Penicillin acylase (penicillin amidohydrolase, EC 3.5.1.11) is widely distributed among microorganisms, including bacteria, yeast and filamentous fungi. It is used on an industrial scale for the production of 6-aminopenicillanic acid, the starting material for the synthesis of semi-synthetic penicillins. Its in vivo role remains unclear, however, and the observation that expression of the Escherichia coli enzyme in vivo is regulated by both temperature and phenylacetic acid has prompted speculation that the enzyme could be involved in the assimilation of aromatic compounds as carbon sources in the organism's free-living mode. The mature E. coli enzyme is a periplasmic 80K heterodimer of A and B chains (209 and 566 amino acids, respectively) synthesized as a single cytoplasmic precursor containing a 26-amino-acid signal sequence to direct export to the cytoplasm and a 54-amino-acid spacer between the A and B chains which may influence the final folding of the chains. The N-terminal serine of the B chain reacts with phenylmethylsulphonyl fluoride, which is consistent with a catalytic role for the serine hydroxyl group. Modifying this serine to a cysteine inactivates the enzyme, whereas threonine, arginine or glycine substitution prevents in vivo processing of the enzyme, indicating that this must be an important recognition site for cleavage. Here we report the crystal structure of penicillin acylase at 1.9 A resolution. Our analysis shows that the environment of the catalytically active N-terminal serine of the B chain contains no adjacent histidine equivalent to that found in the serine proteases. The nearest base to the hydroxyl of this serine is its own alpha-amino group, which may act by a new mechanism to endow the enzyme with its catalytic properties.

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A formal discussion of the archival journal requirements for data deposition was held at the International Seminar-cum-School on Macromolecular Crystallographic Data at Calcutta, India in November. The current policy of the International Union of Crystallography (IUCr) is that on publication of a crystal structure determination of a macromolecule, the atomic parameters used or represented in the publication must be deposited in the Protein Data Bank. The deposition of structure amplitudes is recommended but not insisted on. The policy provides crystallographers with the option to delay the release of atomic parameters for one year and of structure amplitudes for up to four years from the date of publication. Participants strongly supported this policy and felt it should be strictly applied by the journals (referees). Recent developments in x-ray crystallographic experimental and refinement techniques and the huge expansion in computing power and networking, however, necessitate the review of deposition arrangements. It was noted that the new validation procedures are much more effective, but require the experimental structure amplitudes as well as the atomic parameters. In addition the technical arrangements for deposition, analysis, and validation of macromolecular crystal structures are now much easier. The undersigned consider it vital for the macromolecular crystallographers to respond to these developments in their deposition practices. We recommend therefore that publication of macromolecular crystal structures should be accompanied by deposition of atomic parameters and structure amplitudes. Amongst the many reasons identified for this practice, the two following are critical. 1) Rigorous validation of the structure determinations results can only be carried out using both atomic parameters and experimental structure amplitudes. It is important that journals ensure that referees have sufficient information to prevent incorrect structures from being published.