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Biophys J, June 2000, p. 3240-3251, Vol. 78, No. 6
*Istituto di Scienze Fisiche and Istituto Nazionale per la Fisica
della Materia, Università, I-60131 Ancona, Italy;
Facoltà di Agraria and Istituto Nazionale per la
Fisica della Materia, Università, I-60131 Ancona, Italy;
MPI-Polymerforschung, D-55021 Mainz, Germany;
§Dipartimento di Biologia, Università di Bologna,
I-40100 Bologna, Italy; and ¶Dipartimento di Biochimica e
Biologia Molecolare, Università di Ferrara, I-44100 Ferrara,
Italy
Small-angle neutron and x-ray scattering experiments have
been performed on type 2 tissular transglutaminase to characterize the
conformational changes that bring about Ca2+ activation and
guanosine triphosphate (GTP) inhibition. The native and a proteolyzed
form of the enzyme, in the presence and in the absence of the two
effectors, were considered. To describe the shape of transglutaminase
in the different conformations, a Monte Carlo method for calculating
small-angle neutron scattering profiles was developed by taking into
account the computer-designed structure of the native transglutaminase,
the results of the Guinier analysis, and the essential role played by
the solvent-exposed peptide loop for the conformational changes of the
protein after activation. Although the range of the neutron scattering
data is rather limited, by using the Monte Carlo analysis, and because
the structure of the native protein is available, the distribution of
the protein conformations after ligand interaction was obtained.
Calcium activation promotes a rotation of the C-terminal with respect
to the N-terminal domain around the solvent-exposed peptide loop that
connects the two regions. The 
angle between the longest axes of the
two pairs of domains is found to be above 50°, larger than the value of 35° calculated for the native transglutaminase. On the other
hand, the addition of GTP makes possible conformations characterized by
angles lower than 34°. These results are in good agreement with
the proposed enzyme activity regulation: in the presence of GTP, the
catalytic site is shielded by the more compact protein structure, while
the conformational changes induced by Ca2+ make the active
site accessible to the substrate.
Biophys J, June 2000, p. 3240-3251, Vol. 78, No. 6
© 2000 by the Biophysical Society 0006-3495/00/06/3240/12 $2.00
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