Transcriptionis the process by which the DNA-based genetic message is read and
carried out of the nucleus to an organelle called the ribosome, at which protein synthesis
occurs.Translationis the process by which the genetic messages are decoded and used
to construct proteins. To permit the biochemistry of this dogma, many types of RNA are
required: messenger RNA (mRNA) carries genetic messages from DNA to ribosomes;
ribosomal RNA (rRNA) is complexed with protein to provide the physical makeup of
the ribosomes; and transfer RNA (tRNA) transports amino acids to the ribosomes
where they are joined to produce proteins. The specific nucleotide sequence in mRNA
forms a message that determines the order in which the amino acid residues are assem-
bled. Each word, or codon, along the mRNA chain consists of a defined sequence of
three nucleotides that is specific for a given amino acid.
Nucleotides and nucleosides have emerged as important molecules in medicinal
chemistry. In the 1950s, Elion and Hitchings discovered that 6-mercaptopurine had
antitumor properties. This pioneering discovery opened the door for many subsequent
studies of nucleotide derivatives as therapeutics. Acyclovir (8.3), a nucleoside that lacks
two carbon atoms of its ribose ring, is effective in the treatment of herpes infections.
Allopurinol, a purine derivative, is useful in the treatment of gout.
8.1.4 LipidsLipidsare naturally occurring organic molecules, isolated from animal or plant cells by
extraction with nonpolar organic solvents. This definition defines lipids in terms of a
physical property (solubility) and differs from structural definitions used for proteins or
carbohydrates. Not surprisingly, lipids are highly varied in their structure; from the
medicinal chemistry perspective, there are five classes of lipids:
- Waxes, fats, and oils
- Phospholipids
- Prostaglandins
- Terpenes
- Steroids
Each of these classes has some relevance to medicinal chemistry and drug design.
8.1.4.1 Lipids: Waxes, Fats, and Oils
Waxes, fats, and oils contain ester linkages that can be hydrolyzed. Waxes are mixtures
of esters of long-chain carboxylic acids with long-chain alcohols. Animal fats and
vegetable oils, the most widely occurring lipids, are collectively referred to as
ENDOGENOUS MACROMOLECULES 471