Polymerases for Biosynthesis of Storage Compounds

Abstract

enzyme catalyzing the polymerization of two peptidoglycan precursors in bacterial cell wall biogenesis.1. The best investigated mechanism, distributed ubiquitously in living matter, is the template -dependent ribosomal synthesis of proteins. Here the amino acids are activated by adenylation catalyzed by aminoacyl -tRNA synthetases [1] .2. The second mechanism is performed by nonribosomal peptide synthetases ( NRPS ) which are multienzyme complexes consisting of four domains [2, 3] . The adenylation domain required for activation of the substrate at the expense of ATP, via the formation of an enzyme -stabilized aminoacyl adenylate. As the formed adenylate is not stable, the energy is further conserved by transfer of the peptide to: the thiolation domain giving rise to a thioester bond formed between the amino acid and a cysteine of the enzyme complex.The condensation domain is required for formation of peptide bonds between the itemized monomers, and the thioesterase domain catalyzes the release of the fi nal product from the NRPS by cyclization to an amide or ester, or by hydrolysis to the free acid [3] . The transfer of the intermediates is mediated by the cofactor pantetheine. Numerous peptide antibiotics such as penicillin, bacitracin, and actinomycin are synthesized by NRPS (reviewed by [2] ). These compounds are known to contain non -proteinogenic amino acids, d -amino acids, hydroxy acids, methylated or cyclic forms, and other unusual constituents; these modifi cations are catalyzed by the NRPS. As the enzyme complex itself functions as matrix, the resulting peptides have a strictly defi ned length which is in contrast to poly(amino acids).3. The third mechanism is represented by nonmodular one -step peptide synthesis. Enzymes belonging to this group catalyze the biosynthesis of poly(amino acids). Naturally occurring poly(amino acids) comprise cyanophycin [multil -arginyl -poly -( l -aspartic acid); cyanophycin granule polypeptide , ( CGP )], ( poly -( ε -lysine) ( PL ), and poly -( γ -glutamate) ( PGA ). As a consequence of non -ribosomal biosynthesis these peptides reveal a polydisperse mass distribution.Further aspects distinguishing poly(amino acids) from proteins are the following:proteins consist of a mixture of 22 amino acids, whereas poly(amino acids) consist of one amino acid in the polymer backbone;their biosynthesis is not constrained by translational inhibitors such as chloramphenicol [4] ;the amide bonds formed between the monomers are not exclusively linked between the α -carboxylic and α -amino groups, as for proteins, but also between β -or γ -carboxylic groups or ε -amino groups. Polyhydroxyalkanoate Synthases 249