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Lysozyme from T4 bacteriophage is comprised of two domains that are both involved in binding substrate. Although wild-type lysozyme has been exclusively crystallized in a closed form that is similar to the peptidoglycan-bound conformation, a more open structure is thought to be required for ligand binding. To determine the relative arrangement of domains within T4 lysozyme in the solution state, dipolar couplings were measured in several different dilute liquid crystalline media by solution NMR methods. The dipolar coupling data were analyzed with a domain orientation procedure described previously that utilizes high-resolution X-ray structures. The cleft between the domains is significantly larger in the average solution structure than what is observed in the X-ray structure of the ligand-free form of the protein (∼17° closure from solution to X-ray structures). A comparison of the solution domain orientation with X …