Page 89 - Color Atlas of Biochemistry

P. 89

80 Biomolecules

Bases and nucleotides residues, it yields nucleoside diphosphates
and triphosphates—e. g., ADP and ATP, which
The nucleic acids play a central role in the are important coenzymes in energy metabo-
storage and expression of genetic information lism (see p.106). All of these nucleoside phos-
(see p. 236). They are divided into two major phates are classified as nucleotides.
classes: deoxyribonucleic acid (DNA) func- In nucleosides and nucleotides, the pentose
tions solely in information storage, while ri- residues arepresent in thefuranoseform (see
bonucleic acids (RNAs) are involved in most p. 34). The sugars and bases are linked by an
steps of gene expression and protein biosyn- N-glycosidic bond between the C-1 of the
thesis. All nucleic acids are made up from sugar and either the N-9 of the purine ring
nucleotide components, which in turn consist or N-1 of the pyrimidine ring. This bond al-
of a base,a sugar,and a phosphate residue. ways adopts the β-configuration.
DNA and RNA differ from one another in the
type of the sugar and in one of the bases that
they contain. C. Oligonucleotides, polynucleotides
Phosphoric acid molecules can form acid–an-
hydride bonds with each other. It is therefore
A. Nucleic acid bases
possible for two nucleotides to be linked via
The bases that occur in nucleic acids are the phosphate residues. This gives rise to di-
aromatic heterocyclic compounds derived nucleotides with a phosphoric acid–anhydride
from either pyrimidine or purine.Fiveofthese structure. This group includes the coenzymes
bases are the main components of nucleic NAD(P) + and CoA, as well as the flavin
acids in all living creatures. The purine bases derivative FAD (1;see p.104).
adenine (abbreviation Ade, not “A”!) and gua- If the phosphate residue of a nucleotide
nine (Gua) and the pyrimidine base cytosine reacts with the 3 -OH group of a second nu-
(Cyt) are present in both RNA and DNA. In cleotide, the result is a dinucleotide with a
contrast, uracil (Ura) is only found in RNA. In phosphoric acid diester structure.Dinucleo-
DNA, uracil is replaced by thymine (Thy), the tides ofthis typehavea free phosphateresi-
5-methyl derivative of uracil. 5-methylcyto- due at the 5 end and a free OH group at the 3
sine also occurs in small amounts in the DNA end. They can therefore be extended with
of the higher animals. A large number of other additional mononucleotides by adding fur-
modified bases occur in tRNA (see p. 82) and ther phosphoric acid diester bonds. This is
in other types of RNA. the way in which oligonucleotides,and ulti-
mately polynucleotides, are synthesized.
Polynucleotides consisting of ribonucleo-
B. Nucleosides, nucleotides
tide components are called ribonucleic acid
When a nucleic acid base is N-glycosidically (RNA),while thoseconsistingof deoxyribonu-
linked to ribose or 2-deoxyribose (see p. 38), it cleotide monomers are called deoxyribonu-
yields a nucleoside. The nucleoside adenosine cleic acid (DNA;see p. 84). To describe the
(abbreviation: A) is formed in this way from structure of polynucleotides, the abbrevia-
adenine and ribose, for example. The corre- tions for the nucleoside components are writ-
sponding derivatives of the other bases are ten from left to right in the 5 3 direction.
called guanosine (G), uridine (U), thymidine The position of the phosphate residue is also
(T) and cytidine (C). When the sugar compo- sometimes indicated by a “p”. In this way, the
nent is 2-deoxyribose, the product is a structure of the RNA segment shown Fig. 2
deoxyribonucleoside—e. g., 2 -deoxyadeno- can be abbreviated as ..pUpG.. or simply as
sine (dA, not shown). In the cell, the 5 OH .. UG .. .
group of the sugar component of the nucleo-
side is usually esterified with phosphoric acid.
2 -Deoxythymidine (dT) therefore gives rise
to 2 -deoxythymidine-5 -monophosphate
(dTMP), one of the components of DNA (2).
If the 5 phosphate residue is linked via an
acid–anhydride bond to additional phosphate

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