Ribonucleotide

Ribonucleotide refers to any nucleotide that contains a ribose sugar and a phosphate group, linked to one of the four nitrogenous bases: adenine, cytosine, guanine, or uracil. In RNA, ribonucleotides are the monomer units that polymerize to form ribonucleic acid via phosphodiester bonds between the 5' phosphate of one nucleotide and the 3' hydroxyl of the next.

The sugar is ribose, which carries a hydroxyl group at the 2' position, distinguishing ribonucleotides from deoxyribonucleotides found in DNA. Depending on the number of phosphate groups, ribonucleotides exist as monophosphates (NMPs), diphosphates (NDPs), or triphosphates (NTPs). Common examples include AMP, GMP, CMP, and UMP, as well as their di- and triphosphates such as ADP/ATP, GDP/GTP, and CDP/CTP.

Functions: in cells, ribonucleotides are substrates for RNA synthesis and also act as energy carriers (ATP, GTP, UTP, and CTP). They participate in metabolism as signaling molecules: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) regulate diverse processes. Ribonucleotide triphosphates also function as cofactors in various enzymatic reactions.

Metabolism: ribonucleotide pools are maintained by de novo synthesis and salvage pathways. Purine and pyrimidine biosynthesis pathways produce ribonucleotides, which can be interconverted by kinases and phosphatases. Nucleoside salvage recovers free bases and nucleosides to re-form NMPs. Degradation yields ribose-containing metabolites that feed central carbon metabolism.

Distinction: ribonucleotides differ from deoxyribonucleotides by the presence of a 2' hydroxyl group and are primarily associated with RNA and cytosolic signaling, whereas deoxyribonucleotides are the building blocks of DNA.