The crystal structure of Thermus thermophilus seryl-transfer RNA synthetase, a class 2 aminoacyl-tRNA synthetase, complexed with a single tRNA(Ser) molecule was solved at 2.9 A resolution. The structure revealed how insertion of conserved base G20b from the D loop into the core of the tRNA determines the orientation of the long variable arm, which is a characteristic feature of most serine specific tRNAs. On tRNA binding, the antiparallel coiled-coil domain of one subunit of the synthetase makes contacts with the variable arm and T psi C loop of the tRNA and directs the acceptor stem of the tRNA into the active site of the other subunit. Specificity depends principally on recognition of the shape of tRNA(Ser) through backbone contacts and secondarily on sequence specific interactions.

The dimeric form of the kinesin motor and neck domain from rat brain with bound ADP has been solved by X-ray crystallography. The two heads of the dimer are connected via a coiled-coil alpha-helical interaction of their necks. They are broadly similar to one another; differences are most apparent in the head-neck junction and in a moderate reorientation of the neck helices in order to adopt to the coiled-coil conformation. The heads show a rotational symmetry (approximately 120 degrees) about an axis close to that of the coiled-coil. This arrangement is unexpected since it is not compatible with the microtubule lattice. In this arrangement, the two heads of a kinesin dimer could not have equivalent interactions with microtubules.

The crystal structure of plant acetohydroxy acid isomeroreductase complexed with NADPH, two magnesium ions and a herbicidal transition state analog determined at 1.65 Å resolution nones and sulfonylureas, which act at very low dose rates, investigations showed that selective and highly potent inhibitors of acetohydroxy acid isomeroreductase, (EC Acetohydroxy acid isomeroreductase catalyzes the con-1.1.1.86), the second enzyme in the pathway, such as version of acetohydroxy acids into dihydroxy valerates.2-dimethylphosphinoyl-2-hydroxy acetic acid (Hoe 704) This reaction is the second in the synthetic pathway of and N-hydroxy-N-isopropyloxamate (IpOHA), exhibit the essential branched side chain amino acids valine herbicidal activity (Schultz et al., 1988; Aulabaugh and and isoleucine. Because this pathway is absent from Schloss, 1990). However, because these compounds bind animals, the enzymes involved in it are good targets very slowly to the enzyme and behave as competitive for a systematic search for herbicides. The crystal inhibitors with respect to the acetohydroxy acid isomerostructure of acetohydroxy acid isomeroreductase comreductase substrates, their herbicidal effectiveness is much plexed with cofactor NADPH, Mg 2ϩ ions and a competsmaller than that exhibited by non-competitive inhibitors itive inhibitor with herbicidal activity, N-hydroxy-Ntargeting acetohydroxy acid synthase (Dumas et al., isopropyloxamate, was solved to 1.65 Å resolution and 1994a). Therefore, biochemical and structural characterrefined to an R factor of 18.7% and an R free of 22.9%.ization of plant acetohydroxy acid isomeroreductase is The asymmetric unit shows two functional dimers needed for the design of new molecules inhibiting this related by non-crystallographic symmetry. The active enzyme. site, nested at the interface between the NADPHBesides this potential agrochemical importance, acebinding domain and the all-helical C-terminus domain, tohydroxy acid isomeroreductase from both plants and shows a situation analogous to the transition state. It microorganisms presents several unique catalytic features. contains two Mg 2ϩ ions interacting with the inhibitorThe enzyme catalyzes an unusual two-step reaction molecule and bridged by the carboxylate moiety of an (Figure 1) consisting of an alkyl migration in which aspartate residue. The inhibitor-binding site is well the substrate, either 2-acetolactate (AL) or 2-aceto-2-adjusted to it, with a hydrophobic pocket and a polar hydroxybutyrate (AHB), is converted to 3-hydroxy-3-region. Only 24 amino acids are conserved among methyl-2-oxobutyrate or 3-hydroxy-3-methyl-2-oxopenknown acetohydroxy acid isomeroreductase sequences tanoate, followed by a NADPH-dependent reduction to and all of these are located around the active site.give 2,3-dihydroxy-3-isovalerate or 2,3-dihydroxy-3-Finally, a 140 amino acid region, present in plants but methylvalerate respectively. The enzyme-catalyzed reacabsent from other species, was found to make up most tion obeys an ordered mechanism in which NADPH ...