Crystal Structure Analysis of the Tetragonal Crystal Form and Preliminary Molecular Model of Pig‐Heart Citrate Synthase

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Abstract
The crystal structure of pig heart citrate synthase was analyzed at 0.35‐nm resolution. Chain tracing was possible and an initial molecular model constructed. The dimensions of the dimer molecule (located on a crystallographic diad) are 7.5 × 6.0 × 9.0 nm. The chain folding is characterized by the predominance of helices and the absence of sheet structure. The electron density accounts for 355 residues per monomer, so that about 80 residues must be disordered in the crystal. The disordered segment is probably N‐terminal. The ordered part consists of two closely associated domains, a large domain with 300 residues and a C‐terminal domain of 55 residues consisting of 3 (anti)parallel helices. The large domain is built from 12 helical segments, some of which are buried in the interior of the molecule. Inhibitor binding studies with citrate and CoA revealed citrate binding sites but showed no electron density for CoA. It is suggested that CoA binds to the disordered, flexible N‐terminal domain. Experiments of limited proteolysis with trypsin showed that under certain conditions a segment of Mr 9000 is cleaved off selectively. The remaining 35000‐Mr part is dimeric.