Chemistry Reference
In-Depth Information
whale. The four globin chains of haemoglobin have a very similar three-dimensional structure to myoglobin. It
later became apparent that oxygen-binding hemoproteins from invertebrate insect larvae (Chironomus thummi)
and from the lamprey, a blood-sucking eel-like cyclostome (a subclass of jawless parasitic fish), also have the
globin fold. The pairwise arrangements of the bundle of eight
-helices in the globin fold are quite different from
that found in the four helix bundle domains. The helices, which vary in length from 7 (helix C) to 28 (helix H)
residues, wrap around the core of the molecule, with its haem-binding pocket, such that most adjacent helices in
the 3-D structure are not adjacent in the amino acid sequence.
a
The Structural Building Blocks of Nucleic Acids
Nucleic acids are made up of three components
nitrogen-rich bases of the pyrimidine and purine families,
illustrated in
Figure 3.16
by the DNA bases adenine and guanine, and thymine and cytosine respectively, which are
e
FIGURE 3.16
The structures of the four bases (A, G, T and C) found in DNA and of a nucleoside (deoxyadenosine) and a nucleotide
(deoxyadenosine-5
0
-triphosphate).
linked in an N-glycosidic bond to a sugar (either ribose or deoxyribose in RNA and DNA respectively) and
phosphate groups, which link the sugar residues. The combination of a nucleobase with a sugar generates a
nucleoside
hence, adenine becomes deoxyadenosine when bound to deoxyribose, while a nucleobase plus
a sugar plus one or more phosphate residues (usually on the 5
0
-hydroxyl of the ribose or deoxyribose) constitutes a
nucleotide (in the example given, deoxyadenosine 5
0
-triphosphate).
e
