Fractionation of the mycobacillin-synthesizing enzyme system

Biosynthesis of bacillomycin D by Bacillus subtilis. Evidence for amino acid-activating enzymes by the use of affinity chromatography

Bacillomycin D is an antifungal lipopeptide produced by B. subtilis. The formation of the peptidyl bonds of bacillomycin D occurs non-ribosomally, as demonstrated by the use of chloramphenicol, an inhibitor of protein biosynthesis. Amino acid-activating enzymes were found in B. subtilis cell lysates purified by affinity chromatography on a gel containing L-Pro, an amino acid of bacillomycin D. Presence of ATP during this purification increases the binding of enzymatic proteins and their activity. An enzyme, with an apparent molecular weight of 230 kDa, catalyzed ATP-PPi exchange reactions, which were mediated by specific amino acids, corresponding to a partial sequence of bacillomycin D.

Nonribosomal biosynthesis of peptide antibiotics

Peptide antibiotics are known to contain non-protein amino acids, D-amino acids, hydroxy acids, and other unusual constituents. In addition they may be modified by N-methylation and cyclization reactions. Their biosynthetic origin has been connected in many cases to an enzymatic system referred to as the 'thiotemplate multienzymic mechanism'. This mechanism includes the activation of the constituent residues as adenylates on the enzymic template, the acylation of specific template thiol groups, epimerization or N-methylation at this thioester stage, and polymerization in the sequence directed by the multienzymic structure with the aid of 4'-phosphopantetheine as a cofactor, including possible cyclization or terminal modification reactions. The reaction sequences leading to gramicidin S, tyrocidine, cyclosporine, bacitracin, polymyxin, actinomycin, enniatin, beauvericin, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine and linear gramicidin are discussed. The structures of the multienzymes, their genetic organization, the biological functions of these peptides and results on related systems are discussed.

Studies on the biosynthesis of surfactin, a lipopeptide antibiotic from Bacillus subtilis ATCC 21332

The biosynthesis of the lipopeptide antibiotic surfactin was studied in whole cells of Bacillus subtilis ATCC 21332 which incorporate 14C-labeled precursor amino acids directly into the product. [14C]Acetate appeared in the fatty acid portion of surfactin and was also partially converted into leucine. An enzyme was isolated and partially purified from a cell-free extract of the bacillus which catalyzes ATP-Pi-exchange reactions which are mediated by the amino acid components of surfactin. This activation pattern is consistent with a peptide synthesizing multienzyme which activates its substrate amino acids simultaneously as reactive aminoacyl phosphates.

Peptide antibiotics, beta-lactams, and related compounds

In the field of natural peptides, beta-lactams, and related compounds, recent exciting developments are discussed. The increasing interest in this class of bioactive amino-acid derived structures has been attributed to the use of new directed screens (enzyme inhibition assays, beta-lactam detection, immunomodulator studies), new and improved applications (antibiotic, transplantation, and cancer chemotherapy), and advances in functional studies (DNA binding peptides, nucleotide complexones, cell wall and protein processing inhibitors). Peptides offer unique access to modifications and analog production by in vivo (directed biosynthesis) and in vitro procedures (enzymatic synthesis) due to their general linear precursors permitting point replacements. Of special interest are recent developments in the genetics of these compounds (cyclic peptides and beta-lactams), which will find applications in production methods in the near future.

Role of ATP and enzyme-bound nascent peptides in the control of elongation for mycobacillin synthesis

The enzyme fraction A, a constituent of the three-fraction (A, B and C) enzyme complex mycobacillin synthetase, elongated tri- and tetra-peptides, under enzyme-bound conditions, to tetra- and penta-peptides respectively in the presence of the 'next' amino acid (in the mycobacillin sequence). The enzyme fraction B synthesized hexapeptide from free pentapeptide and the next amino acid, but synthesized heptapeptide from hexapeptide only under enzyme-bound conditions in the presence of the next amino acid. Similarly, the enzyme fraction C synthesized decapeptide from free nonapeptide in the presence of the next amino acid, but undecapeptide only from enzyme-bound decapeptide in the presence of the next amino acid during the elongation process. The Km values for the initiating reactions for each of the three enzyme fractions were 6-7-fold lower than those for the succeeding reactions catalysed by each of the enzyme fractions. The specificity of the initiation and elongation is discussed in the light of these findings.

Characterization of three-fraction mycobacillin synthetase

Mycobacillin synthetase lacks aspartic acid racemase, alanine racemase and glutamic acid racemase activities. The enzyme also does not respond to ATP-[32P]Pi exchange, nor does it catalyse the antibiotic synthesis in presence of amino acids of configuration opposite to that present in the molecule. Preincubation with optical isomers of opposite configuration inhibited the ATP-[32P]Pi exchange reaction to the extent of 60-90%. None of the three fractions of mycobacillin synthetase contained a pantothenic acid arm. Two molecules of ATP are required to synthesize one peptide bond of mycobacillin. Intermediate peptides of mycobacillin are not covalently linked to the three-fraction mycobacillin synthetase.

Purification of the constituent enzyme fractions of mycobacillin synthetase

The final purification of the three-fraction enzyme complex mycobacillin synthetase was done by hydroxyapatite column chromatography and sucrose-density-gradient centrifugation; each of the fractions obtained migrates as a single component in SDS/polyacrylamide-gel electrophoresis and gel electrofocusing. The Mr of the enzyme fractions A, B and C by gel filtration is 260 000, 190 000 and 105 000, and that by SDS/polyacrylamide-gel electrophoresis is 252 000, 198 000 and 108 000 respectively. None of the enzyme fractions appears to possess subunit structure.

Functional characterization of constituent enzyme fractions of mycobacillin synthetase

The enzyme fraction A, a constituent enzyme of the three-fraction enzyme mycobacillin synthetase, independently and sequentially activated five amino acids starting from L-proline, producing the pentapeptide Pro(Asp1,Glu1,Tyr1)Asp. The fractions B and C were unable to function independently. However, the fraction B synthesized the nonapeptide Pro(Asp3,Glu1,Tyr2,Ser1)Leu, sequentially activating the pentapeptide and next four amino acids, whereas the fraction C synthesized mycobacillin by the sequential activation of the nonapeptide and the remaining four amino acids. The pH optima of the above enzymes are almost identical (pH 7.8), but their Km values are a little different.

Translocation of mycobacillin synthetase in Bacillus subtilis

The extracellular release of mycobacillin from Bacillus subtilis first occurred in the medium at the onset of stationary phase and continued at a high rate even after 6 days. Mycobacillin synthetase activity appeared earlier than late-exponential phase in the cytosol of producer cells and was not sedimentable even at 105 000 g. The activity then quickly reached the maximum late in the stationary phase. With further increase in the age of the culture, the activity gradually disappeared from the cytosol, to reappear concomitantly in the membrane in an insoluble particulate form, even in absence of protein synthesis. The membrane-bound synthetase activity was sedimentable at 10 000 g and was fairly active even after 5 days.

Standardization of parameters for the mycobacillin synthetase activity

An effective method of preparation involving sonication was developed for cell-free mycobacillin synthetase from Bacillus subtilis. The enzyme showed optimum activity at a buffer concentration of 50 mM (Tris-HCl) and pH 7.5. ATP and Mg2+ which were essential for synthesis showed an optimum requirement at a ratio of 1:1. The synthetase was markedly inhibited by ADP whereas AMP was without any effect. ATP or ATP-generating system could not be replaced by GTP, UTP or CTP. Co2+ and Mn2+ could to some extent substitute Mg2+. Mercapto reagents inhibited the antibiotic synthesis. Exogenous addition of pantothenic acid had no effect.