The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K 12. Relaxations of the allosteric equilibrium

The structural basis for allosteric inhibition of a threonine-sensitive aspartokinase

The commitment step to the aspartate pathway of amino acid biosynthesis is the phosphorylation of aspartic acid catalyzed by aspartokinase (AK). Most microorganisms and plants have multiple forms of this enzyme, and many of these isofunctional enzymes are subject to feedback regulation by the end products of the pathway. However, the archeal species Methanococcus jannaschii has only a single, monofunctional form of AK. The substrate l-aspartate binds to this recombinant enzyme in two different orientations, providing the first structural evidence supporting the relaxed regiospecificity previously observed with several alternative substrates of Escherichia coli AK ( Angeles, T. S., Hunsley, J. R., and Viola, R. E. (1992) Biochemistry 31, 799-805 ). Binding of the nucleotide substrate triggers significant domain movements that result in a more compact quaternary structure. In contrast, the highly cooperative binding of the allosteric regulator l-threonine to multiple sites on this dimer of dimers leads to an open enzyme structure. A comparison of these structures supports a mechanism for allosteric regulation in which the domain movements induced by threonine binding causes displacement of the substrates from the enzyme, resulting in a relaxed, inactive conformation.

LOOKING BACK

My encounter with Jacques Monod has shaped my scientific career. After a short incursion in the biochemistry of strict anaerobes, and after elucidating the biosynthetic pathway leading from aspartate to threonine in Escherichia coli, I joined his laboratory. With him and Howard Rickenberg, I discovered the stereospecific permeability of galactosides and amino acids (permeases). After this intermezzo, I returned to the analysis of biosynthetic pathways and of their regulation by allosteric feedback inhibition and repression in E. coli. Among others, my studies led to the discovery of the tryptophan and methionine repressors, to the incorporation of amino acid analogues in proteins, including selenomethionine (which much later led to progress in protein crystallography), to the definition of isofunctional and multifunctional enzymes, and to the elucidation of the primary structure of most of the enzymes leading to threonine and methionine.

Aspartokinase-homoserine dehydrogenase I from Escherichia coli: pH and chemical modification studies of the kinase activity

The pH variation of the kinetic parameters was examined for the kinase activity of the bifunctional enzyme aspartokinase--homoserine dehydrogenase I isolated from Escherichia coli. The V/K profile for L-aspartic acid indicates the loss of activity upon protonation of a cationic acid type group with a pK value near neutrality. Incubation of the enzyme with diethyl pyrocarbonate at pH 6.0 results in a loss of enzymic activity. The reversal of this reaction by neutral hydroxylamine, the appearance of a peak at 242 nm for the inactivated enzyme, and the observation of a pK value of 7.0 obtained from variation of the inactivation rate with pH all suggest that enzyme inactivation occurs by modification of histidine residues. The substrate L-aspartic acid protects one residue against inactivation, which implies that this histidine may participate in substrate binding or catalysis. Activity loss was also observed at high pH due to the ionization of a neutral acid group with a pK value of 9.8. The reactions of AK-HSD I with N-acetylimidazole and tetranitromethane have been investigated to obtain information about the functional role of tyrosyl residues in the enzyme. The acylation of tyrosines leads to inactivation of the enzyme, which can then be fully reversed by treatment with hydroxylamine. Incubation of the enzyme with tetranitromethane at pH 9.5 also leads to rapid inactivation, and the substrates of the kinase reaction provide substantial protection against inactivation. However, three tyrosines are protected by substrates, implying a structural role for these amino acids.

Reversible dissociation of aspartokinase I/homoserine dehydrogenase I from Escherichia coli K 12. The active species is the tetramer

Dimers of aspartokinase I/homoserine dehydrogenase I from Escherichia coli K 12 have been isolated under very mild conditions. The dimers which cannot be distinguished from the tetramers by their kinetic properties, reassociate in the presence of potassium ions or L-aspartate. The selective sensitivity of aspartokinase I/homoserine dehydrogenase I to mild proteolytic digestion of dimers has been used to probe the reassociation reaction under the conditions of aspartokinase assay. We demonstrate that rapid reassociation occurs and that the protein species present in the assay when dimers are used to test the activity is tetrameric. These results confirm the previously proposed model for the subunit association of aspartokinase I/homoserine dehydrogenase I.

Active enzyme gel chromatography. I. Experimental aspects

Transport properties of active enzyme species can be studied effectively by layering a small band of enzyme-containing sample on a gel chromatographic column previously saturated with substrate. The column is optically scanned at successive time intervals to yield profiles representing the appearance of chromophoric product or disappearnce of chromophoric substrate. These profiles permit determination of the specific activity and rate of transport of the active species. Initial studies on mechanic of the technique establish the feasibility of accurately determining transport properties of active enzyme species chromatographed on gel columns. Illustrative results are presented for L-glutamate dehydrogenase and for homoserine dehydrogenase studied in both forward and reverse reactions. It is shown that the partititon cross sections derived from the rates of motion of catalytic activity are the same as those determined by equilibrium saturation experiments which directly measure the degree of partitioning by the protein. These results establish the validity of the technique for a variety of future studies. Active enzyme gel chromatography appears comparable in precision to the active enzyme sedimentation technique at current stages of development.

Cobalt(III) affinity-labeled aspartokinase. Formation of substrate and inhibitor adducts

The kinase active site of the aspartokinase-homoserine dehydrogenase enzyme complex of Excherichia coli has been affinity labeled both with substrates aspartate and adenosine triphosphate and feedback inhibitor threonine. Co(III) exchange-inert adducts of aspartokinase and inhibitor or substrates were produced in situ by oxidation of Co(II) with H2O2. Emzyme-Co(III)-adenosine 5'-triphosphate (ATP), enzyme-Co(III)-aspartate, and enzyme-Co(III)-threonine ternary adducts were produced in this manner. The formation of the enzyme-Co(III)-threonine adduct leads us to conclude that threonine inhibits the kinase activity of this enzyme complex by binding in the first coordination sphere of the catalytic metal ion cofactor, a conclusion which is consistent with evidence derived from previous nuclear magnetic resonance data obtained in this laboratory. The quaternary adducts formed by H2O2 oxidation in the presence of aspartokinase, Co(II), ATP, aspartate, and threonine comprised a mixture of both ezyme-Co(III)-ATP-aspartate and enzyme-Co(III)-ATP-threonine adducts. The formation of the quaternary aspartate-containing adduct was unexpected, since the presence of threonine was expected to prevent access of the aspartate to the active site; most significantly however, the the sum of the numbers of aspartate plus threonine molecules incorporated per active site is one. We believe that this shows direct steric overlap between the metal-adjacent binding sites for aspartate and threonine. Aspartate or threonine can not occupy the kinase active site simultaneously; this conclusion is consistent with the direct competitive inhibition of aspartate by threonine observed in steady-state kinetic studies.

Molecular interactions and structure as analysed by fluorescence relaxation spectroscopy

Spectroscopic probes have become powerful tools in analysing the correlation between structure and function of biological macromolecules. Though these spectral methods cannot give as circumstantial information about the anatomy of a biological structure as, for example, X-ray diffraction they can provide information on physical properties at defined loci in a macromolecule which are not accessible by other techniques. Most important, spectroscopic studies have provided means to study the dynamics of structural changes and interactions in time domains spanning from nanoseconds and less up to infinite time.

A quantitative description of end-plate currents

1. End-plate currents have been studied in glycerol-treated frog sartorius nerve-muscle preparations with the voltage clamp technique.2. The effects of temperature on the decay rate of end-plate currents were investigated over a temperature range from 10 to 30.5 degrees C. The Q(10) for the decay constant of end-plate currents depends somewhat on membrane potential; at - 100 mV the decay constant has a Q(10) of 2.7.3. Peak end-plate current depends non-linearly on membrane potential with a decreasing slope conductance associated with hyperpolarization.4. The ;instantaneous' voltage-current relationship for end-plate channels was determined by causing step changes in membrane potential during end-plate current flow. This relationship appears to be linear.5. The interaction of acetylcholine with its receptor is viewed as being analogous to the first step in enzymic catalysis. On this view, acetylcholine binds to its receptor and induces a conformational change which is responsible for opening end-plate channels. By analogy to the first steps in the catalytic sequence of enzymes, the binding step is very rapid, almost diffusion-limited, and the conformational change is rate-limiting.6. Equations describing this process have been derived. Expressions for the rate constants have also been derived by considering changing dipole moments of the transmitter-receptor complex associated with the conformational change. As the transmitter-receptor complex is in the membrane field, different conformational states have different energies, and the rate of conformational change thus depends on membrane potential. The equations thus derived are shown to account adequately for the time course of end-plate conductance change.