Sialyltransferase active sites

The stimulatory effect of nonionic detergents on the sialyltransferase reactions may reflect an interaction of the hydrophobic environments of the active sites with the detergents, possibly by the insertion of the latter into the lipid bilayer surrounding the enzymes or by the formation of detergent-enzyme complexes, thus inducing more active enzyme conformations (51,52,53). The effect of nonionic detergents may be similar to the previously reported effects of phosphatidyl ethanolamine (34), CDP-choline and lysolecithin (54,55), phospho-diglycerol and cardiolipid (56). 

The requirements for the rat and pork liver sialyltransferases are shown in Table VI. The need for exogenous acceptor is almost absolute and neuraminidase-treated i-acid glycoprotein is far more eflFective than other derivatives, indicating that a terminal galactose residue is required for acceptor activity. The sialyltransferases show no requirements for cations, and the pH optima are 5.7 and 7.0 for the rat and pork liver enzymes, respectively. The Km values for the pork liver enzyme are 0.19 mM for the nucleotide sugar and 0.9 mM for the glycoprotein acceptor (calculated on the basis of available acceptor sites). 

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