During the past decade, ionic-liquid-based Aqueous Biphasic Systems (ABS) have been the focus of a significant amount of research. Based on a compilation and analysis of the data hitherto reported, this critical review provides a judicious assessment of the available literature on the subject. We evaluate the quality of the data and establish the main drawbacks found in the literature. We discuss the main issues which govern the phase behaviour of ionic-liquid-based ABS, and we highlight future challenges to the field. In particular, the effect of the ionic liquid structure and the various types of salting-out agents (inorganic or organic salts, amino acids and carbohydrates) on the phase equilibria of ABS is discussed, as well as the influence of secondary parameters such as temperature and pH. More recent approaches using ionic liquids as additives or as replacements for common salts in polymer-based ABS are also presented and discussed to emphasize the expanding number of aqueous two-phase systems that can actually be obtained. Finally, we address two of the main applications of ionic liquid-based ABS: extraction of biomolecules and other added-value compounds, and their use as alternative approaches for removing and recovering ionic liquids from aqueous media.

Aqueous biphasic systems: a boost brought about by using ionic liquids

This review describes the most recent developments in the biotechnological applications of penicillin acylases. This group of enzymes is involved mainly in the industrial production of 6-aminopenicillanic acid and the synthesis of semisynthetic β-lactam antibiotics. In addition, penicillin acylases can also be employed in other useful biotransformations, such as peptide synthesis and the resolution of racemic mixtures of chiral compounds. Particular emphasis is placed on advances in detection of new enzyme specificities towards other natural penicillins, enzyme immobilization, and optimization of enzyme-catalyzed hydrolysis and synthesis in the presence of organic solvents.

Biotechnological applications of penicillin acylases: state-of-the-art.

Advances in enzymatic transformation of penicillins to 6-aminopenicillanic acid (6-APA).

414 nature structural biology ¥ volume 6 number 5 ¥ may 1999 Two enzyme types, penicillin V acylases (PVA) and penicillin G acylases (PGA), with distinct substrate preferences, account for all the enzymic industrial production of 6-aminopenicillanic acid 1,2. This b-lactam compound is then elaborated into a range of semi-synthetic penicillins. Although their industrial substrates are very similar, representative examples of the two enzyme types differ widely in molecular properties. PVA from Bacillus sphaericus is tetrameric with a monomer M r of 35,000 while PGA from Escherichia coli is a heterodimer of M r 90,000. Furthermore, they have no detectable sequence homology. These differences, which exist in spite of the similarity of their industrial substrates, provoked us to determine the crystal structure of PVA to establish the nature of its catalytic mechanism and to identify any biochemical and structural relationships with PGA and other Ntn (N-terminal nucleophile) hydrolases. The PVA molecule is a well-defined tetramer with 222 organization made up of two obvious dimers (A and D) and (B and C), which generate a flat disc-like assembly (Fig. 1a). The X-ray analysis revealed that the PVA monomer contains two central anti-parallel b-sheets above and below which is a pair of anti-parallel helices (Fig. 1b). There are two extensions , one from the upper pair of helices and the other at the C-terminal segment, that interact with other monomers in the tetramer and help stabilize it. The b-sheet and helix organization and connectivity are characteristic of members of the Ntn hydrolase family, which have an N-terminal catalytic residue that is often created by autocatalytic processing 3,4. In the PVA structure, cysteine was observed as the N-terminal residue, whereas the gene sequence predicts an N-terminal sequence of Met-Leu-Gly-Cys 5. This finding shows that three amino acids are processed from the precursor N-terminus to unmask a nucleophile with a free a-amino group. Since PVA is an Ntn hydro-lase, we can deduce that the N-terminal cysteine in PVA is the catalytic residue. The PVA and PGA enzymes thus share a distinctive structural core but are otherwise unrelated in primary sequence, including the active site residue. Both PGA and PVA have approximately the same angle (+30°) between the b-strands of the two b-sheets, which are decorated by the active site residues in Ntn hydro-lases. Using these b-sheets for structural alignment reveals that the catalytic regions of PVA and PGA overlap (Fig. 1c) with a root …

J. G. Shewale

Papers

Penicillin acylase: enzyme production and its application in the manufacture of 6-apa