question: Why can't humans synthesize Neu5Gc (N-Glycolylneuraminic acid)?
answer: N-Glycolylneuraminic acid (Neu5Gc) is a sialic acid synthesized by animals, but not by humans or birds. Humans lack a functional cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) protein and cannot synthesize the sugar Neu5Gc, an innate mammalian signal of self. Losing this sugar changed how humans interact with some of our deadliest pathogens: malaria, influenza, and streptococcus among others.