Small-Angle X-Ray Scattering on Malate Synthase from Baker's Yeast. The Native Substrate-Free Enzyme-Substrate Complexes

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Abstract
Malate synthase from baker''s yeast was investigated in solution by the small-angle X-ray scattering technique. Size, shape and structure of the native substrate-free enzyme and of various enzyme-substrate complexes were determined. As the enzyme was found to be rather unstable against X-rays, several precautions as well as sophisticated evaluation procedures had to be adopted to make sure that the results were not influenced by radiation damage. These included use of low primary intensity, short time of measurement, the presence of high concentrations of dithiothreitol, combined use of the conventional slit-collimation system and the new cone-collimation system. For the native substrate-free enzyme the following molecular parameters were established: radius of gyration R = 3.96 .+-. 0.02 nm, maximum particle diameter D = 11.2 .+-. 0.6 nm, radius of gyration of the thickness Rt = 1.04 .+-. 0.04 nm, Mr [relative MW] = 187,000 .+-. 3000, correlation volume Vc = 338 .+-. 5 nm3, hydration x = 0.35 .+-. 0.02 g/g, mean intersection length .hivin.l = 5.0 .+-. 0.2 nm. Comparison of the experimental scattering curve with theoretical curves for various models showed that the enzyme is equivalent in scattering to an oblate ellipsoid of revolution rather than to a circular cylinder. The semiaxes of this ellipsoid are a = b = 6.06 nm and c = 2.21 nm. Thus with an axial ratio of about 1:0.36 the enzyme is of very anisometric shape. Binding of the substrates (acetyl-CoA, glyoxylate) or the substrate analog pyruvate causes slight structural changes of the enzyme. These changes are reflected mainly by a slight decrease of the radius of gyration (0.3-1.3%, as established both with the slit-smeared and the desmeared curves). Concomitantly there occurs a decrease of the maximum particle diameter and an increase of the radius of gyration of the thickness. These changes imply an increase of the axial ratio by 2.2-6.9% i.e., substrate binding induces a decrease of anisometry. While the particle volume appears to be unchanged on binding glyoxylate or its analog pyruvate, binding of acetyl-CoA causes slight changes of this parameter. In a similar manner the binding of acetyl-CoA leads to a slight enhancement of the MW; this increase corresponds to the binding of 2.7 .+-. 1 molecules of acetyl-CoA.

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