Ligand-induced conformational changes in tissue transglutaminase: Monte Carlo analysis of small-angle scattering data

Small-angle neutron and x-ray scattering experiments have been performed on type 2 tissular transglutaminase to characterize the conformational change s that bring about Ca2+ activation and guanosine triphosphate (GTP) inhibit ion. The native and a proteolyzed form of the enzyme, in the presence and i n the absence of the two effecters, were considered. To describe the shape of transglutaminase in the different conformations, a Monte Carlo method fo r calculating small-angle neutron scattering profiles was developed by taki ng into account the computer-designed structure of the native transglutamin ase, the results of the Guinier analysis, and the essential role played by the solvent-exposed peptide loop for the conformational changes of the prot ein after activation. Although the range of the neutron scattering data is rather limited, by using the Monte Carlo analysis, and because the structur e of the native protein is available, the distribution of the protein confo rmations 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 psi ang le between the longest axes of the two pairs of domains is found to be abov e 50 degrees, larger than the psi value of 35 degrees calculated for the na tive transglutaminase. On the other hand, the addition of GTP makes possibl e conformations characterized by psi with the proposed enzyme activity regu lation: 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.