polyethylenimine [537], or dextran moieties [538]. Although the cofactor is
freely dissolved, it cannot pass the membrane barrier due to its high molecular
weight.
It has to be emphasized that it is difficult to directly compare the different
methodologies of enzyme immobilization since enzymes differ in their properties
and details of the immobilization of industrial biocatalysts are often not disclosed.
3.4 Artificial and Modified Enzymes
Synthetic chemists have always admired the unparalleled catalytic efficiency and
specificity of enzymes with some envy and have attempted to copy the catalytic
principles which were developed by nature during evolution and to adopt them to
the needs of chemical synthesis. The following strategies can be distinguished.
3.4.1 Non-Proteinogenic Enzyme Mimics
In a biomimetic approach, enzyme models are created de novo from a
nonproteinogenic artificial scaffold, such a (natural) cyclodextrin, or a (synthetic)
dendrimer or polymer, which is endowded with a rationally designed catalytic site
[539]. The catalytic machinery represents a simplified chemical model of the
chemical operators found in enzymes and is often derived from a cofactor (e.g.,
cytochrome, pyridoxal or pyridoxamine phosphate) or a catalytic metal acting as
Lewis acid (e.g., Zn) or as central mediator in a redox reaction (e.g., Cu, Co, Mn,
Fe). Although many of these so-called synzymes show astonishing catalytic
activites, the catalytic efficiencies (expressed as rate accelerations and turnovers)
and (stereo)selectivities are way too low to use them for synthetic transformations.
What we gain from these studies, is a better understanding (and appreciation) of the
numerous subtle contributions which add up in a synergistic fashion in enzymes and
thereby make them as efficient as they are. From these studies we have learned that
the structural complexity of the whole three-dimensional structure of a protein
made up of several hundred amino acids is not out of mere biological luxury, but
due to mechanistic necessity.
3.4.2 Modified Enzymes
Modified enzymes can be created by leaving their protein scaffold basically intact
but altering some of their properties by chemical or genetic methods, which yields
3.4 Artificial and Modified Enzymes
379
freely dissolved, it cannot pass the membrane barrier due to its high molecular
weight.
It has to be emphasized that it is difficult to directly compare the different
methodologies of enzyme immobilization since enzymes differ in their properties
and details of the immobilization of industrial biocatalysts are often not disclosed.
3.4 Artificial and Modified Enzymes
Synthetic chemists have always admired the unparalleled catalytic efficiency and
specificity of enzymes with some envy and have attempted to copy the catalytic
principles which were developed by nature during evolution and to adopt them to
the needs of chemical synthesis. The following strategies can be distinguished.
3.4.1 Non-Proteinogenic Enzyme Mimics
In a biomimetic approach, enzyme models are created de novo from a
nonproteinogenic artificial scaffold, such a (natural) cyclodextrin, or a (synthetic)
dendrimer or polymer, which is endowded with a rationally designed catalytic site
[539]. The catalytic machinery represents a simplified chemical model of the
chemical operators found in enzymes and is often derived from a cofactor (e.g.,
cytochrome, pyridoxal or pyridoxamine phosphate) or a catalytic metal acting as
Lewis acid (e.g., Zn) or as central mediator in a redox reaction (e.g., Cu, Co, Mn,
Fe). Although many of these so-called synzymes show astonishing catalytic
activites, the catalytic efficiencies (expressed as rate accelerations and turnovers)
and (stereo)selectivities are way too low to use them for synthetic transformations.
What we gain from these studies, is a better understanding (and appreciation) of the
numerous subtle contributions which add up in a synergistic fashion in enzymes and
thereby make them as efficient as they are. From these studies we have learned that
the structural complexity of the whole three-dimensional structure of a protein
made up of several hundred amino acids is not out of mere biological luxury, but
due to mechanistic necessity.
3.4.2 Modified Enzymes
Modified enzymes can be created by leaving their protein scaffold basically intact
but altering some of their properties by chemical or genetic methods, which yields
3.4 Artificial and Modified Enzymes
379