Many derivatives of monosaccharides are found in nature. These include
- oxidized forms in which the aldehyde and/or alcohol functional groups are oxidized to carboxylic acids
- phosporylated forms in which phosphate is added by ATP to form phosphoester derivatives
- amine derivatives such as glucosamine or galactosamine
- acetylated amine derivatives such as N-Acetyl-GlcNAc (GlcNAc) or GalNAc
- lactone forms (intramolecular esters) in which an OH group attacks a carbonyl C that was previously oxidized to a carboxylic acid
- condensation products of sugar derivatives with lactate (CH3CHOHCO2-) and pyruvate (CH3COCO2- ), both from the glycolytic pathway, to form muramic acid and neuraminic acids, (also called sialic acids), respectively.
In the above figure, N-acetylmuramic acid, found in bacterial cell walls, consists of GlcNAc ether-linked at C3 with lactate, while N-acetylneuraminic acid results from an intramolecular cyclization of a condensation product of ManNac and pyruvate.
Figure: Is sialic acid the big difference between humans and chimps?
What happens when non-vegan humans eat animal products (meat, milk) with N-glycoyl neuraminic acids (Neu5Gc)? Varki et al found that some gets incorporated into human membrane glycans (see next Chapter). Sialic acids on surface proteins can serve as "receptors" that allow binding of self-cells as well as foreign cells or proteins that have evolved to bind them. Byres et al discovered that a toxin, SubAB, secreted by E. Coli 0157, can bind Neu5Gc. Hence eating meat products can make us more susceptible to bacteria that recognize Neu5Gc.