Both nucleotides and nucleosides are important molecules in living organisms. They are composed of purine or pyrimidine bases, sugars, and phosphates. However, there are some key differences between them.
What are Nucleosides?
Nucleosides are glycoside molecules that are composed of pentose and bases linked by β-N glycosidic bonds. They play a vital role in nucleic acids. The pentose component may be D-ribose or 2-deoxy-D-ribose, both of which are presented in furan-type ring structures. The former constitutes ribonucleosides, while the latter forms deoxynucleosides. In nucleoside molecules, the rotation between the base and the pentose ring is restricted by steric hindrance, which allows nucleosides to exist in both cis and trans conformations. The characteristic of cis nucleosides is that their bases are located on the same side as the pentose ring, while trans nucleosides are the opposite, with the bases and the pentose ring in opposite positions. It is worth noting that pyrimidine nucleosides usually tend to form trans conformations due to the steric hindrance between the pyrimidine ring and the pentose ring. In contrast, purine nucleosides have two possible conformations. Free purine nucleosides are more likely to form a cis conformation, but purine nucleosides in DNA and RNA helices have a trans conformation.
In nucleic acids, pyrimidine nucleosides and purine nucleosides are linked to pentose sugars through β-N glycosidic bonds, forming the basis of nucleoside molecules. In addition to the common pyrimidine nucleosides and purine nucleosides, there are also a variety of modified nucleosides. These modified nucleosides differ in chemical structure, giving them unique properties and functions. For example, some modified nucleosides can affect the specificity of base pairing, thereby playing a key role in DNA replication and RNA transcription.
What are Nucleotides?
Nucleotides are the esterification products of nucleosides and phosphates, where the phosphate esters of ribonucleosides are called ribonucleotides, while the phosphate esters of deoxynucleosides form deoxynucleotides. In theory, the 5′-OH, 3′-OH, and 2′-OH positions of nucleosides can be phosphorylated to generate nucleoside-5′-phosphate, nucleoside-3′-phosphate, and nucleoside-2′-phosphate, respectively. However, in natural nucleic acid molecules, the most common form of nucleotides is nucleoside-5′-phosphate.
Nucleoside monophosphate (NMP) is a simple esterification product of a nucleoside with a phosphate. Through a single anhydride reaction, nucleoside monophosphate can be converted into nucleoside diphosphate (NDP). Subsequently, nucleoside diphosphate undergoes another anhydride reaction to generate nucleoside triphosphate (NTP). In order to more clearly identify the phosphate groups at different positions, the phosphate directly connected to the 5′-hydroxyl group of the pentose is called the α-phosphate, while the other two phosphate groups are called the β-phosphate and the γ-phosphate from the inside to the outside.
In organisms, nucleotides play a variety of important roles. They are not only the main source of energy, usually in the form of ATP, but sometimes also use derivatives such as UTP, CTP, and GTP, which participate in key processes such as glycogen synthesis, phospholipid synthesis, and protein synthesis, respectively. In addition, nucleotides are also important precursors for nucleic acid synthesis, such as NTP for RNA synthesis. At the same time, they also play a key role in information transmission. In addition, nucleotides can also serve as precursors of other substances or components of coenzymes/cofactors.
What is the Relationship between Nucleosides and Nucleotides?
There is a direct synthesis and composition relationship between nucleosides and nucleotides. Nucleosides are the basic units that constitute nucleotides, and nucleotides are formed by the synthesis reaction of nucleosides and phosphoric acid.
Structural relationship:
Nucleosides are compounds formed by pentose and bases connected by glycosidic bonds.
Nucleotides are compounds synthesized from nucleosides and phosphoric acid, so nucleosides are an important part of nucleotides.
Synthesis relationship:
Pentose and bases are first connected by glycosidic bonds to form nucleosides.
Subsequently, nucleosides and phosphoric acid are further synthesized to form nucleotides.
