Proc. NatL Acad Sci. USA Vol. 78, No. 9, pp. 5440-5444, September 1981 Biochemistry
Chemical synthesis of acyl thioesters of acyl carrier protein with native structure (lipid synthesis/fatty acid synthesis/chemical modification/imidazole catalysis/sulfhydryl groups) JOHN E. CRONAN, JR., AND AMY L. KLAGES Department of Microbiology, University of Illinois, Urbana, Illinois 61801 Communicated by P. K. Stumpf, May 29, 1981 ABSTRACT The acyl carrier protein (ACP) ofEscherichia coli flash evaporated. The residue was dissolved in the appropriate was converted to acyl-ACP by imidazole-catalyzed S-acylation solvent and crystallized. with N-acylimidazole. The acylation was specific to the sulfhydryl Method B. A solution of 10 mmol of the fatty acid (V) in 10 group; no acylation oftyrosine or amino groups ofthe protein oc- ml of dry tetrahydrofuran was added to a solution of 10 mmol curred. The acyl-ACP substrates synthesized had a native struc- ofN,N'-carbonyldiimidazole (VI) in the same solvent. This mix- ture as determined by gel electrophoresis, hydrophobic chroma- ture was refluxed until C02 evolution was complete (5-15 min) tography, and enzymatic activity. N-Acylimidazoles are readily and then flash evaporated. The residue was dissolved in ben- synthesized and permit preparation of those acyl-ACP substrates zene, filtered to remove the bulk ofthe imidazole formed, and that cannot be produced enzymatically. then crystallized from the appropriate solvent. The short-chain N-acylimidazoles ( 0 of phenol and is 60- to 80-fold more rapid than the acetylation 1 of the amino groups of glycine and glycylglycine. Although these data gave some promise of a specific S-acy- lation ofACP by N-acylimidazole, the presence ofthe five amino NI groups (a terminus and 4e) and the tyrosine residue (23) meant that either a low conversion of ACP to acyl-ACP or some acy- A) RC Cl C I D lation of residues other than the prosthetic group must be tol- erated. The solution to this problem was the observation by H H Cl- Jencks and Carriuolo (22) that the free base of imidazole can II m IV specifically increase the rate of S-acylation. Jencks and Carriuolo (22) demonstrated that imidazole buf- 0 Na=\ 0 s=N fers greatly increase the rate of S-acetylation of 2-mercaptoeth- B) RCOH + [N -C- NLj L II+IM+CO anol by N-acetylimidazole whereas enhancement ofrate ofacet- ylation ofphenol is very small. Later results showed no catalysis V VI by imidazole of the acylation of phenol or a number of substi- FIG. 1. Structure and synthesis of N-acylimidazole. N-Acylimid- tuted phenols (24). Jencks and Carriuolo (22) also observed sig- azoles (I) can be readily synthesized by either of two routes starting nificant imidazole catalysis of the acetylation of amino groups from the acid chloride (method A) or the free fatty acid (method B). by N-acetylimidazole but this catalysis was largely suppressed at pH values below neutrality. The imidazole-catalyzed reaction Na EDTA. ACP and an equimolar amount of dithioerythritol of N-acetylimidazole with sulfhydryl groups is an example of were added and the final concentrations of the components general base catalysis and is thus pH independent, depending were adjusted to: ACP, 1-2.5 mg/ml; detergent, 10%; and N- only on the concentration of imidazole free base (22). The de- acylimidazole, 5-20 mM (final pH, 6.50). The final N-acylimid- creased rate ofS-acylation by N-acylimidazole observed at lower azole concentrations used were 5mM (C10 or C12), 10mM (C14), pH therefore can be offset by increasing the imidazole concen- or 20 mM (C16 or C18). After incubation for 4 hr at 220C (with tration ofthe buffer. From the data ofJencks and Carriuolo (22) occasional vigorous mixing), the solutions were applied to and the assumption that the nucleophilic groups of ACP and DEAE-cellulose and freed of the nonprotein components; the those of the model compounds have similar reactivities, the protein was eluted as described by Rock and Garwin (12). ratio of the reactivity of the ACP sulfhydryl group relative to The yields ofacyl-ACPs based on ACP were essentially quan- the sum of the reactivities of the other groups (one phenolic titative forchain lengths'12. The yields ofmyristoyl-ACP were hydroxyl group and five amino groups) could be calculated. The about 90%, whereas the yields of the C16 and C18 acids varied ratio calculated for the acylation of 0.1 mM ACP dissolved in widely (30-90%), probably due to variability in the dispersion 1 M imidazole at pH 6.5 with 2.5 mM N-acetylimidazole was of these insoluble compounds. The only unsaturated acyl-ACP about 50, and thus the specific S-acylation ofACP by N-acylim- synthesized, cis-3-decenoyl-ACP, was prepared as described idazole seemed possible. Moreover, the actual specificity for for octanoyl-ACP and gave a similar yield. S-acylation of ACP seemed likely to be greater than that cal- Analytical Techniques. Polyacrylamide gel electrophoresis culated from the model compound data because the amino was done as described by Rock et al. (6) and Rock and Cronan groups ofACP appear to be involved in salt linkages (4) and the (16). High-pressure chromatography was performed on a tyrosine residue appears to be in a nonreactive environment Waters apparatus with a column (50 X 0.46 cm) of Perisorb RP8 (5). (Merck) (C. 0. Rock, personal communication). ACP concen- Studies with Model Compounds. N-Acylimidazoles are read- trations were determined by absorbance at 280 nm (17), by a ily synthesized by starting with either the free fatty acid or the microbiuret procedure (8), or by use of ACP radioactively la- acid chloride (14, 15). The yields are essentially quantitative and beled (by biosynthetic incorporation) in the prosthetic group the concentrations ofN-acylimidazole can be determined either (16). spectrophotometrically or by conversion to the hydroxamate Materials. ACP was purified from E. coli K-12 as described (14). N-Acylimidazoles are readily hydrolyzed at extremes ofpH by Rock and Cronan (16, 17). Organic chemicals were from and are unstable even at neutral pH (25). Imidazole free base Aldrich. Triton X-100 and Brij 35 (from Sigma) were reduced catalyzes hydrolysis (22, 25). The rates of reaction of N-acylim- with NaBH4 and extracted as described by Chang and Bock (18). idazoles with nucleophiles had been reported to vary with the The chloroform extract was then throughly dried and chroma- acyl moiety (26, 27) but no data on long (>C4) acyl chains were tographed on a column of activated alumina F-1 (Alcoa) to re- available. We therefore synthesized a number ofN-acylimidaz- move possible acidic components. The alumina column was oles and compared their reactivities with water and the sulfhy- eluted with chloroform, and the purified detergent was stored dryl of 2-mercaptoethanol in imidazole buffer. as described (18). Solvents were dried with molecular sieve 4A. The rates of hydrolysis of the short-chain N-acylimidazoles varied with chain length (Table 1). Our value for N-acetylimid- RESULTS azole was essentially identical to that previously reported by Rationale ofthe Method. N-Acetylimidazole has been a stan- Jencks and Carriuolo (22) and as expected (26) a similar behavior dard reagent for the modification of protein tyrosine residues was observed for N-butylimidazole. However, the hydrolysis without acetylation of protein amino groups (19, 20). However, rate for the C6 and C8 N-acylimidazoles was more rapid than that it has not been generally appreciated that N-acetylimidazole is of N-acetylimidazole, which may reflect the type of micelle also a powerful reagent for the modification of sulfhydryl these sparingly soluble compounds assume in water. As ex- groups. Stadtman (21) reported the S-acetylation ofglutathione pected from previous work (28), the addition of a nonionic de- by N-acylimidazole, and Riordan and coworkers (19, 20) re- tergent to disperse the longer-chain N-acylimidazoles de- ported the S-acetylation of a number of model compounds and creased the rate of hydrolysis. The rates of S-acylation by N- proteins. Furthermore, Jencks, and Carriuolo (22) demon- acylimidazoles decreased with increased chain length and were strated that the rate of S-acetylation of 2-mercaptoethanol by also slowed by the addition ofdetergent. Detailed study ofthe N-acetylimidazole is 6-fold faster than the rate of O-acetylation reaction ofthe N-butyl- and N-hexanoylimidazoles with sulfhy- Downloaded by guest on September 29, 2021 5442 Biochemistry: Cronan and Klages Proc. Nad Acad. Sci. USA 78 (1981) Table 1. Properties of N-acylimidazoles in 0.5 M imidazole per mol ofACP was reached. Further increases in the N-acetyl- (pH 6.5) imidazole concentration gave increased levels of acylation but k1, iain~' Relative the concentration dependence was weak. For example, a 4-fold increase in N-acetylimidazole concentration (from 5 to 20 mM) Mercaptoethanol S-acylation gave only a 50% increase in the number of acetyl groups in- Chain length Water (10 mM) rate corporated. A similar behavior was observed with N-palmitoyl- C2 0.46 4.1 (100) imidazole but much higher concentrations ofthe N-acylimidaz- C4 0.46 3.4 84 ole were needed for significant incorporation. The time course C6 0.86 2.9 71 of the acylation of ACP depended primarily on the rate of hy- C8 0.92 3.1 76 drolysis ofthe N-acylimidazole. For example, acylation ofACP with 5 mM N-acetylimidazole was complete in 6-8 min (data Plus detergent: not shown), at which time the N-acetylimidazole concentration C6 0.53 1.5 37 by C8 0.27 1.2 28 had decreased to <0.6 mM due to the hydrolysis catalysed C10 0.14 0.8 20 imidazole (Table 1). C12 0.11 0.4 9 Specificity of the Acylation Reaction. The model compound C14 0.12 0.25 6 experiments correctly implied that the acylation occurring at C16 0.07 0.24 6 high N-acylimidazole concentrations was the reaction of phe- C18 0.08 0.26 6 nolic hydroxyl and amino groups after complete S-acylation. Greater than 92% of the [3H]acetyl groups incorporated into C16A9 0.07 0.35 9 ACP atconcentrations ofN-[3H]imidazole <5 mM were cleaved Cj8A9 0.10 0.34 8 from the protein by 1 M hydroxylamine at neutral pH (Table C i8& oS0.09 0.28 7 2). Cleavage by neutral hydroxylamine is consistent with either S-acetylation (9, 10) or acetylation of the tyrosine hydroxyl The k, values were calculated from k, = 0.693/t112. The values for re- the reaction with water or 2-mercaptoethanol were obtained under group (19, 20). Tyrosine acylation can be monitored by the pseudo-first-order conditions essentially as described by Jencks and sulting decrease to one-sixth in the absorption of tyrosine res- Carriuolo (22). The values obtained with various concentrations of 2- idues at 275-280 nm (19, 20). ACP lacks tryptophan (1) and thus mercaptoethanol have been normalized to 10 mM. The detergent used its absorption at 275-280 nm is almost entirely due to the single was Brij 35 at a final concentration of 5%. tyrosine residue (17). The absorption ofthe acylated ACP at 278 nm was therefore a sensitive measure of the extent of tyrosine dryl, amino (both a and e), and hydroxyl (both phenolic and modification. No detectable modification ofthe tyrosine residue aliphatic) groups of model compounds and synthetic peptides was noted in acyl-ACP preparations having a molar ratio ofacyl showed that the S-acylation specificity ofN-acylimidazoles im- groups to ACP '1. plied by the results of Jencks and Carriuolo (22) on the acetyl Amino group modification also did not occur at low concen- derivative held for the longer chain lengths (data not shown). trations ofN-acylimidazole as demonstrated by (i) the sensitivity These model compound data were then used as a basis for the ofthe acyl group cleavage to neutral hydroxylamine, (ii) no loss preparation of acyl-ACP derivatives. of amino groups as measured by trinitrobenzene sulfonate ti- Acylation ofACP. The acetylation ofACP in 0.5 M imidazole tration (29), and (iii) the insensitivity of the acylated protein to buffer (pH 6.5) with N-[acetyl-3H]imidazole had a biphasic de- pH induced denaturation (see below). The specificity of S-acy- pendence on the N-acetylimidazole concentration (Fig. 2). In- lation was also demonstrated by (i) the finding that, at 10 mM creasing concentrations ofN-acylimidazole give an almost pro- portional increase in the number ofacetyl groups incorporated Table 2. Properties of acyl-ACP derivatives per molecule ofACP until a ratio ofabout 0.7-0.8 mol ofacetate Hydrox- Acyl ylamine 1 Acyl Prepa- Yield, group/ACP, Tyrosine/ACP, cleavage, I group rations % mol/mol mol/mol % Chemical synthesis: >1).2 C2 6 95-100 0.85-1.02 0.92-1.2 92-98 0 1 0- C4 2 95-100 0.98-1.06 90 ClOA3 2 95 0.97 95 0 3 60-90 - 0.94-1.21 95 I-¢Cl1).8 / C14 U Ci6 2 50-80 0.4-0.6 90-96 a (0.98)* U,C Enzymatic synthesis: ¢ ).4 t / C14 - 1.0 0.96 95 C16 1.0 - 93 I / The yields were determined by use ofN-[acyl-3H]imidazoles (C2 and 10 20 30 40 C16), of 14C- or 3H-labeled (prosthetic group) ACP (followed by hydro- Acyl-Im, mM phobic chromatography), or by visual inspection of stained polyacryl- amide gels. The ratio of acyl group/ACP was determined by use of FIG. 2. Effect of N-acylimidazole concentration. Acylation of ACP radioactiveN-acylimidazoles orby use of enzymatic synthesis (12, 13). with eitherN-[acetyl-3Hlimidazole (e, A) orN-Lpalmitoyl-5Hlimidazole The tyrosine/ACP values were obtained by comparing the A278 with (o) was performed as described in Materials and Methods except that the protein concentration determined by the microbiuret procedure. the N-acylimidazole concentrations were varied as shown. The two The portion of the acyl groups released by neutral hydroxylamine was different symbols for N-acetylimidazole denote two different experi- ascertained as described (10) for radioactive acyl-ACPs (C2, C16) or by ments. ACP concentrations were determined by the microbiuret re- polyacrylamide gel electrophoresis. action (, o) or by use of ACP labeled with 14C in the prosthetic group * The C16 value in parenthesis is the value obtained after removal of (A). The abscissa is the molar ratio of acyl groups incorporated to ACP. ACP by hydrophobic chromatography. Downloaded by guest on September 29, 2021 Biochemistry: Cronan and Klages Proc. Natl. Acad. Sci. USA 78 (1981) 5443 N-[acetyl-3H]imidazole, myristoyl-ACP (prepared enzymati- octyl-Sepharose (12, 13) or by high-pressure chromatography cally) was labeled to an extent only 15% ofthat ofa parallel sam- on Perisorb RP-8, another octyl group-substituted chromato- ple ofACP and (ii) addition ofa 10-fold molar excess (over ACP) graphic medium, is sensitive to the chain length of the acyl ofthe synthetic peptide L-lysyl-L-tyrosyl-L-serine had no effect moiety (12, 13). Overacylation of ACP should result in an an- on the S-acetylation ofACP (data not shown). omalously late elution from the hydrophobic matrix. Palmitoyl- Physical Properties ofthe Synthesized'Acyl-ACPs. Previous ACP synthesized from N-palmitoylimidazole had an elution studies from this laboratory demonstrated that acyl-ACPs hav- profile identical to that ofenzymatically 'synthesized palmitoyl- ing an acyl moiety 30 ACP C14.0-ACP 60 25 -50 -ACP X 20 --40, 15 3~~~~~--0- ~10 20~ .qmwp.4 omomf -acyl-ACP ...... 1 2 3 4 0 0 U5 10 =20 30 40===C1050 60 70 80 FIG. 3. Polyacrylamide gel electrophoresis of acyl-ACP. Gels (20% Elution volume, ml polyacrylamide) were run at pH 9.5 (final pH) and stained as described (6, 16). Lanes: 1, myristoyl-ACP enzymatically synthesized (12 ,ug); 2, FIG. 4. Hydrophobic chromatography of palmitoyl-ACPR A mix- decanoyl-ACP (68 ,ug); 3, lauroyl-ACP (50 ,ug); and 4, myristoyl-ACP ture (-20 4g, total) of [1-'4C]palmitoyl-ACP (e) prepared enzymati- (40 ,ug). The sample in lane 1 was puriflied free of ACP by octyl-Seph- cally was mixed with [9,10-3H]palmitoyl-ACP (o) prepared by the im- arose chromatography (12, 13); the samples in lanes 2-4 were synthe- idazole method and injected onto a Perisorb RP-8 column equilibrated sized by the imidazole procedure and purified only by DEAE-cellulose with 4% 2-propanol in 50 mM potassium phosphate buffer (pH 7.2). A chromatography. The faint band at the top of the gel is the disulfide linear gradient (broken line) to 45% 2-propanol in the same buffer was dimer of ACP formed during storage of acyl-ACP (6). The faint bands run for 160 min at 0.5 ml/min. Samples (1 ml) were collected and as- above and below ACP are probably complexes of ACP and contami- sayed in a scintillation counter set to discriminate between the two nating trace metal ions (6). The migration position of amino-acylated isotopes. The recoveriesof 14C and 3H were 90% and 87%, respectively. ACP is marked Ac6. All four substrates were active in the P-ketoacyl- The elution volumes of ACP and'myristoyl-ACP (arrows) were estab- ACP synthase reaction. lished in subsequent runs of the same column. Downloaded by guest on September 29, 2021 5444 Biochemistry: Cronan and Klages Proc. Nad Acad. Sci. USA 78 (1981) We thank Dr. Charles 0. Rock for his interest and advice, especially concerning chromatography of acyl-ACP. This work was supported by Research Grants AI 15650 and GM 25616 from the National Institutes of Health. 1. Prescott, D. J. & Vagelos, P. R. (1972) Adv. Enzymot 36, 369-311. 2. Stumpf, P. K. (1977) in .MTP' International. Review of Science, Biochemistry of Lipids 11, ed., Goodwin, T. W. (Butterworth, London), Vol. 14, pp. 215-238. 3. Birge, C. H., Silbert, D. F. & Vagelos, P. R. (1967) Biochem. Biophys. Res. 29, 808-814. 4. Schulz, H. (1975)J. Biol Chem. 250, 2299-2304. 5. Abita, J. P., Lazdunski, M. & Ailhaud, G. (1971) Eur. J. Biochem. 23, 412-420. 6. Rock, C. O., Cronan, J. E., Jr. & Armitage, I. M. (1981)J. Biol Chem. 256, 2669-2674. 7. Spencer, A. K., Greenspan, A. D. & Cronan, J. E., Jr. (1978) J. Biol Chem. 253, 5922-5926. 8. Ray, T. K. & Cronan, J. E., Jr. (1975) J. BioL Chem. 250, 8422-427. 9. Jaworski, J. G. & Stumpf, P. K. (1974) Arch. Biochem. Biophys. 0.02 0.06 0.10 162, 166-173. /C14:0-ACP, pLM-1 10. Ray, T. K. & Cronan, J. E., Jr. (1976) Proc. Natl Acad. Sci. USA 73, 4374-4378. FIG. 5. Enzymatic utilization of myristoyl-ACP. The substrate ac- 11. Rock, C. 0. & Cronan, J. E., Jr. (1979) J. Biol Chem. 254, tivities of myristoyl-ACP samples prepared by either the enzymatic 7116-7122. (e) or the imidazole (o) procedure were assayed at 37TC with purified 12. Rock, C. 0. & Garwin, J. L. (1979) J. Biol Chem. 254, P-ketoacyl-ACP synthase I (specific activity, 0.8 unit/mg) as described 7123-7129. (31). Both acyl-ACP preparations were purified by Perisorb RP-8 chro- 13. Rock, C. O., Garwin, J. L. & Cronan, J. E., Jr. (1980) Methods matography (Fig. 4) to remove unreacted ACP. The Michaelis constant Enzymol 72, 397-403. from the Lineweaver-Burk plot was 82 uM. The reactions with the 14. Staab, H. A. & Rohr, W. (1968) in Newer Methods ofPreparative enzymatic and imidazole synthesized substrates had Km values of 84 Organic Chemistry, ed. Foerst, W. (Academic, New York), Vol. and 78 utM, respectively. 5, pp. 61-108. 15. Staab, H. A. (1962) Angew. Chem. Int. Ed. EngL 1, 351-367. 16. Rock, C. 0. & Cronan, J. E., Jr. (1980) Methods. Enzymol 71, about 4 hr in 2.5 M imidazole (pH 6.5). The reversibility ofthe 349-351. acylation reaction precluded utilizing the somewhat greater 17. Rock, C. 0. & Cronan, J. E., Jr. (1980) AnaL Biochem. 102, for S-acylation that would theoretically (22) result 362-364. specificity 18. Chang, H. W. & Bock, E. (1980) Anal Biochem. 104, 112-117. from increasing the concentration of imidazole buffer. 19. Riordan, J. F., Wacker, W. E. C. & Vallee, B. L. (1965) Bio- chemistry 4, 1758-1765. DISCUSSION 20. Riordan, J. F. & Vallee, B. L. (1967) Methods Enzymol 11, 570-576. The synthesis of acyl-ACP substrates by the N-acylimidazole 21. Stadtman, E. R. (1954) in The Mechanism of Enzyme Action, procedure proceeded smoothly and gave essentially quantita- eds., McElroy, W. D. & Glass, B. (Johns Hopkins Press, Balti- more, MD), pp. 581-598. tive conversion ofACP to acyl-ACP for short-chain (sCj2) acids. 22. Jencks, W. P. & Carriuolo, J. (1959) J. Biol Chem. 234, The ACP moiety of these substrates had native structure, and 1280-1285. the physical and biological properties of these acyl-ACPs were 23. Vanaman, T. C., Wakil, S. J. & Hill, R. L. (1968)J. Biol Chem. indistinguishable from those of enzymatically prepared sub- 243, 6420-6431. strates. The N-acylimidazole method is complementary to the 24. Gerstein, J. & Jencks, W. P. (1964) J. Am. Chem. Soc. 80, method in that short-chain N- 4655-4663. previous acyl-ACP synthetase 25. Jencks, W. P. & Carriuolo, J. (1959) J. Biol Chem. 234, acylimidazoles are the most efficient acyl donors in the chemical 1272-1279.. method whereas the enzymatic synthesis functions only with 26. Fife, T. H. (1965) J. Am. Chem. Soc. 87% 4597-4600. long-chain acids. 27. Staab, H. A. (1956) Chem. Ber. 89, 2088-2093. The main use of the N-acylimidazole method will be in the 28. Getler, C. & Ochoa-Solono, A. (1968) J. Am. Chem. Soc. 90, synthesis of short-chain acyl-ACP substrates and of acyl-ACPs 5004-5009. chains with functional groups. For the long-chain 29. Fields, R. (1972) Methods Enzymol 25, 464-468. having acyl 30. Rock, C. O. & Cronan, J. E., Jr. (1979) J. Biol Chem. 254, acids, the enzymatic method should remain the method of 9778-9785. choice due to its absolute specificity. The synthesis ofthe short- 31. Garwin, J. L., Klages, A. & Cronan, J. E., Jr. (1980) J. Biol chain acyl-ACPs swill allow more definitive studies of the reg- Chem. 255, 11959-11956. ulation of fatty acid synthesis and of the influence of the acyl 32. Greenspan, M. D., Birge, C. H., Powell, G., Hancock, W. S. group on the structure of ACP. & Vagelos, P. R. (1970) Science 70, 1203-1204. Downloaded by guest on September 29, 2021