[H-NMR spectroscopy. Specificity of microbial sialidases against complex substrates]

The specificities of one viral and five bacterial sialidases were investigated by 1H-NMR-spectroscopy with substrates or substrate mixtures containing two sialic acid residues of different linkage types. This technique allows - in contrast to the methods used before - the simultaneous determination of the rates of hydrolysis of both NeuAc linkages in a single experiment. The substrate specificities of the enzymes are discussed on the basis of the relation of the rate constants k/k'. The data obtained are more exact and more informative than those of separate experiments as reported previously. Among the enzymes investigated, i.e. sialidases of fowl plague virus (FPV = VKH), Clostridium perfringens (CP), Vibrio cholerae (VC), Bifidobacterium bifidum var. pennsylvanicum (BBif), Bifidobacterium lactentis (BLac), and Arthrobacter ureafaciens (AU), the activity of the viral sialidase VKH shows the highest, the activities of the Bifidobacterium sialidases the lowest dependence on the nature and on the linkage type of the different substrates. All sialidases preferentially cleave the NeuAc alpha 2-3-Gal linkage with the exception of the enzyme of Arthrobacter ureafaciens (AU) which shows a higher affinity to alpha 2-6 linkages. However, this does not apply to the side-arm-linked NeuAc alpha 2-6 structure in NeuAc alpha 2-3 Gal beta 1-3 (NeuAc alpha 2-6)-GlcNAc beta 1-3Gal beta 1-4Glc (Substrate B). This substrate in generally cleaved very slowly and is hardly affected by the viral enzyme. After the alpha 2-3 linkage, the alpha 2-8 bond in NeuAc alpha 2-8 NeuAc alpha 2-3 Gal beta 1-4Glc(Substrate A) is most susceptible for the sialidases VKH, CP and VC. An elongation of the carbohydrate chain (Substrate D) is accompanied by a reduction of the rate of cleavage for all enzymes. The experiments with alpha 1-acid glycoprotein, fetuin, and with the glycopeptides obtained by proteolytic degradation of the latter, revealed the same specificity towards the alpha 2-3 and the alpha 2-6 linkages as the oligosaccharides. Influenced by the chemical nature and the size of the substrate, NeuAc is released from the native alpha 1-acid glycoprotein more quickly than from the corresponding glycopeptide. All sialidases investigated so far are strictly exo-enzymes as could be demonstrated by the cleavage of NeuAc alpha 2-8 NeuAc alpha 2-3 Gal beta 1-4Glc (Substrate A).