Small-angle x-ray scattering on malate synthase from baker's yeast. Considerations on the effects of bound ligands.

Binding of the substrates to the anisometric enzyme causes slight changes of some mole­ cular parameters (Zipper and Durchschlag, Eur. J. Biochem., 1978). Estimations based on several plausible assumptions allow a separation of the experimentally observed effects into effects caused by the substrates or by the enzyme in the enzyme-substrate complexes or by binding of buffer molecules. The results show that the observed changes of molecular parameters are primarily due to structural changes in the enzyme molecule. From the changes in the radius of gyration upon substrate binding, the binding sites of the substrates may be localized to be at a radial distance of 5.3 nm from the centre of the enzyme particle. Binding of one or both of the substrates induces different structural changes of the enzyme particle. On formation of the [enzyme •acetylC oA ], [enzyme • glyoxylate], or [enzyme-pyruvate] complexes, an increase of the short axis by 4.5 + 1% occurs, while the formation of the [enzyme-acetyl-CoA-pyruvate] complex does not change this axis significantly. At the same time, binding of the substrates leads to a decrease of the long axes of the enzyme particle by 2.0 ± 0.2%, independent on the kind of the complex formed. These changes of the axes correspond to an increase of the axial ratio by 6.7 ± 1% on formation of the [enzyme-acetyl-CoA], [enzyme• glyoxylate], or [enzyme-pyruvate] complexes and by 2 .7.% on formation of the [enzyme-acetyl-CoA-pyruvate] complex, i. e. in all cases to a decrease of anisometry of the enzyme particle.