Chemistry Reference
In-Depth Information
A
B
Figure 10.5
Formation of the pyridoxine ring in vitamin B
6
. (a) deoxyxylu-
lose phosphate-dependent pathway; (b) deoxyxylulose phosphate-independent pathway.
(
43
), 1-deoxy-D-xylulose 5-phosphate; (
47
), 3-amino-1-hydroxyacetone 1-phosphate;
(
48
), pyridoxine 5
-phosphate; (
49
), ribulose 5-phosphate; (
50
), dihydroxyacetone phos-
phate; (
39
), pyridoxal 5
-phosphate.
capacity of humans is limited. On a diet that is low in tryptophan, the combined
contributions of endogenous synthesis and nutritional supply of precursors, such
as nicotinic acid, nicotinamide, and nicotinamide riboside, may be insufficient,
which results in cutaneous manifestation of niacin deficiency under the clinical
picture of pellagra. Exogenous supply of nicotinamide riboside was shown to
promote NAD
+
-dependent Sir2-function and to extend life span in yeast without
calorie restriction (32).
Bacteria and plants use aspartate (
54
) and dihydroxyacetone phosphate (
50
)
as precursors for the biosynthesis of nicotinamide via quinolinic acid (
53
),
(Fig. 10.6b) (33).
The transformation of precursors into NAD (
56
) and NADP (
57
) follow the
same pathway in all organisms. A ribosyl phosphate residue can be transferred
to biosynthetic quinolinic acid or to preformed nicotinamide affording (
55
)(or
its amide) that is converted to NAD by adenylation. Subsequent phosphorylation
yields NADP.
10.2.7 Pantothenate
Pantothenate (also designated vitamin B
5
, Fig. 10.7) (
64
), is biosynthesized
de novo
in plants and many microorganisms but must be obtained from nutri-
tional sources by animals (20, 34). The branched carboxylic acid (
63
) is obtained
from
α
-ketoisovalerate (
61
), which is an intermediate of valine biosynthesis, via
(
62
).
β
-Alanine (
60
) is obtained by decarboxylation of aspartate (
54
) in microor-
ganisms. Plants and yeasts can biosynthesize
β
-alanine from spermine (
58
) (35).
An additional pathway to (
60
) starting from uracil (
59
) has been reported in
