RESUMO
As a highly regulated enzyme at the core of nitrogen metabolism, glutamine synthetase has been studied intensively. We review structural and functional studies of both bacterial and eukaryotic glutamine synthetases, with emphasis on enzymatic inhibitors.
Assuntos
Inibidores Enzimáticos/química , Glutamato-Amônia Ligase/química , Sequência de Aminoácidos , Animais , Bactérias , Sítios de Ligação , Catálise , Células Eucarióticas , Regulação da Expressão Gênica , Glutamato-Amônia Ligase/antagonistas & inibidores , Glutamato-Amônia Ligase/metabolismo , Humanos , Dados de Sequência Molecular , Plantas , Alinhamento de Sequência , Relação Estrutura-Atividade , Especificidade por SubstratoRESUMO
The etiologic agent of tuberculosis, Mycobacterium tuberculosis, has been shown to secrete the enzyme glutamine synthetase (TB-GS) which is apparently essential for infection. Four crystal forms of a recombinant TB-GS were grown. The one chosen for synchrotron X--ray data collection belongs to space group P212121 with unit-cell dimensions 208 x 258 x 274 A, yielding 2.4 A resolution data. A Matthews number of 2.89 A3 Da-1 is found, corresponding to 24 subunits of molecular mass 1300 kDa in the asymmetric unit. From earlier work, the structure of Salmonella typhimurium GS, which is 51% identical in sequence to TB-GS, is known to be dodecameric with 622 symmetry. Self-rotation calculations on the TB-GS X-ray data reveal only one set of sixfold and twofold axes of symmetry. A Patterson map calculated from the native X-ray data confirms that there are two dodecamers in the asymmetric unit, having both their sixfold and twofold axes parallel to one another.
Assuntos
Proteínas de Bactérias/química , Glutamato-Amônia Ligase/química , Mycobacterium tuberculosis/enzimologia , Cristalização , Cristalografia por Raios XRESUMO
In the course of refining atomic protein structures, one often encounters difficulty with molecules that are unusually flexible or otherwise disordered. We approach the problem by combining two relatively recent developments: simultaneous refinement of multiple protein conformations and highly constrained refinement. A constrained Langevin dynamics refinement is tested on two proteins: neurotrophin-3 and glutamine synthetase. The method produces closer agreement between the calculated and observed scattering amplitudes than standard, single-copy, Gaussian atomic displacement parameter refinement. This is accomplished without significantly increasing the number of fitting parameters in the model. These results suggest that loop motion in proteins within a crystal lattice can be extensive and that it is poorly modeled by isotropic Gaussian distributions for each atom.
