The biosynthesis of penicillin and cephalosporin antibiotics in microorganisms requires the formation of the bicyclic nucleus of penicillin. Isopenicillin N synthase (IPNS), a non-haem iron-dependent oxidase, catalyses the reaction of a tripeptide, delta-(L-alpha-aminoadipoyl)-L-cysteinyl-D-valine (ACV), and dioxygen to form isopenicillin N and two water molecules. Mechanistic studies suggest the reaction is initiated by ligation of the substrate thiolate to the iron centre, and proceeds through an enzyme-bound monocyclic intermediate. Here we report the crystal structure of IPNS complexed to ferrous iron and ACV, determined to 1.3 A resolution. Based on the structure, we propose a mechanism for penicillin formation that involves ligation of ACV to the iron centre, creating a vacant iron coordination site into which dioxygen can bind. Subsequently, iron-dioxygen and iron-oxo species remove the requisite hydrogens from ACV without the direct assistance of protein residues. The crystal structure of the complex with the dioxygen analogue, NO and ACV bound to the active-site iron supports this hypothesis.

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The binding of penicillin to penicillin acylase was studied by X-ray crystallography. The structure of the enzyme-substrate complex was determined after soaking crystals of an inactive betaN241A penicillin acylase mutant with penicillin G. Binding of the substrate induces a conformational change, in which the side chains of alphaF146 and alphaR145 move away from the active site, which allows the enzyme to accommodate penicillin G. In the resulting structure, the beta-lactam binding site is formed by the side chains of alphaF146 and betaF71, which have van der Waals interactions with the thiazolidine ring of penicillin G and the side chain of alphaR145 that is connected to the carboxylate group of the ligand by means of hydrogen bonding via two water molecules. The backbone oxygen of betaQ23 forms a hydrogen bond with the carbonyl oxygen of the phenylacetic acid moiety through a bridging water molecule. Kinetic studies revealed that the site-directed mutants alphaF146Y, alphaF146A and alphaF146L all show significant changes in their interaction with the beta-lactam substrates as compared with the wild type. The alphaF146Y mutant had the same affinity for 6-aminopenicillanic acid as the wild-type enzyme, but was not able to synthesize penicillin G from phenylacetamide and 6-aminopenicillanic acid. The alphaF146L and alphaF146A enzymes had a 3-5-fold decreased affinity for 6-aminopenicillanic acid, but synthesized penicillin G more efficiently than the wild type. The combined results of the structural and kinetic studies show the importance of alphaF146 in the beta-lactam binding site and provide leads for engineering mutants with improved synthetic properties.

In a pilot study, the feasibility of immune whey protein concentrate (40 %; immune WPC-40) to aid the prevention of relapse of Clostridium difficile diarrhoea was evaluated. Immune WPC-40 was made from milk after immunization of Holstein-Frisian cows with C. difficile-inactivated toxins and killed whole-cell C. difficile. Immune WPC-40 contained a high concentration of specific sIgA antibodies, and was effective in neutralizing the cytotoxic effect of C. difficile toxins in cell assays in vitro. Immune WPC-40 conferred protection from otherwise lethal C. difficile-associated caecitis in hamsters. To obtain preliminary data in humans, 16 patients (10 male; median 57 years) with toxinand culture-confirmed C. difficile diarrhoea were enrolled in an uncontrolled cohort study. Nine had a history of relapsing C. difficile diarrhoea. After completion of standard antibiotic treatment, the patients received immune WPC-40 TID for 2 weeks; it was well tolerated and no treatment-related adverse effects were observed. In all but one case, C. difficile toxins had disappeared from the faeces upon completion of treatment. During a follow-up period of median 333 days (range 35 days to 1 year), none of the patients had suffered another episode of C. difficile diarrhoea. These preliminary data suggest that immune whey protein concentrate-40 may be of help in the prevention of relapse of C. difficile diarrhoea.