2,4,6-Trinitrobenzenesulfonate labels an essential amino group near the bound phosphate at the catalytic site of mitochondrial F1-ATPase

Near stoichiometric, irreversible inactivation of bacterial collagenases by o-chloranil (3,4,5,6-tetrachloro-1,2-benzoquinone)

The hydrogen-abstracting quinone derivative 3,4,5,6-tetrachloro-1,2-benzoquinone (o-chloranil) caused a strong, near stoichiometric, irreversible inactivation of the collagenases from Bacillus cereus, Clostridium histolyticum and Achromobacter iophagus. p-Chloranil was a weaker inactivator. o-Chloranil reacted rapidly with a site that affected substrate binding. Amino acid analyses of native and totally inactivated enzymes, and the pH-profile of inactivation suggest that the dissociated form of a tyrosine residue was modified.

Biochemical studies on a novel vanadate- and molybdate-sensitive acid phosphatase from human epidermis

A novel vanadate- and molybdate-sensitive human skin epidermal acid phosphatase was purified and characterized. The enzyme was extracted from epidermal sheets with a 0.1% Triton X-100 solution buffered at pH 7.0. The purification procedure consisted of molecular permeation chromatography on Sephadex G-200 followed by chromatography on hydroxylapatite using an ammonium sulfate gradient. The molecular weight of the enzyme was 82,000 and the isoelectric point was at pH 5.6. At the optimum pH (5.1) the enzyme hydrolyzed most rapidly 1-naphthyl phosphate (Km = 0.28 mM) and 4-nitrophenyl phosphate (Km = 0.28 mM). In general, the best substrates had an aromatic leaving group. Fluoride (Ki = 39 microM; noncompetitive) and phosphate (competitive) inhibited by binding to different binding sites of the enzyme. The most potent inhibitors were vanadate (Ki = 1.9 X 10(-6)M), tungstate (Ki = 1.4 X 10(-7)M), and molybdate (Ki = 2.0 X 10(-9)M). Chemical modification and kinetic experiments suggested that the activity of the enzyme is based on imidazole, tyrosyl, and carboxyl groups. Benzoyl peroxide was a relatively potent inhibitor (Ki = 5.0 X 10(-5)M; noncompetitive). This enzyme resembled the prostatic acid phosphatase with regard to substrate specificity, inhibition characteristics, and functional groups.

The stoichiometry of proton translocation through H+-ATPase of rat-liver mitochondria

The ratio of the number of protons transported directly by H+-ATPase of intact mitochondria and that of the hydrolyzed ATP (the proton translocation quotient) is determined. A special kinetic method which makes possible determination of ATPase and H+-translocase activities in the same experiment has been used to estimate the proton translocation quotient for mitochondrial H+-ATPase. The quotient is found to be, on average, equal to 3.31 mol/mol. It is shown that the protonophore 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile can greatly decrease the proton translocation quotient. This supports our assumption concerning a labile coupling between H+-translocase and ATPase in mitochondria [I. A. Gorskaya, K. F. Sholtz, S. A. Moreva, A. V. Kotelnikova (1979) Biochemistry (Engl. Transl. Biokhimiya) 44, 765-770]. A decrease in the translocation quotient in the presence of the protonophore is likely to be due to the ability of this uncoupler to take back the translocated protons into the mitochondrial matrix before their release into the medium. An electrostatic model of the molecular mechanism of H+-translocase and ATPase coupling in the H+-ATPase complex is discussed. The model is in agreement with the results obtained: transport of more than two protons per each hydrolyzed ATP molecule, and variable efficiency of the process.

Changes in chemical properties of mitochondrial adenosinetriphosphatase upon removal of tightly bound nucleotides

The removal of tightly bound nucleotides from mitochondrial F1-ATPase was found to affect the inhibition by ADP and chemical reactivity toward 7-chloro-4-nitro-2,1,3-benzoxadiazole (NBD-C1) and sulfhydryl reagents. Preincubation of nucleotide-depleted F1 with 40 microM ADP in the presence of ethylenediaminetetraacetic acid (EDTA) resulted in a 51% inhibition of the steady-state level of ATPase activity whereas only a 25% inhibition was observed for native F1. Both partially inhibited states of the enzyme could be reversed by the subsequent addition of ATP. Measurement of [14C]ADP binding to nucleotide-depleted F1 in the presence of EDTA reveals three equivalent ADP binding sites with a Kd of 0.45 microM, and a fourth site of lower affinity. The sulfhydryl reagents 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM) were found to inhibit the ATPase activity of nucleotide-depleted F1 but not native F1 or nucleotide-depleted F1 in the presence of ADP or ATP. Polyacrylamide gel electrophoresis of nucleotide-depleted F1 labeled with [14C]NEM gave a 2-fold increase in incorporation into the (alpha + beta) subunits and a 7-fold increase in label in the gamma subunit after 90 min compared to when ADP was present during the reaction. ADP binding to the noncatalytic sites enhanced the rate of inhibition of nucleotide-depleted F1 by NBD-C1 about 2-fold while retarding the subsequent intramolecular transfer from an essential phenol group to an amino group about 2.8-fold. The results suggest a conformational change in F1 caused by changes in nucleotide--protein interaction at the noncatalytic sites.

Functional groups at the catalytic site of BF1 adenosinetriphosphatase from Escherichia coli

The rates of inactivation of BF1 adenosinetriphosphatase (BF1-ATPase) from Escherichia coli by 7-chloro-4-nitro-2,1,3-benzoxadiazole, 1-fluoro-2,4-dinitrobenzene, 2,4,6-trinitrobenzenesulfonate, phenylglyoxal, and N,N'-dicyclohexylcarbodiimide have been measured in the presence and absence of various concentrations of inorganic phosphate, ADP, ATP, or magnesium ion. Dissociation equilibrium constants and rate constants for the labeling reactions have been deduced from a quantitative treatment of the kinetic data. The results suggest that the essential Tyr, Lys, Arg, and Glu or Asp residues are probably located at the catalytic site of BF1-ATPase and that in addition to steric interference, the effect of charge interaction should also be considered in interpreting the kinetic data on the protection of BF1-ATPase by substrate molecules against inactivation by the above labeling reagents. Examination of the relative values of the rate constants for the labeling reactions in the presence and absence of inorganic phosphate, ADP, ATP, or magnesium ion, respectively, and the effect of NBD label on the rates of labeling of the essential guanidinium, amino, and carboxyl groups suggest that the arrangement of these four functional groups at the catalytic site of BF1 may be similar to that previously proposed for MF1-ATPase from bovine heart; namely, the essential amino group and the unusually reactive phenol group are probably located near the bound inorganic phosphate or the gamma-phosphate group of the bound ATP, the essential guanidinium group is probably located nearer to the alpha- or beta-phosphate group than to the gamma-phosphate group of the bound ATP or the bound inorganic phosphate, and the essential carboxylate group is probably complexed with a magnesium ion which it shares with the bound inorganic phosphate.