Chemistry Reference
In-Depth Information
fatty acid and amino acid metabolism (6). Notably, because plants do not gener-
ate corrinoids, animals depend on bacteria for their supply of vitamin B
12
(which
may be obtained in recycled form via nutrients such as milk and meat) (7).
However, the sheer complexity of a pathway is not an indicator for the vitamin
status of a given class of compounds, as opposed to endogenous biosynthesis
in mammals. Thus, animals biosynthesize molybdopterin, which is a cofactor
involved in certain redox reactions, from basic building blocks using at least
eight enzymes, whereas folic acid has vitamin status and must be obtained by
animals from nutritional sources (8).
It should also be noted that some pathways to be discussed below depend on
highly unusual chemical reactions. To give just one example, the formation of the
pyridine ring system of vitamin B
6
depends on a protein that catalyzes a com-
plex series of reactions, including carbohydrate isomerization, imine formation,
ammonia addition, aldol-type condensation, cyclization, and aromatization (9).
10.2 SPECIFIC BIOSYNTHETIC PATHWAYS
10.2.1 Biosynthesis of Iron/Sulfur Clusters
Iron/sulfur clusters are inorganic cofactors that are used in all cells (10). They
comprise S
2
−
ions and iron ions in the
+
2or
+
3 state (Fig. 10.1). Iron/sulfur
clusters are essential cofactors for numerous redox and nonredox enzymes, alone
or in tandem with organic cofactors such as flavocoenzymes and/or pyridine
nucleotides. The simplest structural type is the rhombic [2Fe-2 S] cluster (
3
).
[3Fe-4 S] (
4
) and [4Fe-4 S] (
5
) clusters are characterized by distorted cubic
Figure 10.1
Formation of protein-bound persulfide and its delivery to sulphur-containing
natural compounds. (
1
), cysteine; (
2
), persulfide of a protein bound cysteine; (
3
), rhombic
[2Fe-2 S] cluster; (
4
), [3Fe-4 S] cluster; (
5
), [4Fe-4 S] cluster; (
6
), thiamine;(
7
), lipoic
acid; (
8
), molybtopterin; (
9
), biotin.
