C.-I. Brändén

The VitalBook e-book of Introduction to Protein Structure, Second Edition is inly available in the US and Canada at the present time. To purchase or rent please visit https://store.vitalsource.com/show/9780815323051Introduction to Protein Structure provides an account of the principles of protein structure, with examples of key proteins in their bio

The three-dimensional structure of the electron transport protein thioredoxin-S2 from E. coli has been determined from a 2.8 A resolution electron density map. The molecule is built up of a central core of three parallel and two antiparallel strands of pleated sheet surrounded by four helices. Thr residues involved in the active center 14-membered disulfide ring of thioredoxin form a protrusion between one of the helices and the middle strand of the pleated sheet. This region of the molecule, comprising two parallel strands joined by the protrusion and a helix, is structurally very similar to corresponding functionally important regions in the nucleotide-binding domains of flavodoxin and the dehydrogenases. The molecule has about 75% of the residues in well-defined secondary structures. The structure indicates that the carboxy-terminal third of the molecule forms an independent folding unit consisting of two strands of antiparallel pleated sheet and a terminal alpha-helix. This agress with the noncovalent reconstitution experiments from thioredoxin peptide fragments. Thioredoxin is an example of a protein with the active center located on a protrusion rather than in a cleft, thus demonstrating the existence of male proteins.

Horse liver alcohol dehydrogenase was crystallized from an equilibrium mixture containing predominantly NAD+ and p-bromobenzyl alcohol. X-ray diffractometer data to a resolution of 2.9 A were collected and used to compute electron density maps with phases calculated from the isomorphous enzyme . NADH . dimethyl sulfoxide complex, which has been refined to an R value of 25.6%. The electron density maps were readily interpreted in a graphics display system. Both subunits of the dimer bind coenzyme and alcohol in essentially the same manner; there is no evidence of asymmetry between subunits. The bromophenyl group is accommodated in a large hydrophobic pocket that has the side chain of Leu-116 rotated into a different position than in the complex with dimethyl sulfoxide. The alcohol oxygen is directly ligated to the catalytic zinc atom. The zinc is tetracoordinate and there is no room for a water molecule to make the zinc pentacoordinate. A hydrogen-bonded system formed with the hydroxyl groups of the alcohol, Ser-48 and nicotinamide ribose (2'), and the imidazole of His-51 may provide a proton relay system that links the buried alcohol to solvent. The insertion of the coenzyme's hydroxyl group into this system appears to install the catalytically active species. The observed structure has the pro-R hydrogen on C1 of the alcohol pointing away from C4 of the nicotinamide ring. This is probably a nonproductive complex that easily becomes productive by a rapid rotation of the alcohol to put the pro-R hydrogen within 3 A of C4 of the nicotinamide ring and in position for a direct transfer of hydrogen. A model of the productive complex readily explains the stereospecificity of hydride transfer observed for ethanol.