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The reactivity of a range of Keggin and Wells-Dawson type heteropolyacids (HPAs): H 3 PW 12 O 40 H 4 SiW 12 O 40 , H 3 PMo 12 O 40 , K 6 P 2 W 18 O 62 , and NaH 2 W 12 O 4 , towards the heavily glycosylated α-1-acid glycoprotein (AGP) is reported. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) show that after incubation of the protein with HPAs at 80 °C and pH 2.8 complete hydrolysis of terminal glycosidic bond has been achieved, resulting in the removal of sialic acids with no observed destruction of the protein core or the residual glycan chains. The 1 H NMR spectroscopy confirmed that the released sialic acids preserve intact structure upon their excision from the protein, which makes the reported method suitable for the analysis of sialic acid modifications which play an important role in numerous biological processes. The presence of other sugars was not detected by 1 H NMR and HPAEC-PAD, suggesting that HPAs hydrolyze only the terminal glycosidic bond in the glycoprotein, resulting in the selective release of sialic acid from AGP. The kinetic results have shown that under equal temperature and pH conditions, the hydrolysis of the terminal glucosidic bond occurred faster in the presence of HPAs compared to conventional mineral acids. The observed rate constants were in the range 6,7×10 -2 -11,9×10 -2  min -1 and the complete and selective excision of sialic acids could be achieved within 60 min of incubation. The Trp fluorescence and CD spectroscopy show that non-covalent interaction between HPA and protein takes place in solution which could lead to stabilization of the sialosyl cation that is formed during the glycosidic bond hydrolysis by anionic HPA cluster., (© 2020 Wiley-VCH GmbH.)

The hydrolysis of the iron-binding blood plasma glycoprotein transferrin (Tf) has been examined at pH = 7.4 in the presence of a series of Zr-substituted polyoxometalates (Zr-POMs) including Keggin (Et 2 NH 2 ) 10 [Zr(PW 11 O 39 ) 2 ]∙7H 2 O ( Zr-K 1:2 ), (Et 2 NH 2 ) 8 [{ α -PW 11 O 39 Zr-( μ -OH) (H 2 O)} 2 ]∙7H 2 O ( Zr-K 2:2 ), Wells-Dawson K 15 H[Zr( α 2 -P 2 W 17 O 61 ) 2 ]·25H 2 O ( Zr-WD 1:2 ), Na 14 [Zr 4 ( α -P 2 W 16 O 59 ) 2 ( μ 3 -O) 2 ( μ -OH) 2 (H 2 O) 4 ]·57H 2 O ( Zr-WD 4:2 ) and Lindqvist (Me 4 N) 2 [ZrW 5 O 18 (H 2 O) 3 ] ( Zr-L 1:1 ), (nBu 4 N) 6 [(ZrW 5 O 18 ( μ -OH)) 2 ]∙2H 2 O ( Zr-L 2:2 )) type POMs. Incubation of transferrin with Zr-POMs resulted in formation of 13 polypeptide fragments that were observed on sodium dodecyl sulfate poly(acrylamide) gel electrophoresis (SDS-PAGE), but the hydrolysis efficiency varied depending on the nature of Zr-POMs. Molecular interactions between Zr-POMs and transferrin were investigated by using a range of complementary techniques such as tryptophan fluorescence, circular dichroism (CD), 31 P-NMR spectroscopy, in order to gain better understanding of different efficiency of investigated Zr-POMs. A tryptophan fluorescence quenching study revealed that the most reactive Zr-WD species show the strongest interaction toward transferrin. The CD results demonstrated that interaction of Zr-POMs and transferrin in buffer solution result in significant secondary structure changes. The speciation of Zr-POMs has been followed by 31 P-NMR spectroscopy in the presence and absence of transferrin, providing insight into stability of the catalysts under reaction condition.

Polyoxometalates have long been studied in a variety of biological applications. Interactions between the highly charged POM molecules and biological molecules frequently occur through hydrogen-bonding and electrostatic interactions. Tellurium-centred Anderson-Evans POMs show exceptional promise as crystallization agents, while acidic and metal-substituted POMs may provide interesting alternatives to enzymes in proteomics applications. While POMs also show interesting results in a number of medicinal applications, for example as anti-amyloid agents for the treatment of Alzheimer's disease and as anti-tumoral agents, their use is often impeded by their toxicity. Many recent studies have therefore focussed on POM-functionalization to reduce toxicity and increase activity by addition of biological targeting molecules., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The selective hydrolysis of the glycosidic bond between the terminal sialic acid and the penultimate sugar has been achieved in the alpha-2-HS-glycoprotein (Fetuin-A) in the presence of H 3 PW 12 O 40 , a Keggin type polyoxometalate. A controlled liberation of sialic acid from the glycoprotein could be achieved at pH 3.0 and 37 °C without the destruction of the protein backbone.