoffmann-La Roche AG, PRPI-S 65/308
CH-4070 Basel, Switzerland
3 IPK Gatersleben, Corrensstr. 3, 06466
Gatersleben, Germany
Chitinases hydrolyze the ß-1,4-glycosidic bond between N-Acetylglucosamines in chitin, one of the most abundant biomolecules. Chitinases in plants function as defence proteins against fungi. The structure of barley chitinase is known to 2.0 A. (Song et al., 1996). It is a a-helical protein with three disulfide bridges and a putative substrate-binding groove along the molecule.
The chitinase from jack bean (canavalia ensiformis) was
purified from seeds and crystallized in space group P61. The
X-ray data are 92% complete, the Rmerge is 5.9% for
24446 independent reflections. It is a monomeric enzyme of 243
amino acids with 66% identity in sequence to the chitinase from
barley and was solved with molecular replacement. The refined
structure has a Rfactor of 18.7% (R free:
22.2%) at a resolution of 1.8 A. R.m.s. deviations from standard
bond lengths/bond angles are 0.008 A/1.43°, respectively. 143
water molecules were located. The overall fold and the secondary
structures are the same as in the barley enzyme. 10 helices were
found, ranging in length from 4 to 18 amino acids. The
superposition of all C-positions of the two enzymes gives a
r.m.s. deviation of 0.98 A. The conserved amino acids in
comparison to the barley chitinase are located in the interior of
the protein and in the binding groove, whereas mutated positions
are mostly found on the protein surface. The mutated positions
have a larger structural deviation than the conserved ones (1.23
A against 0.81 A). Also the deviations in loop regions are larger
than in the helical parts (1.15 A against 0.60 A). This is in
accordance with the observation that the rate of conserved amino
acids is higher in the helices (74.0%) than in the loops (61.6%).
Song, HK & Suh, SW. Refined structure of the chitinase from barley seeds at 2.0 A resolution. Acta Cryst. D52, 289-298, (1996).