Proline-dependent oligomerization with arm exchange.

BACKGROUND: Oligomerization is often necessary for protein activity or regulation and its efficiency is fundamental for the cell. The quaternary structure of a large number of oligomers consists of protomers tightly anchored to each other by exchanged arms or swapped domains. However, nothing is known about how the arms can be kept in a favourable conformation before such an oligomerization. RESULTS: Upon examination of such quaternary structures, we observe an extremely frequent occurrence of proline residues at the point where the arm leaves the protomer. Sequence alignment and site-directed mutagenesis confirm the importance of these prolines. The conservation of these residues at the hinge regions can be explained by the constraints that they impose on polypeptide conformation and dynamics: by rigidifying the mainchain, prolines favour extended conformations of arms thus favouring oligomerization, and may prevent interaction of the arms with the core of the protomer. CONCLUSIONS: Hinge prolines can be considered as 'quaternary structure helpers'. The presence of a proline should be considered when searching for a determinant of oligomerization with arm exchange and could be used to engineer synthetic oligomers or to displace a monomers to oligomers equilibrium by mutation of this proline residue.

Microenvironmental effects on enzyme catalysis. A kinetic study of hydrophobic derivatives of chymotrypsin.

The aim of this work was to study the role of hydrophobic interactions in the enzymic activity of chymotrypsin. The amino groups of chymotrypsin were chemically modified by aliphatic aldehydes of various chain lengths - acetaldehyde, butyraldehyde, hexanal - and with two aldehydes of different steric hindrance - benzaldehyde and trimethyl acetaldehyde. After a rapid study of the derivated enzymes, the hexylchymotrypsin has been chosen for its new catalytic properties: the Michaelis constant is not modified and the maximal velocity with N-glutaryl-L-phenylalanine-4-nitroaniline is increased to 164%. The increase is due to the increase of the acylation constant, k2, by 230%. The value of k3 is not modified or less modified. In the modified enzyme, 85% of free amino acids are still able to react with trinitrobenzenesulphonic acid. The optimum pH is shifted by one pH unit towards the alkaline pH. The thermodynamic study shows that the catalytic process itself is not modified. The increase in Vm could be a simple increase of k2 linked to a modification of the site or of the protein. The phenomenon described is very specific and obtained only with one modification, hexanal, and with one enzyme, alpha-chymotrypsin.

Kinetics of amino acid esterification catalyzed by hydrophobic derivatives of alpha-chymotrypsin.

Hexyl-alpha-chymotrypsin, a hydrophobic derivative of the enzyme, is explored for the proteinase-catalyzed ester synthesis reaction with N-acetyl-L-tyrosine and ethanol. To guarantee the preservation of the enzyme activity and to allow for the extraction of the product in the organic phase, a biphasic system was used. The Vm increased for the modified enzyme. This phenomenon was linked to the modification of the protein as demonstrated by its chemical denaturation with sodium dodecyl sulfate.

Studies on haemoglobin immobilized by cross-linking with glutaraldehyde. Cross-linked soluble polymers and artificial membranes.

Human haemoglobin was immobilized by cross-linking with glutaraldehyde as soluble polymers and artificial membranes. Effects of pH and 2,3-diphosphoglycerate on oxygen binding and cross-linking were studied with haemoglobin immobilized in both the oxy and deoxy states. The cooperativity is suppressed and the affinity is increased when compared with native haemoglobin. Haemoglobin immobilized in the oxy state exhibited a higher oxygen affinity than that immobilized in the deoxy state. The alkaline Bohr effect is not significantly different from that of native haemoglobin. The 2,3-diphosphoglycerate influence on oxygen binding was reduced by one third with immobilization. In order to separate the chemical and the "conformation freezing' effects on the properties of immobilized haemoglobin, glutaraldehyde-modified haemoglobin in oxy and deoxy states was produced. Oxygen binding was studied and chemical modifications were checked by electrophoresis and gel filtration. This chemically modified haemoglobin without polymerization and without intra-chain bridging exhibits a behaviour similar to that of cross-linked soluble polymers or membranes of haemoglobin.

Acetylcholinesterases from Elapidae snake venoms: biochemical, immunological and enzymatic characterization.

We analyzed 45 batches of venom from 20 different species belonging to 11 genera from the 3 main families of venomous snakes (Elapidae, Viperidae and Crotalidae). We found high acetylcholinesterase (AChE) activity in all venoms from Elapidae, except in those from the Dendroaspis genus. AChE was particularly abundant in Bungarus venoms which contain up to 8 mg of enzyme per gram of dried venom. We could not detect acetylcholinesterase activity in any batch of venom from Viperidae or Crotalidae. Titration of active sites with an organophosphorous agent (MPT) revealed that the AChE of all venoms have similar turnovers (6000 to 8000 s(-1)) which are clearly higher than those of Torpedo and mammalian enzymes but lower than that of Electrophorus. AChEs from the venom of elapid snakes of the Bungarus, Naja, Ophiophagus and Haemacatus genera were purified by affinity chromatography. SDS-PAGE analysis and sucrose gradient centrifugation demonstrated that AChE is exclusively present as a nonamphiphilic monomer. These enzymes are true AChEs, hydrolyzing acetylthiocholine faster than propionylthiocholine and butyrylthiocholine and exhibiting excess substrate inhibition. Twenty-seven different monoclonal antibodies directed against AChE from Bungarus fasciatus venom were raised in mice. Half of them recognized exclusively the Bungarus enzyme while the others cross-reacted with AChEs from other venoms. Polyspecific mAbs were used to demonstrate that venoms from Dendroaspis, which contain the AChE inhibitor fasciculin but lack AChE activity, were also devoid of immunoreactive AChE protein. AChE inhibitors acting at the active site (edrophonium, tacrine) and at the peripheral site (propidium, fasciculin), as well as bis-quaternary ligands (BW284C51, decamethonium), were tested against the venom AChEs from 11 different species. All enzymes had a very similar pattern of reactivity with regard to the different inhibitors, with the exception of fasciculin. AChEs from Naja and Haemacatus venoms were relatively insensitive to fasciculin inhibition (IC50 >> 10(-6) M), while Bungarus (IC50 approximately 10(-8) M) and especially Ophiophagus (IC50 < 10(-10) M) AChEs were inhibited very efficiently. Ophiophagus and Bungarus AChEs were also efficiently inhibited by a monoclonal antibody (Elec-410) previously described as a specific ligand for the Electrophorus electricus peripheral site. Taken together, these results show that the venoms of most Elapidae snakes contain large amounts of a highly active non-amphiphilic monomeric AChE. All snake venom AChEs show strong immunological similarities and possess very similar enzymatic properties. However, they present quite different sensitivity to peripheral site inhibitors, fasciculin and the monoclonal antibody Elec-410.

Immunogenicity and antigenicity of soluble cross-linked enzyme/albumin polymers: Advantages for enzyme therapy.

Soluble cross-linked polymers of hog-liver uricase and an excess of either rabbit or dog albumin were injected repeatedly into rabbits in order to determine their antigenicity and immunogenicity. Whereas the enzyme in its free form induces antibody production, homologous albumin polymerised with the enzyme renders the complex non-immunogenic and non-antigenic. Dog-albumin/uricase polymers injected into rabbits induce antibody formation against the dog albumin but not against the uricase. The results suggest an important advantage of these soluble enzyme polymers in enzyme-replacement therapy.

Characterization of monoclonal antibodies that strongly inhibit Electrophorus electricus acetylcholinesterase.

In this study, we describe three different monoclonal antibodies (mAbs Elec-403, Elec-408, and Elec-410) directed against Electrophorus electricus acetylcholinesterase (AChE) which were selected as inhibitors for this enzyme. Two of these antibodies (Elec-403 and Elec-410), recognized overlapping but different epitopes, competed with snake venom toxin fasciculin for binding to the enzyme, and thus apparently recognized the peripheral site of AChE. In addition, the binding of Elec-403 was antagonized by 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284C51) and propidium, indicating that the corresponding epitope encompassed the anionic site involved in the binding of these low-molecular-mass inhibitors. The third mAb (Elec-408), was clearly bound to another site on the AChE molecule, and its inhibitory effect was cumulative with those of Elec-403, Elec-410, and fasciculin. All mAbs bound AChE with high affinity and were as strong inhibitors with an apparent Ki values less than 0.1 nM. Elec-403 was particularly efficient with an inhibitory activity similar to that of fasciculin. Inhibition was observed with both charged (acetylthiocholine) and neutral substrates (o-nitrophenyl acetate) and had the characteristics of a non-competitive process. Elec-403 and Elec-410 probably exert their effect by triggering allosteric transitions from the peripheral site to the active site. The epitope recognized by mAb Elec-408 has not been localized, but it may correspond to a new regulatory site on AChE.

Topographical and functional investigation of Escherichia coli penicillin-binding protein 1b by alanine stretch scanning mutagenesis.

Penicillin-binding proteins (PBPs) are the targets of beta-lactam antibiotics. We have used a systematic five-alanine substitution method (called ASS [alanine stretch scanning] mutagenesis) to investigate the functional or structural role of various stretches of amino acids in the PBP1b of Escherichia coli. To probe the specific activity of each variant, the antibiotic discs assay was used with strain QCB1 (delta ponB) in the presence of cefaloridine, which totally inhibits the complementing action of PBP1a. This in vivo test has been combined with a quick and efficient in vitro test of the penicillin-binding activity of each of these variants with fluorescent penicillin. This approach has enabled us to show an unexpected role of the N-terminal and C-terminal tails of PBP1b. Moreover, we have established the correct position in PBP1b of the SMN motif that, with the SXXK and the KTG motifs, constitutes the signature of the penicilloyl serine transferases family. Finally, we have shown that the transglycosylase and the transpeptidase domains are separated by an inert linker region, where substitutions and insertions can be made without hindering the in vivo and in vitro activity of the protein.

Alanine-stretch scanning mutagenesis: a simple and efficient method to probe protein structure and function.

We have developed a foolproof method to substitute a stretch of residues by alanines. After the introduction of aPstI site by IPCR, thus creating two alanine codons, additional codons are introduced at this site through the use of an 'alanine-stretch cartridge'. These cartridges comprise an antibiotic resistance gene flanked on both sides by alanine codons. Excision of the resistance gene byPvuII then yields the correct insertion of codons. The method is both highly reliable and flexible and should be of general use.

Disulfide bridges are not involved in penicillin-binding protein 1b dimerization in Escherichia coli.

PBP1b can be found as a dimer in Escherichia coli. Previous results suggested that dimerization involved the cysteine(s) in an intermolecular disulfide bond. We show that either deletion mutants or a mutant without cysteines is fully active and still binds penicillin and that the latter can also form dimers.

Differential responses of Escherichia coli cells expressing cytoplasmic domain mutants of penicillin-binding protein 1b after impairment of penicillin-binding proteins 1a and 3.

Penicillin-binding protein 1b (PBP1b) is the major high-molecular-weight PBP in Escherichia coli. Although it is coded by a single gene, it is usually found as a mixture of three isoforms which vary with regard to the length of their N-terminal cytoplasmic tail. We show here that although the cytoplasmic tail seems to play no role in the dimerization of PBP1b, as was originally suspected, only the full-length protein is able to protect the cells against lysis when both PBP1a and PBP3 are inhibited by antibiotics. This suggests a specific role for the full-length PBP1b in the multienzyme peptidoglycan-synthesizing complex that cannot be fulfilled by either PBP1a or the shorter PBP1b proteins. Moreover, we have shown by alanine-stretch-scanning mutagenesis that (i) residues R(11) to G(13) are major determinants for correct translocation and folding of PBP1b and that (ii) the specific interactions involving the full-length PBP1b can be ascribed to the first six residues at the N-terminal end of the cytoplasmic domain. These results are discussed in terms of the interactions with other components of the murein-synthesizing complex.

Monoclonal anti-idiotypic antibodies as functional internal images of enzyme active sites: production of a catalytic antibody with a cholinesterase activity.

Monoclonal antibody 9A8 was selected by immunizing mice with AE-2, a monoclonal antibody directed against the active site of acetylcholinesterase. In accordance with the idiotypic network theory, monoclonal anti-idiotypic antibody 9A8 displayed internal-image properties of the original immunogen, the acetylcholinesterase active site. Hydrolysis of acetylthiocholine and related esters of thiocholine by 9A8 follows saturation kinetics and kinetic parameters were determined. The hydrolytic activity is characterized by a lowered kcat value (81 s-1) and an increased Km value (0.6 mM) when compared with the original enzyme. However, the rate acceleration (kcat/kuncat = 4.15 x 10(8) remains higher than for the esterase activities usually described for catalytic antibodies directed against transition-state analogs. The 9A8 activity exhibits a relaxation of specificity toward both substrates and inhibitors. This specificity does not correspond to a known enzymatic activity. The anti-idiotypic approach should be valuable for producing different structural and functional copies of the same enzyme active site. This should allow further insights into structure-activity relationships. Furthermore, use of chemically modified enzymes as immunogens may result in anti-idiotypic antibodies with catalytic activities not found in the native enzymes.

Kinetic behaviour of acid phosphatase-albumin co-polymers in homogeneous phase and under gel-immobilized conditions.

1. An analysis of the kinetic behaviour of immobilized acid phosphatase (EC 3.1.3.2) layers, gelled on the active surface of an ultrafiltration membrane, was carried out. 2. Two possible forms of such immobilized-enzyme systems were dealt with, namely enzyme-polyalbumin co-gelation through an ultrafiltration process, and enzyme co-polymerization to the same albumin polymers and subsequent gelation. 3. A preliminary analysis was also performed on both the corresponding homogeneous-phase (soluble systems to provide reference kinetics. 4. The main conclusions drawn are: (i) the enzyme-albumin co-polymers show a decrease in specific activity compared with the corresponding free enzyme in both soluble and immobilized forms; (ii) in the homogeneous phase a slight increase in the apparent Michaelis constant was measured for the co-polymerized enzyme compared with the free one, which suggests a decrease in affinity towards substrate; (iii) the activation energy in the immobilized phase is halved, compared with that in the homogeneous phase, which indicates that the combined mass-transfer/reaction step is rate-controlling.

Retrograde axonal transport of an exogenous enzyme covalently linked to B-IIb fragment of tetanus toxin.

Attempt to replace enzymes in a number of fatal lysosomal storage disease involving the central nervous system have as yet been unsuccessful owing to the impermeability of the blood/brain barrier to macromolecules. In order to treat storage disease due to enzyme deficiencies, we investigated the feasibility of transporting an enzyme into the central nervous system without crossing the blood/brain barrier. Using the B-IIb fragment of tetanus toxin (because it is involved in recognition by the nerve-cell endings), retrograde axonal transport toward the spinal cord and trans-synaptic movement, and glucose oxidase as a marker, we demonstrated that a non-toxic enzyme-vector conjugate was taken up by axon terminals. After injection into the gastrocnemius muscle, the B-IIb-glucose oxidase conjugate was detected, both histologically and electrochemically, distally to a ligature on the sciatic nerve. Thus the B-IIb fragment could serve as a vector for glucose oxidase transport into the central nervous system. It was also verified that the transported enzyme retained its activity. Transport of this 150 kDa molecule by fragment B-IIb of tetanus toxin suggests that other enzymes of a lesser molecular mass may also be transported.

Direct electrochemical determination of glucose oxidase in biological samples.

An electrochemical method for the quantitation of glucose oxidase in murine plasma and tissues has been developed. Instead of oxygen, this method uses benzoquinone as an artificial cosubstrate of glucose oxidase. The quantitative detection of the enzymatically produced hydroquinone by controlled-potential amperometry allows measurement of glucose oxidase concentrations in biological samples. The use of an internal standard corrects for all possible interfering effects. We demonstrated a 10-fold increase in sensitivity, as well as the ability to work in turbid media, in comparison to spectrophotometric methods.

An extracorporeal hollow-fiber reactor for phenylketonuria using immobilized phenylalanine ammonia lyase.

A hollow-fiber hemodialyzer with immobilized phenylalanine ammonia lyase was tested in vitro for depletion of blood phenylalanine in a recirculating system. A sustained reduction of phenylalanine was obtained in less than 1 h. The product of phenylalanine deamination, trans-cinnamic acid, is a nontoxic compound metabolized to benzoic acid by the liver and eliminated in the urine as hippuric acid. As a model, this reactor may be relevant not only for the short-term management of hyperphenylalaninemia (particularly in pregnant phenylketonuric mothers), but for other metabolic diseases as well, provided that a biocatalyst effective on the accumulating substance is available.