Deflavination of flavo-oxidases by nucleophilic reagents

Green Oxidation of Amines by a Novel Cold-Adapted Monoamine Oxidase MAO P3 from Psychrophilic Fungi Pseudogymnoascus sp. P3

The use of monoamine oxidases (MAOs) in amine oxidation is a great example of how biocatalysis can be applied in the agricultural or pharmaceutical industry and manufacturing of fine chemicals to make a shift from traditional chemical synthesis towards more sustainable green chemistry. This article reports the screening of fourteen Antarctic fungi strains for MAO activity and the discovery of a novel psychrozyme MAOP3 isolated from the Pseudogymnoascus sp. P3. The activity of the native enzyme was 1350 ± 10.5 U/L towards a primary (n-butylamine) amine, and 1470 ± 10.6 U/L towards a secondary (6,6-dimethyl-3-azabicyclohexane) amine. MAO P3 has the potential for applications in biotransformations due to its wide substrate specificity (aliphatic and cyclic amines, pyrrolidine derivatives). The psychrozyme operates at an optimal temperature of 30 °C, retains 75% of activity at 20 °C, and is rather thermolabile, which is beneficial for a reduction in the overall costs of a bioprocess and offers a convenient way of heat inactivation. The reported biocatalyst is the first psychrophilic MAO; its unique biochemical properties, substrate specificity, and effectiveness predispose MAO P3 for use in environmentally friendly, low-emission biotransformations.

The molten-globule residual structure is critical for reflavination of glucose oxidase

Glucose oxidase (GOX) is a homodimeric glycoprotein with tightly bound one molecule of FAD cofactor per monomer of the protein. GOX has numerous applications, but the preparation of biotechnologically interesting GOX sensors requires a removal of the native FAD cofactor. This process often leads to unwanted irreversible deflavination and, as a consequence, to the low enzyme recovery. Molecular mechanisms of reversible reflavination are poorly understood; our current knowledge is based only on empiric rules, which is clearly insufficient for further development. To develop conceptual understanding of flavin-binding competent states, we studied the effect of deflavination protocols on conformational properties of GOX. After deflavination, the apoform assembles into soluble oligomers with nearly native-like holoform secondary structure but largely destabilized tertiary structure presumambly due to the packing density defects around the vacant flavin binding site. The reflavination is cooperative but not fully efficient; after the binding the flavin cofactor, the protein directly disassembles into native homodimers while the fraction of oligomers remains irreversibly inactivated. Importantly, the effect of Hofmeister salts on the conformational properties of GOX and reflavination efficiency indicates that the native-like residual tertiary structure in the molten-globule states favorably supports the reflavination and minimizes the inactivated oligomers. We interpret our results by combining the ligand-induced changes in quaternary structure with salt-sensitive, non-equilibrated conformational selection model. In summary, our work provides the very first steps toward molecular understanding the complexity of the GOX reflavination mechanism.

Menaquinone reduction by an HMT2-like sulfide dehydrogenase from Bacillus stearothermophilus

The gene-encoding HMT2-like sulfide dehydrogenase from Bacillus stearothermophilus JCM2501 was amplified and expressed in Escherichia coli and the enzymatic features were examined. The enzyme was detected mainly in the membrane fraction. It catalyzed the sulfide-dependent menaquinone (MK) reduction showing special enzymatic features distinct from other sulfide–quinone oxidoreductases (SQRs) from autotrophic bacteria. The purified protein from E. coli brought about the sulfide-dependent 2,3-dimethyl-1,4-naphthoquinone (DMN) reduction in vitro. The reduction was accelerated in the presence of either cyanide or 2-mercaptoethanol and phospholipids. The high reduction was followed by a change in Kmvalues for sulfide and DMN. The purified enzyme utilized MK as an electron acceptor in the membrane fraction from E. coli. Under anaerobic conditions, sulfide was oxidized with reduction of fumarate or nitrate via the MK pool. The dehydrogenase was different from SQR in autotrophic bacteria in terms of the low affinity for sulfide and the activity enhancement in the presence of cyanide or 2-mercaptoethanol. The sulfide oxidation via MK in the cellular membrane of Gram-positive bacteria was certified.

Detection of structural changes in a cofactor binding protein by using a wheat germ cell-free protein synthesis system coupled with unnatural amino acid probing

A cell-free protein synthesis system is a powerful tool with which unnatural amino acids can be introduced into polypeptide chains. Here, the authors describe unnatural amino acid probing in a wheat germ cell-free translation system as a method for detecting the structural changes that occur in a cofactor binding protein on a conversion of the protein from an apo-form to a holo-form. The authors selected the FMN-binding protein from Desulfovibrio vulgaris as a model protein. The apo-form of the protein was synthesized efficiently in the absence of FMN. The purified apo-form could be correctly converted to the holo-form. Thus, the system could synthesize the active apo-form. Gel filtration chromatography, analytical ultracentrifugation, and circular dichroism-spectra studies suggested that the FMN-binding site of the apo-form is open as compared with the holo-form. To confirm this idea, the unnatural amino acid probing was performed by incorporating 3-azido-L-tyrosine at the Tyr35 residue in the FMN-binding site. The authors optimized three steps in their system. The introduced 3-azido-L-tyrosine residue was subjected to specific chemical modification by a fluorescein-triarylphosphine derivative. The initial velocity of the apo-form reaction was 20 fold faster than that of the holo-form, demonstrating that the Tyr35 residue in the apo-form is open to solvent.

Alcohol oxidase: a complex peroxisomal, oligomeric flavoprotein

Alcohol oxidase (AO) is the key enzyme of methanol metabolism in methylotrophic yeast species. It catalyses the first step of methanol catabolism, namely its oxidation to formaldehyde with concomitant production of hydrogen peroxide. In its mature active form, AO is a molecule of high molecular mass (600 kDa) that consists of eight identical subunits, each of which carry one non-covalently bound flavin adenine nucleotide (FAD) molecule as the prosthetic group. In vivo, the protein is compartmentalized into special cell organelles, termed peroxisomes. AO is an abundant protein and its synthesis is strictly regulated by repression/derepression and induction mechanisms that occur at the transcriptional level. Various aspects of its sorting and assembly/activation render AO a unique protein. Recent developments of AO synthesis, sorting and assembly/activation are highlighted in this paper.

Amperometric cholesterol biosensor based on in situ reconstituted cholesterol oxidase on an immobilized monolayer of flavin adenine dinucleotide cofactor

A new amperometric biosensor for determining cholesterol based on deflavination of the enzyme cholesterol oxidase (ChOx) and subsequent reconstitution of the apo-protein with a complexed flavin adenine dinucleotide (FAD) monolayer is described. The charge transfer mediator pyrroquinoline quinone (PQQ) was covalently bound to a cystamine self-assembled monolayer (SAM) on an Au electrode. Boronic acid (BA) was then bound to PQQ using the carbodiimide procedure, and the BA ligand was complexed to the FAD molecules on which the apo-ChOx was subsequently reconstituted. The effective release of the FAD from the enzyme and the successful reconstitution were verified using molecular fluorescence and cyclic voltammetry. The optimal orientation of FAD toward the PQQ mediator and the distances between FAD and PQQ and between PQQ and electrode enhance the charge transfer, very high sensitivity (about 2,500 nAmM(-1)cm(-2)) being obtained for cholesterol determination. The biosensor is selective toward electroactive interferents (ascorbic acid and uric acid) and was tested in reference serum samples, demonstrating excellent accuracy (relative errors below 3% in all cases). The biosensor activity can be successfully regenerated in a simple process by successive reconstitution with batches of recently prepared apo-ChOx on the same immobilized Au/SAM-PQQ-BA-FAD monolayer (it was tested five times); the lifetime of the biosensor is about 45-60 days.

Modulation of activity of NADH oxidase from Thermus thermophilus through change in flexibility in the enzyme active site induced by Hofmeister series anions

The conformational dynamics of NADH oxidase from Thermus thermophilus was modulated by the Hofmeister series of anions (H2PO4-, SO42-, CH3COO-, Cl-, Br-, I-, ClO4-, SCN-) in the concentration range 0-3 M. Both chaotropic and kosmotropic anions, at high concentration, inhibit the enzyme by different mechanisms. Chaotropic anions increase the apparent Michaelis constant and decrease the activation barrier of the reaction. Kosmotropic anions have the opposite effect. Anions from the middle of the Hofmeister series do not significantly affect the enzyme activity even at high concentration. We detected no significant changes in ellipticity of the aromatic region in the presence of the anions studied. There is a decreased Stern-Volmer quenching constant for FAD fluorescence quenching in the presence of kosmotropic anions and an increased quenching constant in the presence of chaotropic anions. All of this indicates that active site flexibility is important in the function of the enzyme. The data demonstrate that both the high rigidity of the active site in the presence of kosmotropic anions, and its high flexibility in the presence of chaotropic anions have a decelerating effect on enzyme activity. The Hofmeister series of anions proved to be suitable agents for altering enzyme activity through changes in flexibility of the polypeptide chain, with potential importance in modulating extremozyme activity at room temperature.

Deflavination and reconstitution of flavoproteins

Flavoproteins are ubiquitous redox proteins that are involved in many biological processes. In the majority of flavoproteins, the flavin cofactor is tightly but noncovalently bound. Reversible dissociation of flavoproteins into apoprotein and flavin prosthetic group yields valuable insights in flavoprotein folding, function and mechanism. Replacement of the natural cofactor with artificial flavins has proved to be especially useful for the determination of the solvent accessibility, polarity, reaction stereochemistry and dynamic behaviour of flavoprotein active sites. In this review we summarize the advances made in the field of flavoprotein deflavination and reconstitution. Several sophisticated chromatographic procedures to either deflavinate or reconstitute the flavoprotein on a large scale are discussed. In a subset of flavoproteins, the flavin cofactor is covalently attached to the polypeptide chain. Studies from riboflavin-deficient expression systems and site-directed mutagenesis suggest that the flavinylation reaction is a post-translational, rather than a cotranslational, process. These genetic approaches have also provided insight into the mechanism of covalent flavinylation and the rationale for this atypical protein modification.