Page 21 - Color Atlas of Biochemistry
P. 21
12 Basics
Biomolecules II genic amine (see p. 62) formed by decarbox-
ylation of the amino acid cysteine.
Many biomolecules are made up of smaller (2) The amino group of cysteamine is
units in a modular fashion, and they can be bound to the carboxy group of another bio-
broken down into these units again. The con- genic amine via an acid amide bond (-CO-
struction of these molecules usually takes NH-). β-Alanine arises through decarboxyla-
place through condensation reactions involv- tion of the amino acid aspartate, but it can
ing the removal of water. Conversely, their also be formed by breakdown of pyrimidine
breakdown functions in a hydrolytic fash- bases (see p.186).
ion—i. e., as a result of water uptake. The (3) Another acid amide bond (-CO-NH-)
page opposite illustrates this modular princi- creates the compound for the next
ple using the example of an important coen- constituent, pantoinate.This compound con-
zyme. tains a chiral center and can therefore appear
in two enantiomeric forms (see p. 8). In natu-
ral coenzyme A, only one of the two forms is
A. Acetyl CoA
found, the (R)-pantoinate. Human metabo-
Coenzyme A (see also p.106) is a nucleotide lism is not capable of producing pantoinate
with a complex structure (see p. 80). It serves itself, and it therefore has to take up a
to activate residues of carboxylic acids (acyl compound of β-alanine and pantoinate—
residues). Bonding of the carboxy group of the pantothenate (“pantothenic acid”)—in the
carboxylic acid with the thiol group of the form of a vitamin in food (see p. 366).
coenzyme creates a thioester bond (-S-CO-R; (4) The hydroxy group at C-4 of pantoinate
see p.10) in which the acyl residue has a high is bound to a phosphate residue by an ester
chemical potential. It can therefore be trans- bond.
ferred to other molecules in exergonic reac- Thesection of themoleculediscussed so
tions. This fact plays an important role in lipid far represents a functional unit. In the cell, it is
metabolism in particular (see pp.162ff.), as produced from pantothenate. The molecule
well as in two reactions of the tricarboxylic also occurs in a protein-bound form as 4 -
acid cycle(seep.136). phosphopantetheine in the enzyme fatty
As discussed on p.16, the group transfer acid synthase (see p.168). In coenzyme A,
potential can be expressed quantitatively as however, it is bound to 3 ,5 -adenosine di-
the change in free enthalpy (∆G) during hy- phosphate.
drolysis of the compound concerned. This is (5) When two phosphate residues bond,
an arbitrary determination, but it provides they do not form an ester, but an “energy-
important indications of the chemical energy rich” phosphoric acid anhydride bond,as
stored insuchagroup. In the case ofacetyl- also occurs in other nucleoside phosphates.
CoA, the reaction to be considered is: By contrast, (6) and (7) are ester bonds again.
(8) The base adenine is bound to C-1 of
Acetyl CoA + H 2 O acetate + CoA ribose by an N-glycosidic bond (see p. 36). In
addition to C-2 to C-4, C-1 of ribose also rep-
In standard conditions and at pH 7, the resents a chiral center. The E-configuration is
0
change in the chemical potential G (∆G ,see usually found in nucleotides.
p.18) in this reaction amounts to –32 kJ
mol –1 and it is therefore as high as the ∆G 0
of ATP hydrolysis (see p.18). In addition to the
“energy-rich” thioester bond, acetyl-CoA also
has seven other hydrolyzable bonds with dif-
ferent degrees of stability. These bonds, and
the fragments that arise when they are hydro-
lyzed, will be discussed here in sequence.
(1) The reactive thiol group of coenzyme A
is located in the part of the molecule that is
derived from cysteamine.Cysteamine is a bio-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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