DNA contains a deoxygenated form of ribose called deoxyribose. DNA and RNA are built up of the monosaccharides deoxyribose and ribose, respectively. Some species can metabolize pentose sugars such as ribose, alcohols, and organic acids. Nucleosides are molecules formed by attaching a nucleobase to a ribose or deoxyribose ring. GDP consists of a pyrophosphate group, a pentose sugar ribose, and the nucleobase guanine. PARP1 synthesizes polymeric adenosine diphosphate ribose (poly (ADP-ribose) or PAR) chains on itself. His-51 and Ile-269 form hydrogen bonds with the alcohols on nicotinamide ribose. This includes common monosaccharides like galactose, glucose, glyceraldehyde, fructose, ribose, and xylose. Uric acid is formed when GMP is split into the base guanine and ribose. Crystal structures indicate that the His-51 deprotonates the nicotinamide ribose, which deprotonates Ser-48. One important modification occurs at the C2' position of the ribose molecule. Cleavage of the ribose and N-methylation yields 7-methylxanthosine. Purines are biologically synthesized as nucleosides (bases attached to ribose). Remdesivir can be synthesized in multiple steps from ribose derivatives. Purines are synthesized as nucleosides (bases attached to ribose).