The structure of a thermochromic complex, bis(N,N-diethylethylenediamine)copper(II) tetrafluoroborate [CuL&F4)d having the lowest transition temperature (24°C) amongst its isomorphous counterparts (C104-salts of Cu and Ni) has been determined at 22°C thus offering a unique opportunity to study near transition structural parameters. The Cu atom at the crystallographic centre of inversion has square planar coordination from four N-atoms of two Fntrosymmetrically related ligands [Cu-N(secondary) = 2.061(5), Cu-Nbrimary) = 2.003(6) A, bite angle N-Cu-N = 84.6(2)"], the diethyl group orienJations insulate the metal ion from anion coordination [closest approach (Cu.. .F3 = 3.621(4)A)]. The thermal anisotropies of the chelate ring carbons are normal and do not indicate any disorder, although the structure is determined at a temperature (~22°C) very close to the transition. The highest thermal vibrations are shown by the BF4-anion and then to some extent by the terminal C-atoms of the diethyl groups. Factors involved in the thermochromic phase transition in these systems are discussed.
Metallohydrolases catalyse some of the most important reactions in biology and are targets for numerous chemotherapeutic agents designed to combat bacterial infectivity, antibiotic resistance, HIV infectivity, tumour growth, angiogenesis and immune disorders. Rational design of inhibitors of these enzymes with chemotherapeutic potential relies on detailed knowledge of the catalytic mechanism. The roles of the catalytic transition ions in these enzymes have long been assumed to include the activation and delivery of a nucleophilic hydroxy moiety. In the present study, catalytic intermediates in the hydrolysis of L-leucyl-L-leucyl-L-leucine by Vibrio proteolyticus aminopeptidase were characterized in spectrokinetic and structural studies. Rapid-freeze-quench EPR studies of reaction products of L-leucyl-L-leucyl-L-leucine and Co(II)-substituted aminopeptidase, and comparison of the EPR data with those from structurally characterized complexes of aminopeptidase with inhibitors, indicated the formation of a catalytically competent post-Michaelis pre-transition state intermediate with a structure analogous to that of the inhibited complex with bestatin. The X-ray crystal structure of an aminopeptidase-L-leucyl-L-leucyl-L-leucine complex was also analogous to that of the bestatin complex. In these structures, no water/hydroxy group was observed bound to the essential metal ion. However, a water/hydroxy group was clearly identified that was bound to the metal-ligating oxygen atom of Glu152. This water/hydroxy group is proposed as a candidate for the active nucleophile in a novel metallohydrolase mechanism that shares features of the catalytic mechanisms of aspartic proteases and of B2 metallo-beta-lactamases. Preliminary studies on site-directed variants are consistent with the proposal. Other features of the structure suggest roles for the dinuclear centre in geometrically and electrophilically activating the substrate.
We have determined steady-state rate constants and net rate constants for the chemical steps in the catalytic pathway catalyzed by the E370D mutant of glutaryl-CoA dehydrogenase and compared them with those of the wild-type dehydrogenase. We sought rationales for changes in these rate constants in the structure of the mutant cocrystallized with the alternate substrate, 4-nitrobutyric acid. Substitution of aspartate for E370, the catalytic base, results in a 24% decrease in the rate constant for proton abstraction at C-2 of 3-thiaglutaryl-CoA as the distance between C-2 of the ligand and the closest carboxyl oxygen at residue 370 increases from 2.9 A to 3.1 A. The net rate constant for flavin reduction due to hydride transfer from C-3 of the natural substrate, which includes proton abstraction at C-2, to N5 of the flavin decreases by 81% due to the mutation, although the distance increases only by 0.7 A. The intensities of charge-transfer bands associated with the enolate of 3-thiaglutaryl-CoA, the reductive half-reaction (reduced flavin with oxidized form of substrate), and the dienolate following decarboxylation are considerably diminished. Structural investigation suggests that the increased distance and the change in angle of the S-C1(=O)-C2 plane of the substrate with the isoalloxazine substantially alter rates of the reductive and oxidative half-reactions. This change in active site geometry also changes the position of protonation of the four carbon dienolate intermediate to produce kinetically favorable product, vinylacetyl-CoA, which is further isomerized to the thermodynamically stable normal product, crotonyl-CoA.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.