Novel endo-α-N-acetylgalactosaminidases with broader substrate specificity

In an effort to identify novel endo-α-N-acetylgalact- osaminidases (endo-α-GalNAcases), four potential genes were cloned. Three of the expressed proteins EngEF from Enterococcus faecalis, EngPA from Propionibacterium acnes, and EngCP from Clostridium perfringens were purified and characterized. Their substrate specificity was investigated and compared to the commercially available endo-α-GalNAcases from Streptococcus pneumoniae (EngSP) and Alcaligenes sp. (EngAL). All enzymes were incubated with various synthetic substrates, and natural glycoproteins and the released sugars were detected by colorimetric assay and thin layer chromatography analysis. The Core 1 disaccharide Galβ1,3GalNAcα1pNP was the most rapidly hydrolyzed substrate by all enzymes tested. EngEF exhibited the highest kcat for this substrate. EngEF and EngPA were also able to fully hydrolyze the Core 3 disaccharide GlcNAcβ1,3GalNAcα1pNP. This is the first report of endo-α-GalNAcases EngEF and EngPA acting on Core 3 in addition to Core 1 O-glycans. Interestingly, there were no significant differences in transglycosylation activities when Galβ1,3GalNAcα1pNP or GlcNAcβ1,3GalNAcα1pNP was incubated with various 1-alkanols in the presence of the endo-α-GalNAcases tested in this work

Carbohydrate Recognition by an Architecturally Complex α-N-Acetylglucosaminidase from Clostridium perfringens

CpGH89 is a large multimodular enzyme produced by the human and animal pathogen Clostridium perfringens. The catalytic activity of this exo-α-d-N-acetylglucosaminidase is directed towards a rare carbohydrate motif, N-acetyl-β-d-glucosamine-α-1,4-d-galactose, which is displayed on the class III mucins deep within the gastric mucosa. In addition to the family 89 glycoside hydrolase catalytic module this enzyme has six modules that share sequence similarity to the family 32 carbohydrate-binding modules (CBM32s), suggesting the enzyme has considerable capacity to adhere to carbohydrates. Here we suggest that two of the modules, CBM32-1 and CBM32-6, are not functional as carbohydrate-binding modules (CBMs) and demonstrate that three of the CBMs, CBM32-3, CBM32-4, and CBM32-5, are indeed capable of binding carbohydrates. CBM32-3 and CBM32-4 have a novel binding specificity for N-acetyl-β-d-glucosamine-α-1,4-d-galactose, which thus complements the specificity of the catalytic module. The X-ray crystal structure of CBM32-4 in complex with this disaccharide reveals a mode of recognition that is based primarily on accommodation of the unique bent shape of this sugar. In contrast, as revealed by a series of X-ray crystal structures and quantitative binding studies, CBM32-5 displays the structural and functional features of galactose binding that is commonly associated with CBM family 32. The functional CBM32s that CpGH89 contains suggest the possibility for multivalent binding events and the partitioning of this enzyme to highly specific regions within the gastrointestinal tract

Directed evolution on the cold adapted properties of TAB5 alkaline phosphatase

Psychrophilic alkaline phosphatase (AP) from the Antarctic strain TAB5 was subjected to directed evolution in order to identify the key residues steering the enzyme's cold-adapted activity and stability. A round of random mutagenesis and further recombination yielded three thermostable and six thermolabile variants of the TAB5 AP. All of the isolated variants were characterised by their residual activity after heat treatment, Michaelis-Menten kinetics, activation energy and microcalorimetric parameters of unfolding. In addition, they were modelled into the structure of the TAB5 AP. Mutations which affected the cold-adapted properties of the enzyme were all located close to the active site. The destabilised variants H135E and H135E/G149D had 2- and 3-fold higher kcat, respectively, than the wild-type enzyme. Wild-type AP has a complex heat-induced unfolding pattern while the mutated enzymes loose local unfolding transitions and have large shifts of the Tm values. Comparison of the wild-type and mutated TAB5 APs demonstrates that there is a delicate balance between the enzyme activity and stability and that it is possible to improve the activity and thermostability simultaneously as demonstrated in the case of the H135E/G149D variant compared to H135E

A HPLC-based glycoanalytical protocol allows the use of natural O-glycans derived from glycoproteins as substrates for glycosidase discovery from microbial culture

Many disorders are characterised by changes in O-glycosylation, but analysis of O-glycosylation has been limited by the availability of specific endo- and exo-glycosidases. As a result chemical methods are employed. However, these may give rise to glycan degradation, so therefore novel O-glycosidases are needed. Artificial substrates do not always identify every glycosidase activity present in an extract. To overcome this, an HPLC-based protocol for glycosidase identification from microbial culture was developed using natural O-glycans and O-glycosylated glycoproteins (porcine stomach mucin and fetuin) as substrates. O-glycans were released by ammonia-based β-elimination for use as substrates, and the bacterial culture supernatants were subjected to ultrafiltration to separate the proteins from glycans and low molecular size molecules. Two bacterial cultures, the psychrotroph Arthrobacter C1-1 and a Corynebacterium isolate, were examined as potential sources of novel glycosidases. Arthrobacter C1-1 culture contained a β-galactosidase and N-acetyl-β-glucosaminidase when assayed using 4-methylumbelliferyl substrates, but when defucosylated O-glycans from porcine stomach mucin were used as substrate, the extract did not cleave β-linked galactose or N-acetylglucosamine. Sialidase activity was identified in Corynebacterium culture supernatant, which hydrolysed sialic acid from fetuin glycans. When both culture supernatants were assayed using the glycoproteins as substrate, neither contained endoglycosidase activity. This method may be applied to investigate a microbial or other extract for glycosidase activity, and has potential for scale-up on high-throughput platforms