Chemistry Reference
In-Depth Information
(a)
(b)
The structure of yeast tRNA
Phe
(a) the cloverleaf form of the base sequence: tertiary base-pairing interactions are represented
by thin red lines connecting the participating bases. Bases that are conserved in all tRNAs are circled by solid and dashed lines respectively. The
different parts of the structure are colour-coded. (b) The X-ray structure showing how the different base-paired stems are arranged to form an
L-shaped molecule. The sugar-phosphate backbone is represented as a ribbon with the same colour scheme as in (a). (From
Voet & Voet, 2004
.
Reproduced with permission from John Wiley and Sons, Inc.)
FIGURE 3.20
Carbohydrates
Carbohydrates, so-called because they have the empirical formula C
n
H
2n
O
n
, make up an important class of
biomolecules. They are extensively used in the form of simple sugars (monosaccharides) in intermediary
metabolism (Chapter 5), in polymer form (polysaccharides) as storage forms of both carbon and energy (such as
polymers of glucose in the starch of plants and the glycogen of animals), and as structural elements (polymers of
glucose in the cellulose of plant cell walls, and of N-acetylglucasamine in the chitin of invertebrate shells), or even
attached to proteins and lipids in highly specific structure involved in biological recognition (like the blood group
determinants).
We begin by describing very briefly the main classes of monosaccharides, together with some of the stereo-
chemical properties which give them their particularity with regard to molecular recognition. If we stick with the
definition of monosaccharides as (CH
2
O)
n
, it is clear that we need n to be at least 3 (three carbon 'oses', or sugars
called trioses), and we must have one of the carbons at the level of oxidation of an aldehyde or ketone
e
as
illustrated (
Figure 3.21
)
by the structure of the two trioses, glyceraldehyde (an aldose), and dihydroxyacetone
(a ketose). These two molecules are isomers of one another, and can be interconverted by enzymes called
isomerases. We note an important point in addition, namely, that the hydroxyl group on the central carbon of
glyceraldehyde can be either on the right or on the left side of the central carbon chain. In fact, most sugars (with
the exception of the sugar acid ascorbic acid, better known as vitamin C) have the hydroxyl furthest away from
their potential reducing function (the aldehyde or ketone group) to the right, and are therefore called D-sugars. We
will not fatigue the reader with the niceties of the structures of the different aldo- and keto-tetroses, pentoses,
hexoses, etc., but simply illustrate a few more, all of which play an important role in metabolism, often in
a phosphorylated form
D
-ribose,
D
-galactose, and
D
-glucose (aldoses and D-fructose (ketose)). We illustrate our
next important point with glucose (an aldohexose), which can exist in a cyclic form by ring closure between the
aldehyde function and the hydroxyl function on carbon 5
e
e
this forms what is called a hemiacetal, and when the
