Covalent attachment of FAD derivatives to a fusion protein consisting of 6-hydroxy-D-nicotine oxidase and a mitochondrial presequence. Folding, enzyme activity, and import of the modified protein into yeast mitochondria

Apoenzyme reconstitution as a chemical tool for structural enzymology and biotechnology

Many enzymes contain a nondiffusible organic cofactor, often termed a prosthetic group, which is located in the active site and essential for the catalytic activity of the enzyme. These cofactors can often be extracted from the protein to yield the respective apoenzyme, which can subsequently be reconstituted with an artificial analogue of the native cofactor. Nowadays a large variety of synthetic cofactors can be used for the reconstitution of apoenzymes and, thus, generate novel semisynthetic enzymes. This approach has been refined over the past decades to become a versatile tool of structural enzymology to elucidate structure-function relationships of enzymes. Moreover, the reconstitution of apoenzymes can also be used to generate enzymes possessing enhanced or even entirely new functionality. This Review gives an overview on historical developments and the current state-of-the-art on apoenzyme reconstitution.

One-step immobilization of glucose oxidase in a silica matrix on a Pt electrode by an electrochemically induced sol-gel process

We demonstrate here that the electrochemical generation of hydroxyl ions and hydrogen bubbles can be used to induce the synthesis of enzyme- or protein-encapsulated 3D porous silica structure on the surface of noble metal electrodes. In the present work, the one-step synthesis of a glucose oxidase (GOD)-encapsulated silica matrix on a platinum electrode is presented. In this process, glucose oxidase was mixed with ethanol and TEOS to form a doped precursory sol solution. The electrochemically generated hydrogen bubbles at negative potentials assisted the formation of the porous structure of a GOD-encapsulated silica gel, and then the one-step immobilization of enzyme into the silica matrix was achieved. Scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM) characterizations showed that the GOD-encapsulated silica matrix adhered to the electrode surface effectively and had an interconnected porous structure. Because the pores started at the electrode surface, their sizes increased gradually along the distance away from the electrode and reached maximum at the solution side, and effective mass transport to the electrode surface could be achieved. The entrapped enzyme in the silica matrix retained its activity. The present glucose biosensor had a short response time of 2 s and showed a linear response to glucose from 0 to 10 mM with a correlation coefficient of 0.9932. The detection limit was estimated to be 0.01 mM at a signal-to-noise ratio of 3. The apparent Michaelis-Menten constant (K m app) and the maximum current density were determined to be 20.3 mM and 112.4 microA cm-2, respectively. The present method offers a facile way to fabricate biosensors and bioelectronic devices in situ.

Investigating the receptor-independent neuroprotective mechanisms of nicotine in mitochondria

Although nicotine has been associated with a decreased risk of developing Parkinson disease, the underlying mechanisms are still unclear. By using isolated brain mitochondria, we found that nicotine inhibited N-methyl-4-phenylpyridine (MPP(+)) and calcium-induced mitochondria high amplitude swelling and cytochrome c release from intact mitochondria. Intra-mitochondria redox state was also maintained by nicotine, which could be attributed to an attenuation of mitochondria permeability transition. Further investigation revealed that nicotine did not prevent MPP(+)- or calcium-induced mitochondria membrane potential loss, but instead decreased the electron leak at the site of respiratory chain complex I. In the presence of mecamylamine hydrochloride, a nonselective nicotinic acetylcholine receptor inhibitor, nicotine significantly postponed mitochondria swelling and cytochrome c release induced by a mixture of neurotoxins (MPP(+) and 6-hydroxydopamine) in SH-SY5Y cells, suggesting that there is a receptor-independent nicotine-mediated neuroprotective effect of nicotine. These results show that interaction of nicotine with mitochondria respiratory chain together with its antioxidant effects should be considered in the neuroprotective effects of nicotine.

Characterization of a highly conserved FAD-binding site in human monoamine oxidase B

Monoamine oxidase B (MAO B) catalyzes the oxidative deamination of biogenic and xenobiotic amines. The oxidative step is coupled to the reduction of an obligatory cofactor, FAD, which is covalently linked to the apoenzyme at Cys397. Our previous studies identified two noncovalent flavin-binding regions in MAO B (residues 6-34 and 39-46) (Kwan, S.-W., Lewis, D. A., Zhou, B. P., and Abell, C. W. (1995) Arch. Biochem. Biophys. 316, 385-391; Zhou, B. P., Lewis, D. A., Kwan, S.-W., Kirksey, T. J., and Abell, C. W. (1995) Biochemistry 34, 9526-9531). In these regions, Glu34 and Tyr44 were found to be required for the initial binding of FAD. By comparing sequences with enzymes in the oxidoreductase family, we now have found an additional FAD-binding site in MAO B (residues 222-227), which is highly conserved across species (human, bovine, and rat). This conserved sequence contains adjacent glycine and aspartate residues (Gly226 and Asp227). Based on the x-ray crystal structures of several oxidoreductases (Eggink, G., Engel, H., Vriend, G., Terpstra, P., and Witholt, B. (1990) J. Mol. Biol. 212, 135-142; Van Driessche, G., Kol, M., Chen, Z.-W., Mathews, F. S., Meyer, T. E., Bartsch, R. G., Cusanovich, M. A., and Van Beeumen, J. J. (1996) Protein Sci. 5, 1753-1764), the Gly residue at the end of a beta-strand facilitates a sharp turn and extends the beta-carbonyl group of Asp to interact with the 3'-hydroxyl group of the ribityl chain of FAD. To assess the hypothesis that Gly226 and Asp227 are involved in FAD binding in MAO B, site-specific mutants that encode substitutions at these positions were prepared and expressed in mammalian COS-7 cells. Our results indicate that Gly226 and the beta-carbonyl group of Asp227 are required for covalent flavinylation and catalytic activity of MAO B, but not for noncovalent binding of FAD. Our studies also reveal that mutagenesis at Glu34 and Tyr44 not only interferes with covalent flavinylation and catalytic activity of MAO B, but also with noncovalent binding of FAD. Based on these collective results, we propose that the coupling of FAD to the MAO B apoenzyme is a multistep process.

Covalent attachment of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) to enzymes: the current state of affairs

The first identified covalent flavoprotein, a component of mammalian succinate dehydrogenase, was reported 42 years ago. Since that time, more than 20 covalent flavoenzymes have been described, each possessing one of five modes of FAD or FMN linkage to protein. Despite the early identification of covalent flavoproteins, the mechanisms of covalent bond formation and the roles of the covalent links are only recently being appreciated. The main focus of this review is, therefore, one of mechanism and function, in addition to surveying the types of linkage observed and the methods employed for their identification. Case studies are presented for a variety of covalent flavoenzymes, from which general findings are beginning to emerge.