Page 115 - Color Atlas of Biochemistry

P. 115

106 Metabolism

Coenzymes 2 of energetic coupling (see p. 124) also allow
endergonic processes to proceed. Metabolites
are often made more reactive (“activated”) as
A. Redox coenzymes 2
a result of the transfer of phosphate residues
In lipoic acid (6), an intramolecular disulfide (phosphorylation). Bonding with nucleoside
bond functionsasa redox-active structure. As diphosphate residues (mainly UDP and CDP)
a result of reduction, it is converted into the provides activated precursors for polysac-
corresponding dithiol. As a prosthetic group, charides and lipids (see p. 110). Endergonic
lipoic acid is usually covalently bound to a formation of bonds by ligases (enzyme class
lysine residue (R) of the enzyme, and it is 6) also depends on nucleoside triphosphates.
then referred to as lipoamide. Lipoamide is Acyl residues are usually activated by
mainly involved in oxidative decarboxylation transfer to coenzyme A (2). In coenzyme A
of 2-oxo acids (see p. 134). The peptide (see p. 12), pantetheine is linked to 3 -phos-
coenzyme glutathione is a similar disulfide/ pho-ADP by a phosphoric acid anhydride
dithiol system (not shown; see p. 284). bond. Pantetheine consists of three compo-
Iron–sulfur clusters (7)occur as prosthetic nents connected by amide bonds—pantoic
groups in oxidoreductases, but they are also acid, E-alanine,and cysteamine. The latter
found in lyases—e. g., aconitase (see p. 136) two components are biogenic amines formed
and other enzymes. Iron–sulfur clusters con- by the decarboxylation of aspartate and
sist of 2–4 iron ions that are coordinated with cysteine, respectively. The compound formed
cysteine residues of the protein (–SR) and from pantoic acid and β−alanine (pantothenic
with anorganic sulfide ions (S). Structures of acid) has vitamin-like characteristics for hu-
this type are only stable in the interior of mans (see p. 368). Reactions between the
proteins. Depending on the number of iron thiol group of the cysteamine residue and
and sulfide ions, distinctions are made be- carboxylic acids give rise to thioesters,such
tween [Fe 2 S 2 ], [Fe 3 S 4 ], and [Fe 4 S 4 ]clusters. as acetyl CoA. This reaction is strongly ender-
These structures are particularly numerous gonic, and it is therefore coupled to exergonic
in the respiratory chain (see p. 140), and processes. Thioesters represent the activated
they are found in all complexes except com- form of carboxylic acids, because acyl residues
plex IV. of this type have a high chemical potential
Heme coenzymes (8)withredox functions and are easily transferred to other molecules.
exist in the respiratory chain (see p. 140), in This property is often exploited in metabo-
photosynthesis (see p. 128), and in monooxy- lism.
genases and peroxidases (see p. 24). Heme- Thiamine diphosphate (TPP, 3), in coopera-
containing proteins with redox functions are tion with enzymes, is able to activate alde-
also referred to as cytochromes.In cyto- hydes or ketones as hydroxyalkyl groups and
chromes, in contrast to hemoglobin and myo- then to pass them on to other molecules. This
globin, the iron changes its valence (usually type of transfer is important in the transketo-
between +2 and +3). There are several classes lase reaction, for example (see p. 152). Hy-
of heme (a, b, and c), which have different droxyalkyl residues also arise in the decar-
types of substituent – R 1 to – R 3 .Hemoglobin, boxylation of oxo acids. In this case, they are
myoglobin, and the heme enzymes contain released as aldehydes or transferred to lipo-
heme b. Two types of heme a are found in amide residues of 2-oxoacid dehydrogenases
cytochrome c oxidase(seep. 132), while (see p. 134). The functional component of TPP
heme c mainly occurs in cytochrome c,where is the sulfur- and nitrogen-containing thiazole
it is covalently bound with cysteine residues ring.
of the protein part via thioester bonds.

B. Group-transferring coenzymes 1

The nucleoside phosphates (1) are not only
precursors for nucleic acid biosynthesis; many
of them also have coenzyme functions. They
serve for energy conservation, and as a result

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