Biochemistry and Molecular Biology Resource

 

Web Lessons

 

Eric C. Niederhoffer, Ph.D.

Associate Professor of Biochemistry and Molecular Biology

Southern Illinois University School of Medicine
600 Agriculture Drive, Carbondale, IL 62901-6503
Rm 112 Lindegren, 618-453-6467
eniederhoffer@siumed.edu
Copyright 2000- , E.C. Niederhoffer.
All Rights Reserved. All trademarks and copyrights are the property of their respective owners.

Purines, Pyrimidines, Nucleosides, and Nucleotides


Purines and pyrimidines are the building blocks of the nucleic acids (deoxyribonucleic acid, DNA ; ribonucleic acid, RNA). The various forms also serve as second messengers for signal transduction pathways. Some serve as nucleotide sugar donors in metabolic pathways. There are five nitrogenous bases, the two purines adenine and guanine and the three pyrimidines cytosine, thymine (found primarily in DNA), and uracil (found primarily in RNA). Their structures appear as follows:

 

Building blocks: bases, nucleosides, nucleotides

Purines and pyrimidines representations

 

Adenine, A

 

Cytosine, C

 

Guanine, G

 

Thymine, T

 

Uracil, U

When the five-carbon sugar group (ribose or deoxyribose) is added to the nitrogenous base, we get what is denoted as a nucleoside. Note the position of the 2-C hydroxyl (OH) or hydrogen (H).

Generic deoxyribonucleoside and ribonucleoside representations

 

 

Nucleosides that have a phosphate group at the 3- or 5-C position are denoted nucleotides (nucleoside monophosphate). The following compares the structures of purines, pyrimidine, nucleosides, and nucleotides:

Purines, Pyrimidines, Nucleosides, and Nucleotides

 

Adenine

 

Deoxyadenosine

 

Deoxyadenylate, A, dA, dAMP

 

Deoxyadenylate, A, dA, dAMP

 

Adenosine

 

Adenylate, A, AMP

 

Adenylate, A, AMP

 

Guanine

 

Deoxyguanosine

 

Deoxyguanylate, G, dG, dGMP

 

Deoxyguanylate, G, dG, dGMP

 

Guanosine

 

Guanylate, G, GMP

 

Guanylate, G, GMP

 

Cytosine

 

Deoxycytidine

 

Deoxycytidylate, C, dC, dCMP

 

Deoxycytidylate, C, dC, dCMP

 

Cytidine

 

Cytidylate, C, CMP

 

Cytidylate, C, CMP

 

Thymine

 

Deoxythymidine

 

Deoxythymidylate, T, dT, dTMP

 

Deoxythymidylate, T, dT, dTMP

 

Uracil

 

Uridine

 

Uridylate, U, UMP

 

Uridylate, U, UMP

 

The properties of purine and pyrimidine bases and phosphate groups of nucleic acids and those of proteins allows for hydrogen bonding, non-polar, and polar interactions. We can observe these interactions when DNA-binding proteins (transcription factors) and drugs associate with nucleic acids.

We also find the triphosphate form of nucleic acids involved in energy transfer reactions. The most common example is ATP. GTP can bind to proteins (enzymes), where they serve to facilitate signal transduction (G protein transducin).

Phosphorylated forms of nucleotides

adenosine-5'-monophosphate, AMP

adenosine-5'-diphosphate, ADP

adenosine-5'-triphosphate, ATP

 

Cyclic nucleotides as second messengers

Both ATP and GTP may be cyclized by their respective enzymes, adenylyl cyclase and guanylyl cyclase, to form cyclic AMP and cyclic GMP. The remaining phosphates are lost as pyrophosphate.

Cyclic nucleotides

 

cyclic AMP, cAMP

 

cyclic GMP, cGMP

Cyclic AMP binds to and activates protein kinase A, which is an important enzyme in the regulation of many signal transduction pathway such as those involved in glucose and glycogen metabolism and in taste sensation. Both cAMP and cGMP bind to cyclic nucleotide-gated channels, which allow the transport of ions across cell membranes.

 

Nucleotides as sugar donors

Nucleotide sugars are used to transfer sugars (monosaccharides) to polymer chains (glycogen or starch) or in the synthesis of other biomolecules (sucrose and lactose). The most important one is UDP, which transfers glucosyl units to growing glycogen chains. During lactation, UDP is used to transfer a galactosyl unit to glucose forming lactose

 

Biochemistry and Molecular Biology Home Page


For more information or comments about this page contact:
eniederhoffer@siumed.edu