Finding of an isoleucine derivative of a recombinant protein for pharmaceutical use

Abstract

Protein modification generally occurs by addition to the amino acid side-chains of protein at the post-translational stage, for example, by enzymatic or chemical reactions after polypeptide synthesis. Recently, the recombinant hirudin analog CX-397, a potent thrombin inhibitor, was found to contain methylated Ile residues when it was overproduced by Escherichia coli in the absence of amino acids in the culture medium. The Ile derivatives, deduced to be β-methylnorleucine [βMeNle; (2 S, 3 S)-2-amino-3-methylhexanoic acid] by systematic chromatographic analysis, do not appear to be normal post-translational modifications of the protein because Ile has no functional group in its side-chain. We, therefore, propose that βMeNle is biosynthesized by E. coli, activated by E. coli isoleucyl-tRNA synthetase (IleRS), then incorporated into the overproduced recombinant hirudin analog. The biosynthesis of βMeNle in E. coli is thought to occur as follows: α-ketovalerate is synthesized from α-ketobutyrate by three Leu biosynthetic enzymes, α-isopropylmalate synthase (IPMS) (EC 4.1.3.12), α-isopropylmalate isomerase (ISOM) (EC 4.2.1.33) and β-isopropylmalate dehydrogenase (IPMD) (EC 1.1.1.85), which have broad substrate specificities. α-Ketovalerate is then converted to α-keto-β-methylcaproate by three Ile and Val biosynthetic enzymes, acetohydroxy acid synthase (AS) (EC 4.1.3.18), acetohydroxy acid isomeroreductase (IR) (EC 1.1.1.86) and dihydroxy acid dehydratase (DH) (EC 4.2.1.9). Finally, this is converted to βMeNle by branched-chain amino acid transaminase (EC 2.6.1.42), one of the Ile and Val biosynthetic enzymes.