Inactivation of 3-oxoacyl synthetase activity 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.

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.

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.

Mapping of acyl carrier domain within the subunit of type I bacterial fatty acid synthetase

A fluorescent thiol reagent, N-(7-dimethylamino-4-methylcoumarinyl) maleimide, was used to label the acyl carrier site of the bacterial fatty acid synthetase from Brevibacterium ammoniagenes. The reagent bound preferentially to the 4'-phosphopantetheine thiol group of the acyl carrier domain and irreversively inactivated the enzyme. The modified enzyme was cleaved by proteinases for the mapping of the labeled site. The fluorescent fragment was readily detected on a polyacrylamide gel after electrophoresis. The region of 45 kDa containing the 4'-phosphopantetheine was located on the polypeptide at around two-thirds of the full length from the N-terminal.

Inhibitory effects of sulfhydryl reagents on acetyl-CoA carboxylase from rat mammary gland

Rat mammary gland acetyl-CoA carboxylase (acetyl-CoA:carbon dioxide ligase (ADP forming), EC 6.4.1.2) is rapidly and irreversibly inactivated by micromolar concentrations of S-(4-bromo-2,3-dioxobutyl)-CoA (BDB-CoA) or p-hydroxymercuribenzoate (PHMB). Inhibition of both half reactions (i.e., the biotin carboxylation and the carboxyltransferase) catalyzed by acetyl-CoA carboxylase closely parallels loss in overall activity (malonyl-CoA synthesis). The presence of a substrate or product (acetyl-CoA, ATP, ADP, Pi) or inhibitor (palmitoyl-CoA) does not protect the enzyme from inhibition caused by BDB-CoA or PHMB. On the other hand, citrate, an activator of acetyl-CoA carboxylase, affords substantial protection against inhibition by BDB-CoA and PHMB. Covalent modification by BDB-CoA or PHMB appears to lock acetyl-CoA carboxylase in an inactive conformation (15-30 S) that is unable to undergo citrate-induced self-association into the catalytically competent polymeric form.

Specific modification of the condensation domain of fatty acid synthase and the determination of the primary structure of the modified active site peptides

Fatty acid synthase from the uropygial gland of goose was inactivated by iodoacetamide with a second-order rate constant of 1.3 M-1 S-1 at pH 6.0 and 25 degrees C. Of the seven component activities of the synthase, only the condensation activity was significantly inhibited by iodoacetamide modification. Since preincubation of the enzyme with acetyl-CoA, but not with malonyl-CoA, protected the enzyme from inactivation by iodoacetamide, it is suggested that iodoacetamide probably modified the primer-binding thiol group at the condensation active site. Determination of the stoichiometry of modification was done using [1-14C]iodoacetamide that was purified by high-performance liquid chromatography. Graphical analysis of the data showed that binding of 1.2 carboxamidomethyl groups per subunit of fatty acid synthase would result in complete inhibition of the enzyme activity, suggesting that there is one condensation domain per subunit of fatty acid synthase. Analysis of the tryptic peptide map of the enzyme that was modified with [1-14C]iodoacetamide in the presence and absence of acetyl-CoA revealed that acetyl-CoA prevented the labeling of a major radioactive peptide and a minor radioactive peptide. These two peptides were purified by high-performance liquid chromatography. Amino acid analysis of these two peptides revealed that the major radioactive peptide contained S-carboxymethylcysteine while the minor radioactive peptide did not. However, the latter peptide contained beta-alanine, suggesting that this peptide was from the acyl carrier protein segment of fatty acid synthase and that the iodoacetamide treatment resulted in modification of the pantetheine thiol, although to a lower extent than the primer-binding thiol. The sequence of the primer-binding active site peptide from the condensation domain was H2N-Gly-Pro-Ser-Leu-Ser-Ile-Asp- Thr-Ala-Cys(carboxamidomethyl)-X-Ser-Ser-Leu-Met-Ala-Leu-Glu-Asn-A la-Tyr-Lys- COOH, the first reported sequence of the condensation active site from a vertebrate fatty acid synthase. The acyl carrier protein segment showed extensive sequence homology with the acyl carrier protein of Escherichia coli, particularly in the vicinity of the phosphopantetheine attachment, and the sequence was H2N-Asp-Val-Ser-Ser-Leu- Asn-Ala-Asp-Ser-Thr-Leu-Ala-Asp-Leu-Gly-Leu-Asp-Ser(4'-phosphopanteth ein e) -Leu-Met-Gly-Val-Glu-Val-Arg-COOH.