Conjugation to gold nanoparticles of methionine gamma-lyase, a cancer-starving enzyme. Physicochemical characterization of the nanocomplex for prospective nanomedicine applications.

The pyridoxal 5'-dependent enzyme methionine γ-lyase (MGL) catalyzes the degradation of methionine. This activity has been profitable to develop an antitumor agent exploiting the strict dependence of most malignant cells on the availability of methionine. Indeed, methionine depletion blocks tumor proliferation and leads to an increased susceptibility to anticancer drugs. Here, we explore the conjugation of MGL to gold nanoparticles capped with citrate (AuNPs) as a novel strategy to deliver MGL to cancer cells. Measurements of Transmission Electron Microscopy, Dynamic Light Scattering, Asymmetrical Flow Field-Flow Fractionation, X-ray Photoelectron Spectroscopy, and Circular Dichroism allowed to achieve an extensive biophysical and biochemical characterization of the MGL-AuNP complex including particle size, size distribution, MGL loading yield, enzymatic activity, and impact of gold surface on protein structure. Noticeably, we found that activity retention was improved over time for the enzyme adsorbed to AuNPs with respect to the enzyme free in solution. The acquired body of knowledge on the nanocomplex properties and this encouraging stabilizing effect upon conjugation are the necessary basis for further studies aimed at the evaluation of the therapeutic potential of MGL-AuNP complex in a biological milieu.

Methionine gamma lyase: Structure-activity relationships and therapeutic applications.

Methionine gamma lyase (MGL) is a bacterial and plant enzyme that catalyzes the conversion of methionine in methanthiol, 2-oxobutanoate and ammonia. The enzyme belongs to fold type I of the pyridoxal 5'-dependent family. The catalytic mechanism and the structure of wild type MGL and variants were determined in the presence of the natural substrate as well as of many sulfur-containing derivatives. Structure-function relationship studies were pivotal for MGL exploitation in the treatment of cancer, bacterial infections, and other diseases. MGL administration to cancer cells leads to methionine starvation, thus decreasing cells viability and increasing their vulnerability towards other drugs. In antibiotic therapy, MGL acts by transforming prodrugs in powerful drugs. Numerous strategies have been pursued for the delivering of MGL in vivo to prolong its bioavailability and decrease its immunogenicity. These include conjugation with polyethylene glycol and encapsulation in synthetic or natural vesicles, eventually decorated with tumor targeting molecules, such as the natural phytoestrogens daidzein and genistein. The scientific achievements in studying MGL structure, function and perspective therapeutic applications came from the efforts of many talented scientists, among which late Tatyana Demidkina to whom we dedicate this review.

Post-Translational Modifications of Histone Variants in the Absence and Presence of a Methionine-Depleting Enzyme in Normal and Cancer Cells.

Methionine is an essential amino acid involved in the formation of polyamines and a precursor metabolite for DNA and protein methylation. The dependence of cancer cells on methionine has triggered extensive investigations aimed at its targeting for cancer therapy, including the exploitation as a therapeutic tool of methionine γ-lyase (MGL), a bacterial enzyme that degrades methionine, capable of inhibiting cancer cells growth due to methionine starvation. We have exploited the high-resolution power of mass spectrometry to compare the effects of reduced availability of the methyl donor SAM, induced by MGL treatment, on the post-translational modifications of the histone tails in normal Hs27 and cancer HT-29 cells. In the absence of MGL, our analysis detected a three-fold higher relative abundance of trimethylated K25 of H1.4 in HT-29 than Hs27 cells, and a complex pattern of methylated, unmethylated and acetylated peptides in H2 and H3.3. In the presence of MGL, in HT-29, the peptide H2A1_4_11 is predominantly unmodified with mono-methylated K5 increasing upon treatment, whereas in Hs27 cells, H2A1_4_11 is monomethylated at K5 and K9 with these marks decreasing upon treatment. The time dependence of the effects of MGL-mediated methionine depletion on PTMs of histone variants in HT-29 cancer cells was also monitored. Overall, our present data on histone variants H1, H2A, H2B as well as H3.3 integrated with our previous studies on histones H3 and H4, shed light on the epigenetic modifications associated with methionine starvation and associated cancer cell death.

Use of Exogenous Enzymes in Human Therapy: Approved Drugs and Potential Applications.

The development of safe and efficacious enzyme-based human therapies has increased greatly in the last decades, thanks to remarkable advances in the understanding of the molecular mechanisms responsible for different diseases, and the characterization of the catalytic activity of relevant exogenous enzymes that may play a remedial effect in the treatment of such pathologies. Several enzyme-based biotherapeutics have been approved by FDA (the U.S. Food and Drug Administration) and EMA (the European Medicines Agency) and many are undergoing clinical trials. Apart from enzyme replacement therapy in human genetic diseases, which is not discussed in this review, approved enzymes for human therapy find applications in several fields, from cancer therapy to thrombolysis and the treatment, e.g., of clotting disorders, cystic fibrosis, lactose intolerance and collagen-based disorders. The majority of therapeutic enzymes are of microbial origin, the most convenient source due to fast, simple and cost-effective production and manipulation. The use of microbial recombinant enzymes has broadened prospects for human therapy but some hurdles such as high immunogenicity, protein instability, short half-life and low substrate affinity, still need to be tackled. Alternative sources of enzymes, with reduced side effects and improved activity, as well as genetic modification of the enzymes and novel delivery systems are constantly searched. Chemical modification strategies, targeted-and/or nanocarrier-mediated delivery, directed evolution and site-specific mutagenesis, fusion proteins generated by genetic manipulation are the most explored tools to reduce toxicity and improve bioavailability and cellular targeting. This review provides a description of exogenous enzymes that are presently employed for the therapeutic management of human diseases with their current FDA/EMA-approved status, along with those already experimented at the clinical level and potential promising candidates.

A Key Silencing Histone Mark on Chromatin Is Lost When Colorectal Adenocarcinoma Cells Are Depleted of Methionine by Methionine γ-Lyase.

Methionine is an essential amino acid used, beyond protein synthesis, for polyamine formation and DNA/RNA/protein methylation. Cancer cells require particularly high methionine supply for their homeostasis. A successful approach for decreasing methionine concentration is based on the systemic delivery of methionine γ-lyase (MGL), with in vitro and in vivo studies demonstrating its efficacy in cancer therapy. However, the mechanisms explaining how cancer cells suffer from the absence of methionine more significantly than non-malignant cells are still unclear. We analyzed the outcome of the human colorectal adenocarcinoma cancer cell line HT29 to the exposure of MGL for up to 72 h by monitoring cell viability, proteome expression, histone post-translational modifications, and presence of spurious transcription. The rationale of this study was to verify whether reduced methionine supply would affect chromatin decondensation by changing the levels of histone methylation and therefore increasing genomic instability. MGL treatment showed a time-dependent cytotoxic effect on HT29 cancer cells, with an IC50 of 30 µg/ml, while Hs27 normal cells were less affected, with an IC50 of >460 µg/ml. Although the levels of total histone methylation were not altered, a loss of the silencing histone mark H3K9me2 was observed, as well as a decrease in H4K20me3. Since H3K9me2/3 decorate repetitive DNA elements, we proved by qRT-PCR that MGL treatment leads to an increased expression of major satellite units. Our data indicate that selected histone methylation marks may play major roles in the mechanism of methionine starvation in cancer cells, proving that MGL treatment directly impacts chromatin homeostasis.

Discovery of Substituted (2-Aminooxazol-4-yl)Isoxazole-3-carboxylic Acids as Inhibitors of Bacterial Serine Acetyltransferase in the Quest for Novel Potential Antibacterial Adjuvants.

Many bacteria and actinomycetales use L-cysteine biosynthesis to increase their tolerance to antibacterial treatment and establish a long-lasting infection. In turn, this might lead to the onset of antimicrobial resistance that currently represents one of the most menacing threats to public health worldwide. The biosynthetic machinery required to synthesise L-cysteine is absent in mammals; therefore, its exploitation as a drug target is particularly promising. In this article, we report a series of inhibitors of Salmonella thyphimurium serine acetyltransferase (SAT), the enzyme that catalyzes the rate-limiting step of L-cysteine biosynthesis. The development of such inhibitors started with the virtual screening of an in-house library of compounds that led to the selection of seven structurally unrelated hit derivatives. A set of molecules structurally related to hit compound 5, coming either from the original library or from medicinal chemistry efforts, were tested to determine a preliminary structure-activity relationship and, especially, to improve the inhibitory potency of the derivatives, that was indeed ameliorated by several folds compared to hit compound 5 Despite these progresses, at this stage, the most promising compound failed to interfere with bacterial growth when tested on a Gram-negative model organism, anticipating the need for further research efforts.

Inhibition of Nonessential Bacterial Targets: Discovery of a Novel Serine O-Acetyltransferase Inhibitor.

In ϒ-proteobacteria and Actinomycetales, cysteine biosynthetic enzymes are indispensable during persistence and become dispensable during growth or acute infection. The biosynthetic machinery required to convert inorganic sulfur into cysteine is absent in mammals; therefore, it is a suitable drug target. We searched for inhibitors of Salmonella serine acetyltransferase (SAT), the enzyme that catalyzes the rate-limiting step of l-cysteine biosynthesis. The virtual screening of three ChemDiv focused libraries containing 91 243 compounds was performed to identify potential SAT inhibitors. Scaffold similarity and the analysis of the overall physicochemical properties allowed the selection of 73 compounds that were purchased and evaluated on the recombinant enzyme. Six compounds displaying an IC50 <100 µM were identified via an indirect assay using Ellman's reagent and then tested on a Gram-negative model organism, with one of them being able to interfere with bacterial growth via SAT inhibition.

Is the protein profile of pig Longissimus dorsi affected by gender and diet?

The impact of gender and diet on the proteome of Longissimus dorsi was addressed by 2D-PAGE analysis of male and female pigs, fed with a barley-based control diet and a diet enriched with extruded linseed and plant extracts. No statistically significant difference in protein number between female and male samples was found. Furthermore, PCA excluded gender-dependent protein clusters. For both the control and enriched diet, several spots exhibited at least a 1.5-fold intensity difference, but none showed a statistically relevant variation. Protein profiles PCA for both diets indicated that the first two principal components account up to 47% of total variance, with two diet-dependent separated clusters. Among 176 common spots, 29 exhibited >1.5 fold change, mostly more abundant in the control diet. PMF identified 14 distinct proteins, including myofibrillar proteins, glycolytic enzymes and myoglobin, thus suggesting a diet-dependent meat quality. A statistically significant increase in carbonylated proteins of enriched diet samples was detected using the 2,4-dinitrophenylhydrazine method but not using fluorescein-5-thiosemicarbazide-labeled bands. ROS induction and DNA oxidative damage, detected in a human cell line exposed to digested meat from both diets, further support the notion that the enriched diet does not protect against oxidative stress. SIGNIFICANCE: The comparison of the protein profile of female and male Longissimus dorsi from pigs fed by a control diet and a diet enriched with polyphenols, indicate no gender effect, whereas diet affects the abundance of several proteins, possibly linked to meat quality. Protein carbonylation was statistically higher in meat from the enriched diet, suggesting that polyphenols at the concentration present in the diet did not exert a protective effect against oxidation.

ADIFAB fluorescence data used for the quantification of free fatty acids in media at different pH.

We investigated the pH dependence of the fluorescence spectra of ADIFAB (FFA Sciences), a probe used for the quantification of free fatty acids (FFA). Data reports the change in the emission peak of ADIFAB and in the affinity for FFA as a function of pH. An algorithm based on spectral deconvolution allowed to correct ADIFAB fluorescence spectra for the spectroscopic effect caused by pH. K d values were calculated at each pH based on a calibration with oleic acid. This method allows estimating FFA concentration by ADIFAB in media at different pH. The current data are related to the research article "Phospholipid components of the synthetic pulmonary surfactant CHF5633 probed by fluorescence spectroscopy" (Faggiano et al., 2018) [1].

Engineering methionine γ-lyase from Citrobacter freundii for anticancer activity.

Methionine deprivation of cancer cells, which are deficient in methionine biosynthesis, has been envisioned as a therapeutic strategy to reduce cancer cell viability. Methionine γ-lyase (MGL), an enzyme that degrades methionine, has been exploited to selectively remove the amino acid from cancer cell environment. In order to increase MGL catalytic activity, we performed sequence and structure conservation analysis of MGLs from various microorganisms. Whereas most of the residues in the active site and at the dimer interface were found to be conserved, residues located in the C-terminal flexible loop, forming a wall of the active site entry channel, were found to be variable. Therefore, we carried out site-saturation mutagenesis at four independent positions of the C-terminal flexible loop, P357, V358, P360 and A366 of MGL from Citrobacter freundii, generating libraries that were screened for activity. Among the active variants, V358Y exhibits a 1.9-fold increase in the catalytic rate and a 3-fold increase in KM, resulting in a catalytic efficiency similar to wild type MGL. V358Y cytotoxic activity was assessed towards a panel of cancer and nonmalignant cell lines and found to exhibit IC50 lower than the wild type. The comparison of the 3D-structure of V358Y MGL with other MGL available structures indicates that the C-terminal loop is either in an open or closed conformation that does not depend on the amino acid at position 358. Nevertheless, mutations at this position allosterically affects catalysis.

Phospholipid components of the synthetic pulmonary surfactant CHF5633 probed by fluorescence spectroscopy.

CHF5633 (Chiesi Farmaceutici, Italy) is a synthetic pulmonary surfactant currently under clinical development for the treatment of Respiratory Distress Syndrome in premature infants. The product is composed of phospholipids in liposomal organization, together with two peptide analogues of human surfactant proteins B and C. Phospholipids in liposomes can undergo oxidation of unsaturated lipids and hydrolysis, with formation of fatty acids and lysolipids, both affecting the physico-chemical properties of the formulation. We exploited two fluorescence probes, Prodan and ADIFAB, to evaluate the stability of the phospholipid components of CHF5633. While Prodan enters the phospholipid bilayer and probes the polarity of this environment, ADIFAB binds free fatty acids in the aqueous phase, allowing to determine their concentration. Changes of Prodan fluorescence emission indicated an increase in the polarity of the phospholipid bilayer as a function of time. This behavior is coupled with an increase in fatty acids concentration in the aqueous phase, as determined by ADIFAB, and an increase in lysolipids concentration, as determined by HPLC-MS. Prodan and ADIFAB resulted efficient probes to monitor phospholipids hydrolysis in liposomes, reporting an increased stability of CHF5633 at pH values higher than 6.5.

Insight into GFPmut2 pH Dependence by Single Crystal Microspectrophotometry and X-ray Crystallography.

The fluorescence of Green Fluorescent Protein (wtGFP) and variants has been exploited in distinct applications in cellular and analytical biology. GFPs emission depends on the population of the protonated (A-state) and deprotonated (B-state) forms of the chromophore. Whereas wtGFP is pH-independent, mutants in which Ser65 is replaced by either threonine or alanine (as in GFPmut2) are pH-dependent, with a p Ka around 6. Given the wtGFP pH-independence, only the structure of the protonated form was determined. The deprotonated form was deduced on the basis of the crystal structure of the Ser65Thr mutant at basic pH, assuming that it corresponds to the conformation populated in solution. Here, we present an investigation where structures of the protonated and deprotonated forms of GFPmut2 were determined from crystals grown in either MPD at pH 6 or PEG at pH 8.5, and moved to either higher or lower pH. Both crystal forms of GFPmut2 were titrated monitoring the process via polarized absorption microspectrophotometry in order to precisely correlate the protonation process with the structures. We found that (i) in solution, chromophore titration is not thermodynamically coupled with any residue and Glu222 is always protonated independent of the protonation state of the chromophore; (ii) the lack of coupling is reflected in the structural behavior of the chromophore and Glu222 environments, with only the former showing variations with pH; (iii) titrations of low-pH and high-pH grown crystals exhibit a Hill coefficient of about 0.75, indicating an anticooperative behavior not observed in solution; (iv) structures where pH was changed in the crystal point to Glu222 as the ionizable group responsible for the outset of the anticooperative behavior; and (v) in GFPmut2 the canonical GFP proton wire involving the chromophore is not interrupted at the level of Ser205 and Glu222 at basic pH as in the Ser65Thr mutant. This allows proposing the structure of the deprotonated state of GFPmut2 as an alternative model for the analogous state of wtGFP.

Soluble and Nanoporous Silica Gel-Entrapped C. freundii Methionine γ-Lyase.

Methionine γ-lyase is a pyridoxal 5'-phosphate dependent tetramer that catalyzes the α,γ-elimination of methionine in ammonia, methanethiol and α-ketobutyrate. MGL catalytic power has been exploited as a therapeutic strategy to reduce the viability of cancer cells or bacteria. In order to obtain a stable enzyme to be delivered at the site of action, MGL can be encapsulated in a variety of matrices. As a reference encapsulation strategy we have prepared MGL nanoporous wet silica gels. Immobilized MGL gels were characterized with regards to activity, stability, absorption, circular dichroism and fluorescence properties and compared with soluble MGL. We found that MGL gels exhibit (i) spectroscopic properties very similar to MGL in solution, (ii) a higher stability with respect to the soluble enzyme and (iii) catalytic activity six-fold lower than in solution. These findings prove that MGL encapsulation is a suitable strategy for therapeutic applications.

Human serine racemase is nitrosylated at multiple sites.

Serine racemase is a pyridoxal 5'­phosphate dependent enzyme responsible for the synthesis of d­serine, a neuromodulator of the NMDA receptors. Its activity is modulated by several ligands, including ATP, divalent cations and protein interactors. The murine orthologue is inhibited by S-nitrosylation at Cys113, a residue adjacent to the ATP binding site. We found that the time course of inhibition of human serine racemase by S-nitrosylation is markedly biphasic, with a fast phase associated with the reaction of Cys113. Unlike the murine enzyme, two additional cysteine residues, Cys269, unique to the human orthologue, and Cys128 were also recognized as S-nitrosylation sites through mass spectrometry and site-directed mutagenesis. The effect of S-nitrosylation on the fluorescence of tryptophan residues and on that of the pyridoxal phosphate cofactor indicated that S-nitrosylation produces a partial interruption of the cross-talk between the ATP binding site and the active site. Overall, it appears that the inhibition results from a conformational change rather than the direct displacement of ATP.

The Energy Landscape of Human Serine Racemase.

Human serine racemase is a pyridoxal 5'-phosphate (PLP)-dependent dimeric enzyme that catalyzes the reversible racemization of L-serine and D-serine and their dehydration to pyruvate and ammonia. As D-serine is the co-agonist of the N-methyl-D-aspartate receptors for glutamate, the most abundant excitatory neurotransmitter in the brain, the structure, dynamics, function, regulation and cellular localization of serine racemase have been investigated in detail. Serine racemase belongs to the fold-type II of the PLP-dependent enzyme family and structural models from several orthologs are available. The comparison of structures of serine racemase co-crystallized with or without ligands indicates the presence of at least one open and one closed conformation, suggesting that conformational flexibility plays a relevant role in enzyme regulation. ATP, Mg2+, Ca2+, anions, NADH and protein interactors, as well as the post-translational modifications nitrosylation and phosphorylation, finely tune the racemase and dehydratase activities and their relative reaction rates. Further information on serine racemase structure and dynamics resulted from the search for inhibitors with potential therapeutic applications. The cumulative knowledge on human serine racemase allowed obtaining insights into its conformational landscape and into the mechanisms of cross-talk between the effector binding sites and the active site.

Gene cloning, characterization, and cytotoxic activity of methionine γ-lyase from Clostridium novyi.

The exploitation of methionine-depleting enzyme methionine γ-lyase (MGL) is a promising strategy against specific cancer cells that are strongly dependent on methionine. To identify MGL from different sources with high catalytic activity and efficient anticancer action, we have expressed and characterized MGL from Clostridium novyi and compared its catalytic efficiency with the previously studied MGL from Citrobacter freundii. The purified recombinant MGL exhibits kcat and kcat /Km for methionine γ-elimination reaction that are 2.4- and 1.36-fold higher than C. freundii enzyme, respectively, whereas absorption, fluorescence, and circular dichroism spectra are very similar, as expected on the basis of 87% sequence identity and high conservation of active site residues. The reactivity of cysteine residues with DTNB and iodoacetamide was investigated as well as the impact of their chemical modification on catalytic activity. This information is relevant because for increasing bioavailability and reducing immunogenity, MGL should be decorated with polyethylene glycol (PEG). It was found that Cys118 is a faster reacting residue, which results in a significant decrease in the γ-elimination activity. Thus, the protection of Cys118 before conjugation with cysteine-reacting PEG represents a valuable strategy to preserve MGL activity. The anticancer action of C. novyi MGL, evaluated in vitro against prostate (PC-3), chronic myelogenous leucemia (K562), and breast (MDA-MB-231 and MCF7) cancer cells, exhibits IC50 of 1.3 U mL-1 , 4.4 U mL-1 , 1.2 U mL-1 , and 3.4 U mL-1 , respectively. A higher cytotoxicity of C. novyi MGL was found against cancer cells with respect to C. freundii MGL, with the exception of PC-3, where a lower cytotoxicity was observed. © 2017 IUBMB Life, 69(9):668-676, 2017.

Site-Specific Transglutaminase-Mediated Conjugation of Interferon α-2b at Glutamine or Lysine Residues.

Interferon α (IFN α) subtypes are important protein drugs that have been used to treat infectious diseases and cancers. Here, we studied the reactivity of IFN α-2b to microbial transglutaminase (TGase) with the aim of obtaining a site-specific conjugation of this protein drug. Interestingly, TGase allowed the production of two monoderivatized isomers of IFN with high yields. Characterization by mass spectrometry of the two conjugates indicated that they are exclusively modified at the level of Gln101 if the protein is reacted in the presence of an amino-containing ligand (i.e., dansylcadaverine) or at the level of Lys164 if a glutamine-containing molecule is used (i.e., carbobenzoxy-l-glutaminyl-glycine, ZQG). We explained the extraordinary specificity of the TGase-mediated reaction on the basis of the conformational features of IFN. Indeed, among the 10 Lys and 12 Gln residues of the protein, only Gln101 and Lys164 are located in highly flexible protein regions. The TGase-mediated derivatization of IFN was then applied to the production of IFN derivatives conjugated to a 20 kDa polyethylene glycol (PEG), using PEG-NH2 for Gln101 derivatization and PEG modified with ZQG for Lys164 derivatization. The two mono-PEGylated isomers of IFN were obtained in good yields, purified, and characterized in terms of protein conformation, antiviral activity, and pharmacokinetics. Both conjugates maintained a native-like secondary structure, as indicated by far-UV circular dichroism spectra. Importantly, they disclosed good in vitro antiviral activity retention (about only 1.6- to 1.8-fold lower than that of IFN) and half-lives longer (about 5-fold) than that of IFN after intravenous administration to rats. Overall, these results provide evidence that TGase can be used for the development of site-specific derivatives of IFN α-2b possessing interesting antiviral and pharmacokinetic properties.

Insight of Saffron Proteome by Gel-Electrophoresis.

Saffron is a spice comprised of the dried stigmas and styles of Crocus sativus L. flowers and, since it is very expensive, it is frequently adulterated. So far, proteomic tools have never been applied to characterize the proteome of saffron or identify possible cases of fraud. In this study, 1D-Gel Electrophoresis was carried out to characterize the protein profile of (i) fresh stigmas and styles of the plant; (ii) dried stigmas and styles from different geographical origins (Spanish, Italian, Greek and Iranian) that had been stored for various periods of time after their processing; and (iii) two common plant adulterants, dried petals of Carthamus tinctorius L. and dried fruits of Gardenia jasminoides Ellis. A selective protein extraction protocol was applied to avoid interference from colored saffron metabolites, such as crocins, during electrophoretic analyses of saffron. We succeeded in separating and assigning the molecular weights to more than 20 proteins. In spite of the unavailability of the genome of saffron, we were able to identify five proteins by Peptide Mass Fingerprinting: phosphoenolpyruvate carboxylase 3, heat shock cognate 70 KDa protein, crocetin glucosyltransferase 2, α-1,4-glucan-protein synthase and glyceraldehydes-3-phosphate dehydrogenase-2. Our findings indicate that (i) few bands are present in all saffron samples independently of origin and storage time, with amounts that significantly vary among samples and (ii) aging during saffron storage is associated with a reduction in the number of detectable bands, suggesting that proteases are still active. The protein pattern of saffron was quite distinct from those of two common adulterants, such as the dried petals of Carthamus tinctorius and the dried fruits of Gardenia jasminoides indicating that proteomic analyses could be exploited for detecting possible frauds.

Inhibitors of the sulfur assimilation pathway in bacterial pathogens as enhancers of antibiotic therapy.

The rising emergence of antibiotic resistance urges the search for new strategies to defeat microorganisms that lead to persistent infections of the host. Tolerant to antibiotics, slowly replicating bacteria often cause latent and persistent infections that are the most challenging for pharmacological treatment. Persistence inside the host requires an extensive re-programming of the pathogen metabolic functions, due to the extremely hostile environment they face. Therefore, targeting key metabolic functions could result in better antibiotic treatments, shortened latency periods, and increased susceptibility to traditional antibiotics. Bacteria, differently from mammals, assimilate inorganic sulfur into cysteine, the precursor of a number of key metabolites including reducing agents, cofactors and membrane components. Inhibition of cysteine biosynthesis was proven to interfere heavily with the ability of pathogens to fight oxidative stress, to infect the host and to establish long-term infections. This review has the purpose of i) briefly summarizing the key structural and functional properties of transporters and enzymes involved in sulfur assimilation, ii) presenting biological evidence that supports the exploitation of this pathway for the identification of potential targets and, iii) highlighting intense efforts and advancements in the search of promising candidates for the development of novel compounds that enhance antibiotics therapy.

Site-specific derivatization of avidin using microbial transglutaminase.

Avidin conjugates have several important applications in biotechnology and medicine. In this work, we investigated the possibility to produce site-specific derivatives of avidin using microbial transglutaminase (TGase). TGase allows the modification of proteins at the level of Gln or Lys residues using as substrate an alkyl-amine or a Gln-mimicking moiety, respectively. The reaction is site-specific, since Gln and Lys derivatization occurs preferentially at residues embedded in flexible regions of protein substrates. An analysis of the X-ray structure of avidin allowed us to predict Gln126 and Lys127 as potential sites of TGase's attack, because these residues are located in the flexible/unfolded C-terminal region of the protein. Surprisingly, incubation of avidin with TGase in the presence of alkylamine containing substrates (dansylcadaverine, 5-hydroxytryptamine) revealed a very low level of derivatization of the Gln126 residue. Analysis of the TGase reaction on synthetic peptide analogues of the C-terminal portion of avidin indicated that the lack of reactivity of Gln126 was likely due to the fact that this residue is proximal to negatively charged carboxylate groups, thus hampering the interaction of the substrate at the negatively charged active site of TGase. On the other hand, incubation of avidin with TGase in the presence of carbobenzoxy-l-glutaminyl-glycine in order to derivatize Lys residue(s) resulted in a clean and high yield production of an avidin derivative, retaining the biotin binding properties and the quaternary structure of the native protein. Proteolytic digestion of the modified protein, followed by mass spectrometry, allowed us to identify Lys127 as the major site of reaction, together with a minor modification of Lys58. By using TGase, avidin was also conjugated via a Lys-Gln isopeptide bond to a protein containing a single reactive Gln residue, namely, Gln126 of granulocyte-macrophage colony-stimulating factor. TGase can thus be exploited for the site-specific derivatization of avidin with small molecules or proteins.