Biochemical characterization and crystal structure determination of human heart short chain L-3-hydroxyacyl-CoA dehydrogenase provide insights into catalytic mechanism.

Human heart short chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD) catalyzes the oxidation of the hydroxyl group of L-3-hydroxyacyl-CoA to a keto group, concomitant with the reduction of NAD+ to NADH, as part of the beta-oxidation pathway. The homodimeric enzyme has been overexpressed in Escherichia coli, purified to homogeneity, and studied using biochemical and crystallographic techniques. The dissociation constants of NAD+ and NADH have been determined over a broad pH range and indicate that SCHAD binds reduced cofactor preferentially. Examination of apparent catalytic constants reveals that SCHAD displays optimal enzymatic activity near neutral pH, with catalytic efficiency diminishing rapidly toward pH extremes. The crystal structure of SCHAD complexed with NAD+ has been solved using multiwavelength anomalous diffraction techniques and a selenomethionine-substituted analogue of the enzyme. The subunit structure is comprised of two domains. The first domain is similar to other alpha/beta dinucleotide folds but includes an unusual helix-turn-helix motif which extends from the central beta-sheet. The second, or C-terminal, domain is primarily alpha-helical and mediates subunit dimerization and, presumably, L-3-hydroxyacyl-CoA binding. Molecular modeling studies in which L-3-hydroxybutyryl-CoA was docked into the enzyme-NAD+ complex suggest that His 158 serves as a general base, abstracting a proton from the 3-OH group of the substrate. Furthermore, the ability of His 158 to perform such a function may be enhanced by an electrostatic interaction with Glu 170, consistent with previous biochemical observations. These studies provide further understanding of the molecular basis of several inherited metabolic disease states correlated with L-3-hydroxyacyl-CoA dehydrogenase deficiencies.

Crystal structure of cellular retinoic acid binding protein I shows increased access to the binding cavity due to formation of an intermolecular beta-sheet.

A recombinant form of murine apo-cellular retinoic acid binding protein I (apo-CRABPI) has been purified and crystallized at pH 5.0, and the crystal structure has been refined to an R-factor of 19.6% at a resolution of 2.7 A. CRABPI binds all-trans retinoic acid and some retinoic acid metabolites with nanomolar affinities. Coordinates of the holo form of CRABP were not available during the early stages of the study, and in spite of numerous homologs of known structure, phases were not obtainable through molecular replacement. Instead, an interpretable electron density map was obtained by multiple isomorphous replacement methods after improvement of the heavy-atom parameters with density modified trial phases. Two molecules of apo-CRABPI occupy the P3121 asymmetric unit and are related by pseudo 2-fold rotational symmetry. Unique conformational differences are apparent between the two molecules. In all of the family members studied to date, there is a lack of hydrogen bonds between two of the component beta-strands resulting in a gap in the interstand hydrogen bonding pattern. In the crystallographic dimer described here, a continuous intermolecular beta-sheet is formed by using this gap region. This is possible because of an 8 A outward maximum displacement of the tight turn between the third and fourth beta-strands on one of the molecules. The result is a double beta-barrel containing two apo-CRABPI molecules with a more open, ligand-accessible binding cavity, which has not been observed in other structures of a family of proteins that bind hydrophobic ligands.

Crystal structures of holo and apo-cellular retinol-binding protein II.

Apo and holo-cellular retinol-binding protein II have been crystallized, and their crystal structures have been determined to 2.1 A and 1.9 A respectively. The apo and holo-crystals have different but related triclinic space groups. The X-ray phases for both structures were determined using the molecular replacement method. The crystal co-ordinates were refined to an R-factor of 0.200 for apo, and 0.173 for holo-cellular retinol-binding protein II. The holo and apo-models have nearly the same tertiary structures. Cellular retinol-binding protein II consists of a ten-stranded anti-parallel beta-barrel with the ligand binding cavity within the barrel. Two alpha-helices cover the open end of the beta-barrel making it almost solvent inaccessible. A single portal large enough to admit a water molecule was observed opening into the binding cavity. Exogenously added retinol was found within the cavity of each holo-cellular retinol-binding protein II molecule. Each retinol was surrounded by both polar and non-polar residues. The hydroxyl group of the bound retinol hydrogen bonds to the amide group of glutamine 108. The overall conformation of the bound retinol was derived from the four different molecules of holo-cellular retinol-binding protein II present in the triclinic form. The four copies of bound retinol had essentially the same conformation as found in crystalline retinaldehyde.

Purification and characterization of methylmalonyl-CoA mutase from Ascaris lumbricoides.

Methylmalonyl CoA mutase from Ascaris lumbricoides has been purified to homogeneity. The mutase is homogeneous as judged by equilibrium sedimentation and polyacrylamide gel electrophoresis. The worm mutase is a glycoprotein with a mol. wt of 147,000 +/- 3500 composed of two identical or very similar subunits. One molecule of adenosylcobalamin is tightly bound to each subunit. The mutase from Ascaris is not affected by exposure to light, cyanide ion or intrinsic factor and is not inhibited by iodoacetate and rho-hydroxymercuribenzoate. The kinetic constants of this mutase for (R,S)methylmalonyl CoA are Km = 4.2 X 10(-5) M and Vmax = 4.73 mumol/mg/min.