created from a neutral group (–CONH 2 ). In order to become hydrated, this newly
generated carboxylate moiety pushes towards the protein’s surface causing
irreversible deactivation.
– Thiol groups may interchange the –S–S– disulfide bridges, leading to a modification of covalent bonds within the enzyme.
– Elimination and oxidation reactions (often involving cysteine residues) cause the
destruction of the protein.
Thermostable enzymes from thermophilic microorganisms show an astonishing
upper operation limit of 60–80
C and differ from their mesophilic counterparts by
only small changes in primary structure [110]. The three-dimensional structure of
such enzymes is often the same as those derived from mesophiles [111], but
generally they possess fewer asparagine residues and more salt- or disulfide bridges. More recently, numerous (thermo)stable mutant enzymes have been obtained
by genetic engineering. It is a common phenomenon, that an increased thermostability of proteins often goes in hand with an enhanced tolerance for organic
solvents.
1.4.2 Mechanistic Aspects of Enzyme Catalysis
Generally, numerous groups – occasionally also coordinated metal ions and cofactors – in the active site of an enzyme cooperate to effect catalysis. Individual
enzyme mechanisms have been elucidated in cases where the exact threedimensional structure is known. For many enzymes used for the biotransformation
of nonnatural organic compounds, assumptions are made about their molecular
action. However, the logic of organic reaction mechanisms, which is based on
thinking in terms of polarities – nucleophile/electrophile, acid/base, electron
source/sink –, represents an excellent intellectual basis for explaining protein
catalysis and organic chemists in particular will quickly see that there is nothing
‘magic’ about enzymes: they simply perform excellent organic chemistry.
The unparalleled catalytic power of enzymes has sparked numerous studies on
mechanistic theories to provide a molecular understanding of enzyme catalysis for
almost a century. Among the numerous theories and rationales, the most illustrative
models for the organic chemist are discussed here [112–114].
‘Lock-and-Key’ Mechanism
The first proposal for a general mechanism of enzymatic action was developed by
E. Fischer in 1894 [115, 116]. It assumes that an enzyme and its substrate mechanistically interact like a lock and key
15 (Fig. 1.2). Although this assumption was
quite sophisticated at that time, it assumes a completely rigid enzyme structure.
15 ‘To use a picture I want to say that enzyme and glucoside must go together like key and lock in
order to exert a chemical effect upon each other’, see [115] p. 2992.
1.4 Enzyme Properties and Nomenclature
13
generated carboxylate moiety pushes towards the protein’s surface causing
irreversible deactivation.
– Thiol groups may interchange the –S–S– disulfide bridges, leading to a modification of covalent bonds within the enzyme.
– Elimination and oxidation reactions (often involving cysteine residues) cause the
destruction of the protein.
Thermostable enzymes from thermophilic microorganisms show an astonishing
upper operation limit of 60–80
C and differ from their mesophilic counterparts by
only small changes in primary structure [110]. The three-dimensional structure of
such enzymes is often the same as those derived from mesophiles [111], but
generally they possess fewer asparagine residues and more salt- or disulfide bridges. More recently, numerous (thermo)stable mutant enzymes have been obtained
by genetic engineering. It is a common phenomenon, that an increased thermostability of proteins often goes in hand with an enhanced tolerance for organic
solvents.
1.4.2 Mechanistic Aspects of Enzyme Catalysis
Generally, numerous groups – occasionally also coordinated metal ions and cofactors – in the active site of an enzyme cooperate to effect catalysis. Individual
enzyme mechanisms have been elucidated in cases where the exact threedimensional structure is known. For many enzymes used for the biotransformation
of nonnatural organic compounds, assumptions are made about their molecular
action. However, the logic of organic reaction mechanisms, which is based on
thinking in terms of polarities – nucleophile/electrophile, acid/base, electron
source/sink –, represents an excellent intellectual basis for explaining protein
catalysis and organic chemists in particular will quickly see that there is nothing
‘magic’ about enzymes: they simply perform excellent organic chemistry.
The unparalleled catalytic power of enzymes has sparked numerous studies on
mechanistic theories to provide a molecular understanding of enzyme catalysis for
almost a century. Among the numerous theories and rationales, the most illustrative
models for the organic chemist are discussed here [112–114].
‘Lock-and-Key’ Mechanism
The first proposal for a general mechanism of enzymatic action was developed by
E. Fischer in 1894 [115, 116]. It assumes that an enzyme and its substrate mechanistically interact like a lock and key
15 (Fig. 1.2). Although this assumption was
quite sophisticated at that time, it assumes a completely rigid enzyme structure.
15 ‘To use a picture I want to say that enzyme and glucoside must go together like key and lock in
order to exert a chemical effect upon each other’, see [115] p. 2992.
1.4 Enzyme Properties and Nomenclature
13