Saturation mutagenesis reveals the importance of residues alphaR145 and alphaF146 of penicillin acylase in the synthesis of beta-lactam antibiotics.

Penicillin acylase (PA) from Escherichia coli can catalyze the coupling of an acyl group to penicillin- and cephalosporin-derived beta-lactam nuclei, a conversion that can be used for the industrial synthesis of beta-lactam antibiotics. The modest synthetic properties of the wild-type enzyme make it desirable to engineer improved mutants. Analysis of the crystal structure of PA has shown that residues alphaR145 and alphaF146 undergo extensive repositioning upon binding of large ligands to the active site, suggesting that these residues may be good targets for mutagenesis aimed at improving the catalytic performance of PA. Therefore, site-saturation mutagenesis was performed on both positions and a complete set of all 38 variants was subjected to rapid HPLC screening for improved ampicillin synthesis. Not less than 33 mutants showed improved synthesis, indicating the importance of the mutated residues in PA-catalyzed acyl transfer kinetics. In several mutants at low substrate concentrations, the maximum level of ampicillin production was increased up to 1.5-fold, and the ratio of the synthetic rate over the hydrolytic rate was increased 5-15-fold. Moreover, due to increased tendency of the acyl-enzyme intermediate to react with beta-lactam nucleophile instead of water, mutants alphaR145G, alphaR145S and alphaR145L demonstrated an enhanced synthetic yield over wild-type PA at high substrate concentrations. This was accompanied by an increased conversion of 6-APA to ampicillin as well as a decreased undesirable hydrolysis of the acyl donor. Therefore, these mutants are interesting candidates for the enzymatic production of semi-synthetic beta-lactam antibiotics.

MSCAN: identification of functional clusters of transcription factor binding sites.

Identification of functional transcription factor binding sites in genomic sequences is notoriously difficult. The critical problem is the low specificity of predictions, which directly reflects the low target specificity of DNA binding proteins. To overcome the noise produced in predictions of individual binding sites, a new generation of algorithms achieves better predictive specificity by focusing on locally dense clusters of binding sites. MSCAN is a leading method for binding site cluster detection that determines the significance of observed sites while correcting for local compositional bias of sequences. The algorithm is highly flexible, applying any set of input binding models to the analysis of a user-specified sequence. From the user's perspective, a key feature of the system is that no reference data sets of regulatory sequences from co-regulated genes are required to train the algorithm. The output from MSCAN consists of an ordered list of sequence segments that contain potential regulatory modules. We have chosen the features in MSCAN such that sequence and matrix retrieval is highly facilitated, resulting in a web server that is intuitive to use. MSCAN is available at http://mscan.cgb.ki.se/cgi-bin/MSCAN.

Structural and kinetic studies on ligand binding in wild-type and active-site mutants of penicillin acylase.

Penicillin acylase catalyses the condensation of Calpha-substituted phenylacetic acids with beta-lactam nucleophiles, producing semi-synthetic beta-lactam antibiotics. For efficient synthesis a low affinity for phenylacetic acid and a high affinity for Calpha-substituted phenylacetic acid derivatives is desirable. We made three active site mutants, alphaF146Y, betaF24A and alphaF146Y/betaF24A, which all had a 2- to 10-fold higher affinity for Calpha-substituted compounds than wild-type enzyme. In addition, betaF24A had a 20-fold reduced affinity for phenylacetic acid. The molecular basis of the improved properties was investigated by X-ray crystallography. These studies showed that the higher affinity of alphaF146Y for (R)-alpha-methylphenylacetic acid can be explained by van der Waals interactions between alphaY146:OH and the Calpha-substituent. The betaF24A mutation causes an opening of the phenylacetic acid binding site. Only (R)-alpha-methylphenylacetic acid, but not phenylacetic acid, induces a conformation with the ligand tightly bound, explaining the weak binding of phenylacetic acid. A comparison of the betaF24A structure with other open conformations of penicillin acylase showed that betaF24 has a fixed position, whereas alphaF146 acts as a flexible lid on the binding site and reorients its position to achieve optimal substrate binding.

Regulog analysis: detection of conserved regulatory networks across bacteria: application to Staphylococcus aureus.

A transcriptional regulatory network encompasses sets of genes (regulons) whose expression states are directly altered in response to an activating signal, mediated by trans-acting regulatory proteins and cis-acting regulatory sequences. Enumeration of these network components is an essential step toward the creation of a framework for systems-based analysis of biological processes. Profile-based methods for the detection of cis-regulatory elements are often applied to predict regulon members, but they suffer from poor specificity. In this report we describe Regulogger, a novel computational method that uses comparative genomics to eliminate spurious members of predicted gene regulons. Regulogger produces regulogs, sets of coregulated genes for which the regulatory sequence has been conserved across multiple organisms. The quantitative method assigns a confidence score to each predicted regulog member on the basis of the degree of conservation of protein sequence and regulatory mechanisms. When applied to a reference collection of regulons from Escherichia coli, Regulogger increased the specificity of predictions up to 25-fold over methods that use cis-element detection in isolation. The enhanced specificity was observed across a wide range of biologically meaningful parameter combinations, indicating a robust and broad utility for the method. The power of computational pattern discovery methods coupled with Regulogger to unravel transcriptional networks was demonstrated in an analysis of the genome of Staphylococcus aureus. A total of 125 regulogs were found in this organism, including both well-defined functional groups and a subset with unknown functions.

The role of hydrophobic active-site residues in substrate specificity and acyl transfer activity of penicillin acylase.

Penicillin acylase of Escherichia coli catalyses the hydrolysis and synthesis of beta-lactam antibiotics. To study the role of hydrophobic residues in these reactions, we have mutated three active-site phenylalanines. Mutation of alphaF146, betaF24 and betaF57 to Tyr, Trp, Ala or Leu yielded mutants that were still capable of hydrolysing the chromogenic substrate 2-nitro-5-[(phenylacetyl)amino]-benzoic acid. Mutations on positions alphaF146 and betaF24 influenced both the hydrolytic and acyl transfer activity. This caused changes in the transferase/hydrolase ratios, ranging from a 40-fold decrease for alphaF146Y and alphaF146W to a threefold increase for alphaF146L and betaF24A, using 6-aminopenicillanic acid as the nucleophile. Further analysis of the betaF24A mutant showed that it had specificity constants (kcat/Km) for p-hydroxyphenylglycine methyl ester and phenylglycine methyl ester that were similar to the wild-type values, whereas the specificity constants for p-hydroxyphenylglycine amide and phenylglycine amide had decreased 10-fold, due to a decreased kcat value. A low amidase activity was also observed for the semisynthetic penicillins amoxicillin and ampicillin and the cephalosporins cefadroxil and cephalexin, for which the kcat values were fivefold to 10-fold lower than the wild-type values. The reduced specificity for the product and the high initial transferase/hydrolase ratio of betaF24A resulted in high yields in acyl transfer reactions.

Quantifying the accessibility of the metagenome by random expression cloning techniques.

The exploitation of the metagenome for novel biocatalysts by functional screening is determined by the ability to express the respective genes in a surrogate host. The probability of recovering a certain gene thereby depends on its abundance in the environmental DNA used for library construction, the chosen insert size, the length of the target gene, and the presence of expression signals that are functional in the host organism. In this paper, we present a set of formulas that describe the chance of isolating a gene by random expression cloning, taking into account the three different modes of heterologous gene expression: independent expression, expression as a transcriptional fusion and expression as a translational fusion. Genes of the last category are shown to be virtually inaccessible by shotgun cloning because of the low frequency of functional constructs. To evaluate which part of the metagenome might in this way evade exploitation, 32 complete genome sequences of prokaryotic organisms were analysed for the presence of expression signals functional in E. coli hosts, using bioinformatics tools. Our study reveals significant differences in the predicted expression modes between distinct taxonomic groups of organisms and suggests that about 40% of the enzymatic activities may be readily recovered by random cloning in E. coli.

Kinetics of enzyme acylation and deacylation in the penicillin acylase-catalyzed synthesis of beta-lactam antibiotics.

Penicillin acylase catalyses the hydrolysis and synthesis of semisynthetic beta-lactam antibiotics via formation of a covalent acyl-enzyme intermediate. The kinetic and mechanistic aspects of these reactions were studied. Stopped-flow experiments with the penicillin and ampicillin analogues 2-nitro-5-phenylacetoxy-benzoic acid (NIPAOB) and d-2-nitro-5-[(phenylglycyl)amino]-benzoic acid (NIPGB) showed that the rate-limiting step in the conversion of penicillin G and ampicillin is the formation of the acyl-enzyme. The phenylacetyl- and phenylglycyl-enzymes are hydrolysed with rate constants of at least 1000 s-1 and 75 s-1, respectively. A normal solvent deuterium kinetic isotope effect (KIE) of 2 on the hydrolysis of 2-nitro-5-[(phenylacetyl)amino]-benzoic acid (NIPAB), NIPGB and NIPAOB indicated that the formation of the acyl-enzyme proceeds via a general acid-base mechanism. In agreement with such a mechanism, the proton inventory of the kcat for NIPAB showed that one proton, with a fractionation factor of 0.5, is transferred in the transition state of the rate-limiting step. The overall KIE of 2 for the kcat of NIPAOB resulted from an inverse isotope effect at low concentrations of D2O, which is overridden by a large normal isotope effect at large molar fractions of D2O. Rate measurements in the presence of glycerol indicated that the inverse isotope effect originated from the higher viscosity of D2O compared to H2O. Deacylation of the acyl-enzyme was studied by nucleophile competition and inhibition experiments. The beta-lactam compound 7-aminodesacetoxycephalosporanic acid (7-ADCA) was a better nucleophile than 6-aminopenicillanic acid, caused by a higher affinity of the enzyme for 7-ADCA and complete suppression of hydrolysis of the acyl-enzyme upon binding of 7-ADCA. By combining the results of the steady-state, presteady state and nucleophile binding experiments, values for the relevant kinetic constants for the synthesis and hydrolysis of beta-lactam antibiotics were obtained.

Role of alphaArg145 and betaArg263 in the active site of penicillin acylase of Escherichia coli.

The active site of penicillin acylase of Escherichia coli contains two conserved arginine residues. The function of these arginines, alphaArg145 and betaArg263, was studied by site-directed mutagenesis and kinetic analysis of the mutant enzymes. The mutants alphaArg145-->Leu (alphaArg145Leu), alphaArg145Cys and alphaArg145Lys were normally processed and exported to the periplasm, whereas expression of the mutants betaArg263Leu, betaArg263Asn and betaArg263Lys yielded large amounts of precursor protein in the periplasm, indicating that betaArg263 is crucial for efficient processing of the enzyme. Either modification of both arginine residues by 2,3-butanedione or replacement by site-directed mutagenesis yielded enzymes with a decreased specificity (kcat/K(m)) for 2-nitro-5-[(phenylacetyl)amino]benzoic acid, indicating that both residues are important in catalysis. Compared with the wild type, the alphaArg145 mutants exhibited a 3-6-fold-increased preference for 6-aminopenicillanic acid as the deacylating nucleophile compared with water. Analysis of the steady-state parameters of these mutants for the hydrolysis of penicillin G and phenylacetamide indicated that destabilization of the Michaelis-Menten complex accounts for the improved activity with beta-lactam substrates. Analysis of pH-activity profiles of wild-type enzyme and the betaArg263Lys mutant showed that betaArg263 has to be positively charged for catalysis, but is not involved in substrate binding. The results provide an insight into the catalytic mechanism of penicillin acylase, in which alphaArg145 is involved in binding of beta-lactam substrates and betaArg263 is important both for stabilizing the transition state in the reaction and for correct processing of the precursor protein.