Oligosaccharide Dehydrogenase-Modified Graphite Electrodes for the Amperometric Determination of Sugars in a Flow Injection System

Starch-degrading polysaccharide monooxygenases

Polysaccharide degradation by hydrolytic enzymes glycoside hydrolases (GHs) is well known. More recently, polysaccharide monooxygenases (PMOs, also known as lytic PMOs or LPMOs) were found to oxidatively degrade various polysaccharides via a copper-dependent hydroxylation. PMOs were previously thought to be either GHs or carbohydrate binding modules (CBMs), and have been re-classified in carbohydrate active enzymes (CAZY) database as auxiliary activity (AA) families. These enzymes include cellulose-active fungal PMOs (AA9, formerly GH61), chitin- and cellulose-active bacterial PMOs (AA10, formerly CBM33), and chitin-active fungal PMOs (AA11). These PMOs significantly boost the activity of GHs under industrially relevant conditions, and thus have great potential in the biomass-based biofuel industry. PMOs that act on starch are the latest PMOs discovered (AA13), which has expanded our perspectives in PMOs studies and starch degradation. Starch-active PMOs have many common structural features and biochemical properties of the PMO superfamily, yet differ from other PMO families in several important aspects. These differences likely correlate, at least in part, to the differences in primary and higher order structures of starch and cellulose, and chitin. In this review we will discuss the discovery, structural features, biochemical and biophysical properties, and possible biological functions of starch-active PMOs, as well as their potential application in the biofuel, food, and other starch-based industries. Important questions regarding various aspects of starch-active PMOs and possible economical driving force for their future studies will also be highlighted.

A family of starch-active polysaccharide monooxygenases

The recently discovered fungal and bacterial polysaccharide monooxygenases (PMOs) are capable of oxidatively cleaving chitin, cellulose, and hemicelluloses that contain β(1→4) linkages between glucose or substituted glucose units. They are also known collectively as lytic PMOs, or LPMOs, and individually as AA9 (formerly GH61), AA10 (formerly CBM33), and AA11 enzymes. PMOs share several conserved features, including a monocopper center coordinated by a bidentate N-terminal histidine residue and another histidine ligand. A bioinformatic analysis using these conserved features suggested several potential new PMO families in the fungus Neurospora crassa that are likely to be active on novel substrates. Herein, we report on NCU08746 that contains a C-terminal starch-binding domain and an N-terminal domain of previously unknown function. Biochemical studies showed that NCU08746 requires copper, oxygen, and a source of electrons to oxidize the C1 position of glycosidic bonds in starch substrates, but not in cellulose or chitin. Starch contains α(1→4) and α(1→6) linkages and exhibits higher order structures compared with chitin and cellulose. Cellobiose dehydrogenase, the biological redox partner of cellulose-active PMOs, can serve as the electron donor for NCU08746. NCU08746 contains one copper atom per protein molecule, which is likely coordinated by two histidine ligands as shown by X-ray absorption spectroscopy and sequence analysis. Results indicate that NCU08746 and homologs are starch-active PMOs, supporting the existence of a PMO superfamily with a much broader range of substrates. Starch-active PMOs provide an expanded perspective on studies of starch metabolism and may have potential in the food and starch-based biofuel industries.

A novel enzymic determination of maltose

A novel enzymic determination of maltose with four enzymes (a new enzyme, maltose 1-epimerase [EC 5.1.3.-], maltose phosphorylase [EC 2.4.1.8], beta-phosphoglucomutase [EC 5.4.2.6], and glucose-6-phosphate dehydrogenase [EC 1.1.1.49]) is described. Maltose was rapidly and quantitatively determined within about 2 min by means of maltose 1-epimerase. The standard curve was linear up to 1.5 micromol/mL. The within-run and between-run studies gave precision (CV) values of < 2.0% and < 3.0%, respectively. No significant interferences by mono- and disaccharides were observed with the proposed method under this study. There was a good correlation (r = 0.997) between the results obtained by the enzymic and HPLC methods. This method fulfills the need for an accurate, specific and simple assay of maltose, and it is less time consuming than HPLC and enzymic methods previously reported.

Chemical and biochemical sensors based on advances in materials chemistry

The advance of materials chemistry has influenced the design of analytical sensors, especially those using spectroscopic or electrochemical methods for generating the signal. New methods of immobilizing enzymes, chromophores, and electron-transfer catalysts have resulted from initiatives in materials science. Systems based on sol-gel chemistry are especially noteworthy in this regard, but other important materials for chemical and biochemical sensors include zeolites, organic polymers, and various conducting composites. Applications cited include determinations of inorganic ions, gases, neurotransmitters, alcohols, carbohydrates, amino acids, proteins, and DNA.

Oligosaccharide dehydrogenase-catalyzed assay for the determination of polysaccharides

Oligosaccharide dehydrogenase (ODH), an enzyme known to have a broad selectivity for reducing sugars of low molecular weight, was investigated to determine its catalytic properties with larger polysaccharides. Six substrates were studied: pullulan standards with molecular weights of between 5,400 and 90,900, debranched starch, and dextran. In addition, maltotriose, isomaltotriose, maltose, and glucose were used as substrates for comparison. ODH catalyzed the oxidation of the large pullulans with a degree of polymerization of at least 560. Isomaltotriose and dextran were not oxidized. ODH activity for the pullulans, expressed as the rate constant Kps, was only three times lower than that for maltose. When the oxidation of sugars with ODH was coupled to a color-forming reaction, quantitative spectrophotometric determination of sugars was possible using either Meldola's blue or N-methylphenazinium as electron acceptors in combination with nitrotetrazolium blue. Linear calibration curves for maltose, maltotriose, and debranched starch were obtained using this ODH method and compared with curves from the conventional spectrophotometric copper sulfate method. This work demonstrates that ODH can be advantageously used for the determination of polysaccharides.