I know that in human cell-lines, some of the N-linked sugars that are added to proteins during maturation in the secretory pathway contain sialic-acid residues. This gives the N-linked sugars a net negative charge. Are there also examples where the carbohydrate additions acquire modifications that give them a net positive charge under physiological conditions?
Well, never say never... but I am unaware of any "intentional" cationic carbohydrate PTMs. That said, I am sure that the N-acetyl groups must experience at least some hydrolysis in vivo. At physiological pH, the amine of glucosamine or galactosamine will have some positive charge. This is non-enzymatic and could be likened to the glycation of hemoglobin. In fact, many of the products of amine glycation will have at least some positive charge at physiological pH. But, again, these are not enzymatic modifications.
Hi Anthony, I agree with Joe. Most nitrogens in protein carbohydrates are in amide bonds, so they are not charged.
However carbohydrate glycation produces a Schiff base intermediate that is positively charged.
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