Heterocycles in Nature 163All the tetrapyrrole pigments are biosynthesised by the combination of four molecules of porphobilinogen (PBG), this
in turn is made from -aminolevulinic acid (5-aminolevulinic acid, H 2 NCH 2 CO(CH 2 ) 2 CO 2 H) and this from glycine and
succinic acid. There are complexities late in the tetramerisation sequence which we do not go into here, but the essence of
the process can be easily understood on the basis of the pyrrole chemistry described in Chapter 9: consider the fi rst step
as typical. Protonation of the amino group of PBG converts it into a leaving group generating an electrophilic azafulvene
(compare with page 82) for attachment via a nucleophilic atom, X, on the enzyme. This fi rst enzyme-bound pyrrole is
now attacked, as is typical for all pyrroles, at an -position, by a second molecule of electrophilic azafulvenium ion.
Deoxyribonucleic acid (DNA), the store of genetic information,
and ribonucleic acid (RNA), its deliverer
(The information on pyrimidine and purine bases and their nucleosides and nucleotides, in Chapters 6 and 13, is required
reading prior to this section. Particular note should be taken of the tautomeric forms of the nucleic acid bases, which
are critical in the formation of specifi c hydrogen bonds: N-hydrogens of pendant amino groups and the N-hydrogens of
ring ‘amides’ form hydrogen bonds to the oxygens of carbonyl groups and the nitrogens of ring imine units.)
The genetic information of all life is stored in DNA*. This information is used for protein synthesis and its delivery
to the ‘production site’ for these proteins requires the intervention of three types of RNA: messenger RNA (mRNA),
transfer RNA (tRNA) and ribosomal RNA (rRNA). A simplifi ed explanation of the functions of these types of RNA is
that mRNA reads the information on (a single strand of ) DNA and passes this information to tRNA, which transfers a
specifi c amino acid to rRNA for assembly into the protein.
*Some viruses use RNA to carry genetic information, but this raises a philosophical question about what is life. RNA
viruses include the retroviruses, of which the most notorious is HIV. Retroviruses are so-named because they induce
reverse transcription, which uses the RNA template of the virus to generate DNA, which is then incorporated into the
host’s genome.