Role of cysteine and 4'-phosphopantetheine in the inactivation of pigeon liver fatty acid synthetase by S-(4-bromo-2,3-dioxobutyl)-coenzyme A

Inhibition of pigeon liver fatty acid synthetase by specific modification of lysine residues with 2,4,6-trinitrobenzenesulphonic acid

Pigeon liver fatty acid synthetase was inactivated irreversibly by 2,4,6-trinitrobenzenesulphonic acid (TNBS). Biphasic inactivation of the enzyme was observed with the inhibitor. NADPH provided protection to the enzyme against inactivation by TNBS and the extent of protection increased with NADPH concentration indicating that the essential lysine residues are present at the NADPH binding site. The stoichiometric results with TNBS showed that 4 mol of lysine residues are modified per mole of fatty acid synthetase upon complete inactivation. The rapid reaction of two amino groups per enzyme molecule led to the loss of 60% of the enzyme activity. These approaches suggested that two lysine residues present at the active site are essential for the enzymatic activity of fatty acid synthetase.

Inactivation of enoyl-CoA reductase in pigeon liver fatty acid synthetase by pyridoxal 5'-phosphate: evidence for the presence of one lysine residue at the active site

Pigeon liver fatty acid synthetase (FAS) was rapidly inactivated by pyridoxal 5'-phosphate (PLP). Assays of the partial activities of the PLP-treated synthetase showed that only the enoyl-CoA reductase was decreased significantly. The inactivation of both the overall activity and enoyl-CoA reductase activity of FAS by PLP could be reversed by dialysis or dilution but not by reduction with sodium borohydride. Malonyl-CoA and acetyl-CoA did not protect the enzyme, whereas NADPH provided 68% protection against PLP-inactivation indicating that PLP modified lysine residues present at or near the co-enzyme binding site. PLP-treated enzyme after reduction with sodium borohydride exhibited fluorescence with a maximum at 397 nm (irradiation at 325 nm). Stoichiometric analysis showed that modification of four lysine residues per enzyme molecule resulted in complete inactivation of the overall and enoyl-CoA reductase activities of FAS. NADPH prevented the inactivation by protecting two of these lysine residues from modification, suggesting the presence of two essential lysine residues per enzyme molecule. These results are consistent with the hypothesis that each subunit of the enzyme contains an enoyl-CoA reductase domain in which a lysine residue, at or near the active site, interacts with NADPH.

Evidence for the essential histidine at the NADPH binding site of enoyl-CoA reductase domain of pigeon liver fatty acid synthetase

Pigeon liver fatty acid synthetase was inactivated by stoichiometric concentrations of diethylpyrocarbonate (DEP). The second order rate constant for the loss of synthetase activity was similar to the value for enoyl-CoA reductase indicating that ethoxyformylation destroys the ability of the enzyme to reduce the unsaturated acyl intermediate, without significant effect on beta-ketoacyl reductase activity. NADPH provided protection to the enzyme against inactivation by DEP indicating that essential histidine residues are present at the active site. DEP-modified enzyme showed a characteristic absorption maxima at 240 nm confirming the formation of ethoxyformic histidine. The reversal of inactivation by hydroxylamine and a pKa value of 7.0 obtained from the pH-rate profile for inactivation again confirmed the specificity of DEP for histidine. Stoichiometric results showed that two moles of histidine residue per mole of enzyme are essential for the activity of FAS.

Three new potential cAMP affinity labels. Inactivation of human platelet low Km cAMP phosphodiesterase by 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate

Three new analogues of cAMP have been synthesized and characterized: 2-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (2-BDB-TcAMP), 2-[(3-bromo-2-oxopropyl)thio]-adenosine 3',5'-cyclic monophosphate (2-BOP-tcAMP), and 8-[(4-bromo-2,3-dioxobutyl)thio]adenosine 3',5'-cyclic monophosphate (8-BDB-TcAMP). The bromoketo moiety has the ability to react with the nucleophilic side chains of several amino acids, while the dioxobutyl group can interact with arginine. These cAMP analogues were tested for their ability to inactivate the low Km (high affinity) cAMP phosphodiesterase from human platelets. The 2-BDB-TcAMP and 2-BOP-TcAMP were competitive inhibitors of cAMP hydrolysis by the phosphodiesterase with Ki values of 0.96 +/- 0.12 and 0.70 +/- 0.12 microM, respectively. However, 2-BDB-TcAMP and 2-BOP-TcAMP did not irreversibly inactivate the phosphodiesterase at pH values from 6.0 to 7.5 and at concentrations up to 10 mM. These results indicate that although the 2-substituted TcAMP analogues bind to the enzyme, there are no reactive amino acids in the vicinity of the 2-position of the cAMP binding site. In contrast, incubation of the platelet low Km cAMP phosphodiesterase with 8-BDB-TcAMP resulted in a time-dependent, irreversible inactivation of the enzyme with a second-order rate constant of 0.031 +/- 0.009 min-1 mM1. Addition of the substrates, cAMP and cGMP, and the product, AMP, to the reaction mixture resulted in marked decreases in the inactivation rate, suggesting that the inactivation was due to reaction at the active site of the phosphodiesterase.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for catalytic site cysteine and histidine by chemical modification of beta-hydroxy-beta-methylglutaryl-coenzyme A reductase

S-(4-Bromo-2,3-dioxobutyl)-coenzyme A inactivates both yeast and rat liver beta-hydroxy-beta-methylglutaryl-coenzyme A reductase. The inactivation is irreversible, complete in 15 s, and proportional to the concentration of the reagent. beta-Hydroxy-beta-methylglutaryl-CoA provides protection against inactivation, whereas NADPH does not. Inactivation is attributed to reaction with an essential cysteine at the beta-hydroxy-beta-methylglutaryl-CoA binding site. Experiments with other active site-directed reagents confirm the involvement of a cysteine and support the presence of an active-site histidine, but rule out the participation of arginine or serine.

Affinity labeling of NADP+-specific isocitrate dehydrogenase by a new fluorescent nucleotide analogue, 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate

A new reactive fluorescent adenine nucleotide analogue has been synthesized and characterized: 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate (BDB-T epsilon ADP). This compound reacts irreversibly with NADP+-specific isocitrate dehydrogenase. Biphasic kinetics of inactivation are observed that can be described in terms of a fast initial phase of inactivation resulting in partially active enzyme of 8-10% residual activity, followed by a slower phase leading to total inactivation. NADPH protects completely against the fast phase of the reaction, indicating that modification occurs at the coenzyme binding site, whereas isocitrate with MnSO4 protects totally against the slow phase of reaction. The inactivation rate constants for both phases exhibit nonlinear dependence on BDB-T epsilon ADP concentration, consistent with the formation of a reversible complex with the enzyme prior to irreversible modification. Covalent incorporation of BDB-T epsilon ADP is limited and specific; only 0.99 mol of reagent/mol of subunit is incorporated when the enzyme is 98% inactivated in the absence of ligands. A maximum incorporation of 0.5 mol of reagent/mol of subunit is obtained in the presence of isocitrate and MnSO4, while incorporation in the presence of NADPH equals the difference between the incorporation in the absence of ligands and that measured in the presence of isocitrate and MnSO4. It appears that 0.5 mol of reagent/mol of subunit is responsible for the fast phase of inactivation and the remaining incorporation causes the slow phase. Under all conditions used in this study, isocitrate dehydrogenase has been shown to exist as a dimer by analytical ultracentrifugation and by cross-linking with dimethyl suberimidate followed by analysis on polyacrylamide gels in the presence of sodium dodecyl sulfate. It is proposed that, in the fast phase of inactivation, 2-[(4-bromo-2,3-dioxobutyl)thio]-1,N6-ethenoadenosine 2',5'-bisphosphate reacts at the coenzyme binding site of one subunit of dimeric isocitrate dehydrogenase, causing complete inactivation of the modified subunit and substantial inactivation of the other subunit. This new reagent is likely to have general applicability as an affinity label for other NADP+ binding enzymes.

6-[(4-bromo-2,3-dioxobutyl)thio]-6-deaminoadenosine 5'-monophosphate and 5'-diphosphate: new affinity labels for purine nucleotide sites in proteins

Two new adenine nucleotide analogues have been synthesized and characterized: 6-[(4-bromo-2,3-dioxobutyl)thio]-6-deaminoadenosine 5'-monophosphate and 5'-diphosphate. The bromoketo and dioxobutyl moieties have the ability to react with the nucleophilic side chains of several amino acids, as well as with arginine. 6-[(4-Bromo-2,3-dioxobutyl)thio]-6-deaminoadenosine 5'-monophosphate reacts irreversibly with rabbit muscle pyruvate kinase, causing inactivation. Addition of ADP to the reaction mixture (in the presence of Mg2+) markedly decreases the rate of inactivation. Pig heart NAD-dependent isocitrate dehydrogenase is allosterically activated by ADP, which reduces the Km for isocitrate. 6-[(4-Bromo-2,3-dioxobutyl)thio]-6-deaminoadenosine 5'-diphosphate reacts irreversibly with isocitrate dehydrogenase, causing, rapidly, a loss of the ability of ADP to increase the initial velocity of assays conducted at low isocitrate concentrations and, more slowly, inactivation. Addition of ADP to the reaction mixture (in the presence of Mn2+) protects this enzyme against the loss of allosteric activation. It is proposed that the 6-[(4-bromo-2,3-dioxobutyl)thio]-6-deaminoadenine nucleotides react at the active site of pyruvate kinase and at the ADP activating site of isocitrate dehydrogenase and that these compounds may have general applicability as affinity labels of catalytic and regulatory adenine nucleotide sites in proteins.

Inhibition of the condensing component of chicken liver fatty acid synthase by iodoacetamide and 5,5'-dithiobis-(2-nitrobenzoic acid)

Chicken liver fatty acid synthase is inhibited by the thiol-modifying reagents 5,5'-dithiobis-(2-nitrobenzoic acid) and iodoacetamide. Total inactivation of the activity for fatty acid synthesis requires the modification of about 8 of the nearly 50 freely accessible thiol groups per molecule. The differential binding of iodo[14C]acetamide to phenylmethylsulphonyl fluoride-modified enzyme in the absence and in the presence of excess acetyl-CoA shows complete modification of one cysteine-SH site of the condensing enzyme and partial modification of the pantetheine-SH site for a total of approx. 1.4 mol of iodoacetamide bound per mol of enzyme. The reaction of the enzyme with 5,5'-dithiobis-(2-nitrobenzoic acid) generates disulphide cross-links for each molecule of the reagent added, but 95% of these cross-links are intrasubunit. Both the iodoacetamide- and 5,5'-dithiobis-(2-nitrobenzoic acid)-modified species catalyse all the component partial reactions of fatty acid synthesis except the condensation reaction. The results obtained with iodoacetamide show that in the dimeric fatty acid synthase modification of one cysteine-SH condensing site and/or one pantetheine-SH site per dimer is sufficient to affect inhibition of condensing activity and the activity for fatty acid synthesis, and are in accord with a recently proposed model for the mechanism of action of animal fatty acid synthases [Kumar (1982) J. Theor. Biol. 95, 263-283].

Irreversible inhibition of fatty acid synthase from rat mammary gland with S-(4-bromo-2,3-dioxobutyl)-CoA. Effect on the partial reactions, protection by substrates and stoichiometry studies

Fatty acid synthase from lactating rat mammary gland is rapidly and irreversibly inhibited by S-(4-bromo-2,3-dioxobutyl)-CoA. Of the seven partial reactions catalysed by the enzyme, the inhibition of the overall catalytic activity is closely paralleled only by inhibition of the beta-oxoacyl synthase (condensing) partial reaction. Three partial reactions. Beta-oxoacyl reductase, beta-hydroxyacyl dehydratase and enoyl reductase, are inhibited to a modest degree. The three partial reactions known to involve an acyl-CoA/CoA-binding site, acetyl acyltransferase, malonyl acyltransferase and palmitoyl thioesterase, are not inhibited by S-(4-bromo-2,3-dioxobutyl)-CoA. The modification process does not cause the enzyme to dissociate into catalytically incompetent monomers. Stoichiometric studies suggest that approx. 6 mol of reagent are incorporated per mol of totally inhibited enzyme (dimer). The formation of acylated enzyme from either acetyl-CoA or malonyl-CoA protects the enzyme equally well against S-(4-bromo-2,3-dioxobutyl)-CoA. Also, pretreatment of the enzyme with 5,5'-dithiobis-(2-nitrobenzoic acid), a thiol-specific reagent reported to block essential thiol groups in the condensing partial reaction, protects against inhibition by the reagent. On the other hand, the presence of up to 770 microM-S-acetonyl-CoA or dethio-CoA does not protect the enzyme from irreversible inhibition. Together, the results suggest that the primary inhibitory process is a bimolecular reaction resulting in alkylation of essential thiol groups in the condensing partial reaction: this process does not require the obligatory formation of a Michaelis-Menten complex of enzyme and reagent before the alkylation reaction.