Page 17 - Color Atlas of Biochemistry

P. 17

8 Basics

Isomerism romoleculessuch asproteinsornucleic acids
usually have well-defined (“native”) confor-
Isomers are molecules with the same compo- mations, which are stabilized by interactions
sition (i. e. the same molecular formula), but in themolecule(seep. 74).
with different chemical and physical proper-
ties. If isomers differ in the way in which their
atoms are bonded in the molecule, they are C. Optical isomers
described as structural isomers (cf. citric acid Another type of isomerism arises when a mol-
and isocitric acid, D). Other forms of isomer- ecule contains a chiral center or is chiral as a
ism are based on different arrangements of whole. Chirality (from the Greek cheir, hand)
the substituents of bonds (A, B)or on the leads to the appearance of structures that
presence of chiral centers in the molecule (C). behave like image and mirror-image and
that cannot be superimposed (“mirror” iso-
mers). Themostfrequentcause of chiral be-
A. cis–trans isomers
havior is the presence of an asymmetric C
Double bonds are not freely rotatable (see atom—i. e., an atom with four different sub-
p. 4). If double-bonded atoms have different stituents. Then there are two forms (enan-
substituents, there are two possible orienta- tiomers) with different configurations. Usu-
tions for these groups. In fumaric acid,an ally, the two enantiomers of a molecule are
intermediate of the tricarboxylic acid cycle designated as L and D forms. Clear classifica-
(see p.136), the carboxy groups lie on different tion of the configuration is made possible by
sides of the double bond (trans or E position). the R/S system (see chemistry textbooks).
In its isomer maleic acid, which is not pro- Enantiomers have very similar chemical
duced in metabolic processes, the carboxy properties, but they rotate polarized light in
groups lie on the same side of the bond (cis opposite directions (optical activity,see
or Z position). Cis–trans isomers (geometric pp. 36, 58). Thesameapplies to theenantiom-
isomers) have different chemical and physical ers of lactic acid.The dextrorotatory L-lactic
properties—e. g., their melting points (Fp.) acid occurs in animal muscle and blood, while
and pK a values. They can only be intercon- the D form produced by microorganisms is
verted by chemical reactions. found in milk products, for example (see
In lipid metabolism, cis–trans isomerism is p.148). The Fischer projection is often used
particularly important. For example, double to represent the formulas for chiral centers
bonds in natural fatty acids (see p. 48) usually (cf. p. 58).
have a cis configuration. By contrast, unsatu-
rated intermediates of β oxidation have a
D. The aconitase reaction
trans configuration. This makes the break-
down of unsaturated fatty acids more compli- Enzymes usually function stereospecifically. In
cated (see p.166). Light-induced cis–trans iso- chiral substrates, they only accept one of the
merization of retinal is of central importance enantiomers, and the reaction products are
in the visual cycle (see p. 358). usually also sterically uniform. Aconitate
hydratase (aconitase) catalyzes the conver-
sion of citric acid into the constitution isomer
B. Conformation
isocitric acid (see p.136). Although citric acid
Molecular forms that arise as a result of rota- is not chiral, aconitase only forms one of the
tion around freely rotatable bonds are known four possible isomeric forms of isocitric acid
as conformers. Even small molecules can have (2R,3S-isocitric acid). The intermediate of the
different conformations in solution. In the reaction, the unsaturated tricarboxylic acid
two conformations of succinic acid illustrated aconitate, only occurs in the cis form in the
opposite, the atoms are arranged in a similar reaction. The trans form of aconitate is found
way to fumaric acid and maleic acid. Both as a constituent of certain plants.
forms are possible, although conformation 1
is more favorable due to the greater distance
between the COOH groups and therefore oc-
curs more frequently. Biologically active mac-

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