AMINO-ACID CONFIGURATION OF MYCOBACILLIN

The stereoselectivity and hydrolysis efficiency of recombinant D-hydantoinase from Vigna angularis Against 5-benzylhydantoin derivatives with halogen and methyl substituents

The researches on D-hydantoinase activity and substrate specificity towards dihydropyrimidine and hydantoin derivatives have been carried out intensively over the last few decades. So far, the major efforts have focused on (R,S)-5-phenylhydantoin and (R,S)-5-(4-hydroxyphenyl)hydantoin, the most desirable D-hydantoinase substrates from pharmaceutical industry point of view. However, it was shown that D-hydantoinase is a substrate-dependent enzyme, and its activity and stereoselectivity towards 5-monosubstituted hydantoins varied significantly with the type of substrate and the source of the enzyme. The aim of this study was to estimate the substrate specificity of D-hydantoinase towards series of 5-benzylhydantoin derivatives with halogen and methyl substituents in the phenyl ring. The biotransformations were carried out by using commercial enzyme: immobilized, recombinant, cloned, and expressed in Escherichia coli D-hydantoinase from Vigna angularis (rD-HYD). All reactions were monitored by capillary electrophoresis (CE), and the conversion yields were calculated. Additionally, enantiomeric ratios of the obtained D-phenylalanine derivatives were estimated by chiral high-performance liquid chromatography (HPLC). Interestingly, the differences in the activities of examined enzyme towards particular 5-benzylhydantoin derivatives were observed. CE was also shown as a promising method for monitoring the hydrolysis of new substrates by D-hydantoinase and further analyzing of enzyme substrate specificity.

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.

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.

Stereoconfiguration of amino acids in peptides from a mycobacillin-synthesizing cell-free system

The stereoconfiguration of amino acids, as determined by treatment with L-amino acid oxidase, d-amino acid oxidase and l-glutamate decarboxylase (containing l-aspartate decarboxylase activity), in the peptides from a mycobacillin-synthesizing cell-free system is identical with that of the growing mycobacillin peptide chaid if its synthesis starts from l-proline and is interrupted at various points by amino acid deprivation.

Gamma-glutamyl and D- or L-peptide linkages in mycobacillin, a cyclic peptide antibiotic

Mycobacillin lacks amino groups but contains two free alpha-carboxyl groups, indicating the presence of two side-chain peptide linkages. The five aspartic acid residues of mycobacillin are all in alpha-peptide linkage whereas the two glutamic acid residues are in gamma-linkage. Mycobacillin does not react with hydroxylamine to give hydroxamate, indicating the absence of anhydride, lactone and ester linkages. This is also confirmed by i.r. spectroscopy and titration of the molecule. Of the 15 peptides obtained from partial hydrolysates of mycobacillin, 12 contain aspartic acid. Results obtained by treatment of hydrolysates of aspartic acid-containing peptides with d-amino acid oxidase and l-glutamate decarboxylase (containing l-aspartate decarboxylase activity) indicate that residue 5 is l-aspartic acid and residues 2, 8, 11 and 13 are d-aspartic acid. The d- or l-peptide sequence and nature of peptide linkages in mycobacillin are proposed on the basis of these findings and the amino acid sequence reported earlier.