Human immunodeficiency viral protease is catalytically active as a fusion protein: characterization of the fusion and native enzymes produced in Escherichia coli.

Processing of the gag and pol gene precursor proteins of retroviruses is essential for the production of mature infectious virions. The processing is directed by a viral protease that itself is part of these precursors and is presumed to cleave itself autocatalytically. To facilitate study of this process, the protease was produced as a fusion protein in Escherichia coli. In this construct, the 10,793-Da protease was preceeded by two copies of a modified IgG binding domain derived from protein A. The IgG binding domain was linked to the protease by an Asp-Pro peptide bond which could not be cleaved by the viral protease. A dimer of the 25,400-Da fusion protein was catalytically active, specifically cleaving a substrate peptide at the correct Tyr-Pro bond. Thus, the fusion protein could serve as a model of the viral gag-pol polyprotein. The finding that the fusion protein was catalytically active supports the suggestion that a gag-pol dimer can initiate a proteolytic cascade after budding of the immature virus. The fusion protein also provided a source of authentic protease. The protease was released from the fusion construct by incubation with formic acid, cleaving the Asp-Pro linkage which had been inserted between the IgG binding domain and the protease.

Butyrate kinase from Clostridium acetobutylicum.

Crude extracts of Clostridium acetobutylicum contain a butyrate kinase of high specific activity (5.2 mumol/min/mg of protein). The enzyme has been purified 77-fold in a six-step procedure to a specific activity of 402 mumol/min/mg of protein. The purified butyrate kinase showed a single band with a molecular weight of 85,000 on nondenaturing polyacrylamide gradient gel electrophoresis. Separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the enzyme is a dimer of two apparently identical subunits with molecular weights of 39,000. The pH optimum for the reaction in the butyryl phosphate-forming direction is 7.5, and the pI of the kinase is 5.6. The amino acid composition of the enzyme is also reported. It contains no tryptophan and is low in sulfur-containing amino acids. The kinase has a broad substrate specificity and exhibits its highest relative activities with butyrate and valerate. Butyrate kinase is rapidly inactivated at 50 degrees C in the absence of a fatty acid substrate. Although a reducing agent was required for maximum activity, treatment with several sulfhydryl-modifying agents failed to inhibit the enzyme.

Solubilization of a membrane-bound diol dehydratase with retention of EPR g = 2.02 signal by using 2-(N-cyclohexylamino)ethanesulfonic acid buffer.

A procedure for solubilization of the oxygen-labile, membrane-bound diol dehydratase from Clostridium glycolicum with retention of enzymatic activity is described. The procedure involves sonication of crude membrane preparations anaerobically in 0.1 M 2-(N-cyclohexylamino)ethane-sulfonic acid (CHES) buffer (pH 8.6-9.0) containing 2 mM dithiothreitol. The addition of dimethylsulfoxide (30%) and lysophosphatidylcholine (0.15 mg/ml) to the solubilization buffer resulted in a 10-fold increase in recovery of solubilized diol dehydratase activity. After ultracentrifugation, an overall recovery of 50% of the activity initially present in the crude membrane preparations was achieved. Active membrane preparations and the solubilized enzyme exhibited an EPR signal at g = 2.02. Both enzyme activity and EPR signal were sensitive to oxygen and the radical scavengers, NH2OH and hydroxy-urea.

Diol metabolism and diol dehydratase in Clostridium glycolicum.

Levels of the five enzymes involved in the fermentation of 1,2-ethanediol and 1,2-propanediol in the strictly anaerobic bacterium, Clostridium glycolicum, were investigated. All enzymes with the exception of the first enzyme in the pathway, diol dehydratase, were found to be constitutive, stable to exposure to oxygen, and present in the cytosol. Diol dehydratase was found to be extremely oxygen sensitive and strongly associated with the cell membrane. Treatment with ionic and nonionic detergents, butanol, phospholipase A2, or osmotic shock procedures failed to solubilize any diol dehydratase activity. Limited proteolysis using subtilisin released small amounts of activity. Diol dehydratase was found to be specific for 1,2-ethanediol and 1,2-propanediol and required the addition of a reducing agent for maximal activity. The enzyme was strongly inhibited by low concentrations of EDTA, ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, o-phenanthroline, hydroxylamine, hydroxyurea, and sulfhydryl reagents. Addition of adenosylcobalamin or high levels of intrinsic factor did not affect the reaction rate. Irradiation with light also did not inhibit the enzyme activity. These results suggest that the catalytic mechanism of diol dehydratase from C. glycolicum does not involve a cobamide coenzyme.

Simultaneous single-step purification of thiolase and NADP-dependent 3-hydroxybutyryl-CoA dehydrogenase from Clostridium kluyveri.

Thiolase and NADP-dependent 3-hydroxybutyryl-CoA dehydrogenase from Clostridium kluyveri were purified by ion-exchange chromatography to near homogeneity in a simultaneous, single-step procedure. The yield of both enzymes was greater than 80%. Thiolase was purified approximately 8-fold with sp act 115 units/mg, whereas 3-hydroxybutyryl-CoA dehydrogenase was purified 14-fold with sp act 292 units/mg. Isoelectric points of the enzymes are 7.7 for thiolase and 7.8 for 3-hydroxybutyryl-CoA dehydrogenase. Milligram quantities of each of these enzymes are readily obtained from this fatty acid-producing organism.

Intermediary Metabolism in Clostridium acetobutylicum: Levels of Enzymes Involved in the Formation of Acetate and Butyrate.

The levels of seven intermediary enzymes involved in acetate and butyrate formation from acetyl coenzyme A in the saccharolytic anaerobe Clostridium acetobutylicum were investigated as a function of time in solvent-producing batch fermentations. Phosphate acetyltransferase and acetate kinase, which are known to form acetate from acetyl coenzyme A, both showed a decrease in specific activity when the organism reached the solvent formation stage. The three consecutive enzymes thiolase, beta-hydroxybutyrylcoenzyme A dehydrogenase, and crotonase exhibited a coordinate expression and a maximal activity after growth had ceased. Only low levels of butyryl coenzyme A dehydrogenase activity were found. Phosphate butyryltransferase activity rapidly decreased after 20 h from 5 to 11 U/mg of protein to below the detection limit (1 mU/mg). Butyrate no longer can be formed, and the metabolic flux may be diverted to butanol. Butyrate kinase showed a 2.5- to 10-fold increase in specific activity after phosphate butyryltransferase activity no longer could be detected. These results suggest that the uptake of acetate and butyrate during solvent formation can not proceed via a complete reversal of the phosphate transferase and kinase reactions. The activities of all enzymes investigated as a function of time in vitro are much higher than the metabolic fluxes through them in vivo. This indicates that none of the maximal activities of the enzymes assayed is rate limiting in C. acetobutylicum.

Isolation of a selenium-containing thiolase from Clostridium kluyveri: identification of the selenium moiety as selenomethionine.

Clostridium kluyveri grown in the presence of 1 muM Na2(75)SeO3 produces a thiolase that copurifies with 75Se. Based on several criteria, the selenium moiety in this protein is selenomethionine. The 75Se-labeled amino acid in acid hydrolysates of the radioactive protein cochromatographed with authentic selenomethionine on an amino acid analyzer and on TLC plates in acidic and basic solvents. Incubation with S-adenosylmethionine synthetase and ATP converted the 75Se-labeled amino acid to a radioactive basic product that was indistinguishable from authentic Se-adenosylselenomethionine by ion exchange and TLC. The native selenoenzyme, Mr 155,000-158,000, is composed of four subunits of Mr 38,000-40,000. Thiolase of similar molecular weight that is less acidic and lacks selenium is also produced by C. kluyveri. The factors that control the relative levels of the two enzymes in the cell have not been identified.