Identification of a new active site for autocatalytic processing of penicillin acylase precursor in Escherichia coli ATCC11105

Effect of Concentrated Salts Solutions on the Stability of Immobilized Enzymes: Influence of Inactivation Conditions and Immobilization Protocol

This paper aims to investigate the effects of some salts (NaCl, (NH₄)₂SO₄ and Na₂SO₄) at pH 5.0, 7.0 and 9.0 on the stability of 13 different immobilized enzymes: five lipases, three proteases, two glycosidases, and one laccase, penicillin G acylase and catalase. The enzymes were immobilized to prevent their aggregation. Lipases were immobilized via interfacial activation on octyl agarose or on glutaraldehyde-amino agarose beads, proteases on glyoxyl agarose or glutaraldehyde-amino agarose beads. The use of high concentrations of salts usually has some effects on enzyme stability, but the intensity and nature of these effects depends on the inactivation pH, nature and concentration of the salt, enzyme and immobilization protocol. The same salt can be a stabilizing or a destabilizing agent for a specific enzyme depending on its concentration, inactivation pH and immobilization protocol. Using lipases, (NH₄)₂SO₄ generally permits the highest stabilities (although this is not a universal rule), but using the other enzymes this salt is in many instances a destabilizing agent. At pH 9.0, it is more likely to find a salt destabilizing effect than at pH 7.0. Results confirm the difficulty of foreseeing the effect of high concentrations of salts in a specific immobilized enzyme.

Modifying the substrate specificity of penicillin G acylase to cephalosporin acylase by mutating active-site residues

The penicillin G acylase (PGA) and cephalosporin acylase (CA) families, which are members of the N-terminal (Ntn) hydrolases, are valuable for the production of backbone chemicals like 6-aminopenicillanic acid and 7-aminocephalosporanic acid (7-ACA), which can be used to synthesize semi-synthetic penicillins and cephalosporins, respectively. Regardless of the low sequence similarity between PGA and CA, the structural homologies at their active-sites are very high. However, despite this structural conservation, they catalyze very different substrates. PGA reacts with the hydrophobic aromatic side-chain (the phenylacetyl moiety) of penicillin G (PG), whereas CA targets the hydrophilic linear side-chain (the glutaryl moiety) of glutaryl-7-ACA (GL-7-ACA). These different substrate specificities are likely to be due to differences in the side-chains of the active-site residues. In this study, mutagenesis of active-site residues binding the side-chain moiety of PG changed the substrate specificity of PGA to that of CA. This mutant PGA may constitute an alternative source of engineered enzymes for the industrial production of 7-ACA.

Regulation of penicillin G acylase gene expression in Escherichia coli by repressor PaaX and the cAMP-cAMP receptor protein complex

The pga gene of Escherichia coli W ATCC11105 encodes a penicillin G acylase whose expression is regulated at both the transcriptional and post-transcriptional level. In this work we have shown that PaaX is the repressor of pga expression, and we have identified its binding consensus as TGATTC(N27)GAATCA. We conclude that the process of "PAA induction" actually involves relief of pga from repression by PaaX. Other features of the pga promoter have also been characterized. (i) It has a native class III cAMP-receptor protein (CRP)-dependent promoter with two CRP-binding sites. (ii) The downstream CRP-binding site II has higher affinity. (iii) Binding of cAMP-CRP to both sites (I + II) is required for maximal expression. We have also shown that the PaaX-binding site overlaps with the CRP-binding site I. This implies that PaaX and the cAMP-CRP compete for binding to the region around the CRP-binding site I and therefore have antagonistic effects on pga expression.

Recent biotechnological interventions for developing improved penicillin G acylases

Penicillin G acylase (PAC; EC 3.5.1.11) is the key enzyme used in the industrial production of beta-lactam antibiotics. This enzyme hydrolyzes the side chain of penicillin G and related beta-lactam antibiotics releasing 6-amino penicillanic acid (6-APA), which is the building block in the manufacture of semisynthetic penicillins. PAC from Escherichia coli strain ATCC 11105, Bacillus megaterium strain ATCC 14945 and mutants of these two strains is currently used in industry. Genes encoding for PAC from various bacterial sources have been cloned and overexpressed with significant improvements in transcription, translation and post-translational processing. Recent developments in enzyme engineering have shown that PAC can be modified to gain conformational stability and desired functionality. This review provides an overview of recent advances in the production, stabilization and application of PAC, highlighting the recent biotechnological approaches for the improved catalysis of PAC.

Fragments of pro-peptide activate mature penicillin amidase of Alcaligenes faecalis

Penicillin amidase from Alcaligenes faecalis is a recently identified N-terminal nucleophile hydrolase, which possesses the highest specificity constant (kcat/Km) for the hydrolysis of benzylpenicillin compared with penicillin amidases from other sources. Similar to the Escherichia coli penicillin amidase, the A. faecalis penicillin amidase is maturated in vivo from an inactive precursor into the catalytically active enzyme, containing one tightly bound Ca2+ ion, via a complex post-translational autocatalytic processing with a multi-step excision of a small internal pro-peptide. The function of the pro-region is so far unknown. In vitro addition of chemically synthesized fragments of the pro-peptide to purified mature A. faecalis penicillin amidase increased its specific activity up to 2.3-fold. Mutations were used to block various steps in the proteolytic processing of the pro-peptide to obtain stable mutants with covalently attached fragments of the pro-region to their A-chains. These extensions of the A-chain raised the activity up to 2.3-fold and increased the specificity constants for benzylpenicillin hydrolysis mainly by an increase of the turnover number (kcat).

Molecular evolution of proteasomes

Proteasomes are large, multisubunit proteases that are found, in one form or another, in all domains of life and play a critical role in intracellular protein degradation. Although they have substantial structural similarity, the proteasomes of bacteria, archaea, and eukaryotes show many differences in architecture and subunit composition. This article discusses possible paths by which proteasomes may have evolved from simple precursors to the highly complicated and diverse complexes observed today.