Page 119 - Color Atlas of Biochemistry

P. 119

110 Metabolism

Activated metabolites 3. Phosphoadenosine phosphosulfate (PAPS)
Sulfate residues occur as strongly polar
Many coenzymes (see pp. 104ff.) serve to ac-
tivate molecules or groups that are poorly groups in various biomolecules—e. g., in gly-
cosaminoglycans (see p. 346) and conjugates
reactive. Activation consists of the formation
of reactive intermediate compounds in which of steroid hormones and xenobiotics (see
p. 316). In the synthesis of the “activated sul-
the group concerned is located at a higher
chemical potential and can therefore be fate” PAPS, ATP first reacts with anorganic
sulfate to form adenosine phosphosulfate
transferred to other molecules in an exer-
gonic reaction (see p. 124). Acetyl-CoA is an (APS, a). This intermediate already contains
exampleofthis typeofcompound (seep. 12). the “energy-rich” mixed anhydride bond be-
tween phosphoric acid and sulfuric acid. In
ATP and the other nucleoside triphosphate
coenzymes not only transfer phosphate resi- the second step, the 3 -OH group of APS is
phosphorylated, with ATP being used again.
dues, but also provide the nucleotide compo-
nents for this type of activation reaction. On After transfer of the sulfate residue to OH
this page, we discuss metabolites or groups groups (c), adenosine-3 ,5 -bisphosphate re-
mains.
that are activated in the metabolism by bond-
ing with nucleosides or nucleotides. Inter-
mediates of this type are mainly found in 4. S-adenosyl methionine (SAM)
the metabolism of complex carbohydrates
and lipids. The coenzyme tetrahydrofolate (THF) is the
main agent by which C 1 fragments are trans-
ferred in the metabolism. THF can bind this
A. Activated metabolites type of group in various oxidation states and
pass it on (see p. 108). In addition, there is
1. Uridine diphosphate glucose (UDPglucose)
“activated methyl,” in the form of S-adenosyl
The inclusion of glucose residues into poly- methionine (SAM). SAM is involved in many
mers such as glycogen or starches is an ender- methylation reactions—e. g., in creatine syn-
gonic process. The activation of the glucose thesis (see p. 336), the conversion of norepi-
building blocks that is required for this takes nephrine into epinephrine (see p. 352), the
places in several steps, in which two ATPs are inactivation of norepinephrine by methyla-
used per glucose. After the phosphorylation of tion of a phenolic OH group (see p. 316), and
free glucose, glucose 6-phosphate is isomer- in the formation of the active form of the
ized to glucose 1-phosphate (a), reaction with cytostatic drug 6-mercaptopurine (see
UTP (b) then gives rise to UDPglucose, in p. 402).
which the anomeric OH group at C-1 of the SAM is derived from degradation of the
sugar is bound with phosphate. This “energy- proteinogenic amino acid methionine,to
rich” compound (an acetal phosphate) allows whichthe adenosyl residue ofanATP mole-
exergonic transfer of glucose residues to gly- cule is transferred. After release of the acti-
cogen (c; see pp. 156, 408) or other acceptors. vated methyl group, S-adenosyl homocys-
teine (SAH) is left over. This can be converted
back into methionine in two further steps.
2. Cytidine diphosphate choline (CDPcholine)
Firstly, cleavage of the adenosine residue
The amino alcohol choline is activated for in- gives rise to the non-proteinogenic amino
clusion in phospholipids following a similar acid homocysteine,to which a methylgroup
5
principle (see p. 170). Choline is first phos- is transferred once again with the help of N -
phorylated by ATP to form choline phosphate methyl-THF (see p. 418). Alternatively, homo-
(a), which by reaction with CTP and cleavage cysteine canalso be brokendownintopro-
of diphosphate, then becomes CDPcholine. In pionyl-CoA.
contrast to (1), it is not choline that is trans-
ferred from CDPcholine, but rather choline
phosphate, which with diacylglycerol yields
phosphatidylcholine (lecithin).

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